Vulkan Image Operations are operations performed by those SPIR-V Image
Instructions which take an OpTypeImage (representing a
VkImageView) or OpTypeSampledImage (representing a
(VkImageView, VkSampler) pair).
Read, write, and atomic operations also take texel coordinates as operands,
and return a value based on a neighborhood of texture elements (texels)
within the image.
Query operations return properties of the bound image or of the lookup
itself.
The “Depth” operand of OpTypeImage is ignored.
Texel is a term which is a combination of the words texture and element.
Early interactive computer graphics supported texture operations on
textures, a small subset of the image operations on images described here.
The discrete samples remain essentially equivalent, however, so we retain
the historical term texel to refer to them.
Image Operations include the functionality of the following SPIR-V Image
Instructions:
OpImageSample* and OpImageSparseSample* read one or more
neighboring texels of the image, and filter
the texel values based on the state of the sampler.
Instructions with ImplicitLod in the name
determine the LOD used in the
sampling operation based on the coordinates used in neighboring
fragments.
Instructions with ExplicitLod in the name
determine the LOD used in the
sampling operation based on additional coordinates.
Instructions with Proj in the name apply homogeneous
projection to the coordinates.
OpImageFetch and OpImageSparseFetch return a single texel of
the image.
No sampler is used.
OpImage*Gather and OpImageSparse*Gather read neighboring
texels and return a single component of each.
OpImageRead (and OpImageSparseRead) and OpImageWrite read
and write, respectively, a texel in the image.
No sampler is used.
OpImageSampleFootprintNV identifies and returns information about
the set of texels in the image that would be accessed by an equivalent
OpImageSample* instruction.
OpImage*Dref* instructions apply
depth comparison on the texel
values.
OpImageSparse* instructions additionally return a
sparse residency code.
OpImageQuerySize, OpImageQuerySizeLod,
OpImageQueryLevels, and OpImageQuerySamples return properties
of the image descriptor that would be accessed.
The image itself is not accessed.
OpImageQueryLod returns the LOD parameters that would be used in a
sample operation.
The actual operation is not performed.
OpImageWeightedSampleQCOM reads a 2D neighborhood of texels and
computes a weighted average using weight values from a separate weight
texture.
opImageBlockMatchSADQCOM and opTextureBlockMatchSSD compare 2D
neighborhoods of texels from two textures.
OpImageBoxFilterQCOM reads a 2D neighborhood of texels and computes
a weighted average of the texels.
opImageBlockMatchWindowSADQCOM and
opImageBlockMatchWindowSSDQCOM compare 2D neighborhoods of texels
from two textures with the comparison repeated across a window region in
the target texture.
opImageBlockMatchGatherSADQCOM and
opImageBlockMatchWindowSSDQCOM compares four 2D neighborhoods of
texels from a target texture with a single 2D neighborhood in the
reference texture.
The R component of each comparison is gathered and returned in the
output.
Texel Coordinate Systems
Images are addressed by texel coordinates.
There are three texel coordinate systems:
SPIR-V OpImageFetch, OpImageSparseFetch, OpImageRead,
OpImageSparseRead,
opImageBlockMatchSADQCOM, opImageBlockMatchSSDQCOM,
opImageBlockMatchWindowSADQCOM, opImageBlockMatchWindowSSDQCOM,
and OpImageWrite instructions use integer texel coordinates.
Other image instructions can use either normalized or unnormalized texel
coordinates (selected by the unnormalizedCoordinates state of the
sampler used in the instruction), but there are
limitations on what operations, image
state, and sampler state is supported.
Normalized coordinates are logically
converted to unnormalized as part of
image operations, and certain steps are
only performed on normalized coordinates.
The array layer coordinate is always treated as unnormalized even when other
coordinates are normalized.
Normalized texel coordinates are referred to as (s,t,r,q,a), with the
coordinates having the following meanings:
s: Coordinate in the first dimension of an image.
t: Coordinate in the second dimension of an image.
r: Coordinate in the third dimension of an image.
(s,t,r) are interpreted as a direction vector for Cube images.
q: Fourth coordinate, for homogeneous (projective) coordinates.
a: Coordinate for array layer.
The coordinates are extracted from the SPIR-V operand based on the
dimensionality of the image variable and type of instruction.
For Proj instructions, the components are in order (s, [t,] [r,]
q), with t and r being conditionally present based on the
Dim of the image.
For non-Proj instructions, the coordinates are (s [,t] [,r]
[,a]), with t and r being conditionally present based on the
Dim of the image and a being conditionally present based on the
Arrayed property of the image.
Projective image instructions are not supported on Arrayed images.
Unnormalized texel coordinates are referred to as (u,v,w,a), with the
coordinates having the following meanings:
u: Coordinate in the first dimension of an image.
v: Coordinate in the second dimension of an image.
w: Coordinate in the third dimension of an image.
a: Coordinate for array layer.
Only the u and v coordinates are directly extracted from the
SPIR-V operand, because only 1D and 2D (non-Arrayed) dimensionalities
support unnormalized coordinates.
The components are in order (u [,v]), with v being conditionally
present when the dimensionality is 2D.
When normalized coordinates are converted to unnormalized coordinates, all
four coordinates are used.
Integer texel coordinates are referred to as (i,j,k,l,n), with the
coordinates having the following meanings:
i: Coordinate in the first dimension of an image.
j: Coordinate in the second dimension of an image.
k: Coordinate in the third dimension of an image.
l: Coordinate for array layer.
n: Index of the sample within the texel.
They are extracted from the SPIR-V operand in order (i [,j] [,k] [,l]
[,n]), with j and k conditionally present based on the Dim
of the image, and l conditionally present based on the Arrayed
property of the image.
n is conditionally present and is taken from the Sample image
operand.
If an accessed image was created from a view using
VkImageViewSlicedCreateInfoEXT and accessed through a
VK_DESCRIPTOR_TYPE_STORAGE_IMAGE descriptor, then the value of k
is incremented by VkImageViewSlicedCreateInfoEXT::sliceOffset,
giving k ← sliceOffset + k.
The image’s accessible range in the third dimension is k < sliceOffset
+ sliceCount.
If VkImageViewSlicedCreateInfoEXT::sliceCount is
VK_REMAINING_3D_SLICES_EXT, the range is inherited from the image’s
depth extent as specified by Image Mip Level Sizing.
For all coordinate types, unused coordinates are assigned a value of zero.
Figure 1. Texel Coordinate Systems, Linear Filtering
The Texel Coordinate Systems - For the example shown of an 8×4 texel
two dimensional image.
Normalized texel coordinates:
The s coordinate goes from 0.0 to 1.0.
The t coordinate goes from 0.0 to 1.0.
Unnormalized texel coordinates:
The u coordinate within the range 0.0 to 8.0 is within the image,
otherwise it is outside the image.
The v coordinate within the range 0.0 to 4.0 is within the image,
otherwise it is outside the image.
Integer texel coordinates:
The i coordinate within the range 0 to 7 addresses texels within
the image, otherwise it is outside the image.
The j coordinate within the range 0 to 3 addresses texels within
the image, otherwise it is outside the image.
Also shown for linear filtering:
Given the unnormalized coordinates (u,v), the four texels
selected are i0j0, i1j0, i0j1, and
i1j1.
The fractions α and β.
Given the offset Δi and Δj, the
four texels selected by the offset are i0j'0,
i1j'0, i0j'1, and i1j'1.
For formats with reduced-resolution components, Δi and
Δj are relative to the resolution of the
highest-resolution component, and therefore may be divided by two relative
to the unnormalized coordinate space of the lower-resolution components.
Texel input instructions are SPIR-V image instructions that read from an
image.
Texel input operations are a set of steps that are performed on state,
coordinates, and texel values while processing a texel input instruction,
and which are common to some or all texel input instructions.
They include the following steps, which are performed in the listed order:
For texel input instructions involving multiple texels (for sampling or
gathering), these steps are applied for each texel that is used in the
instruction.
Depending on the type of image instruction, other steps are conditionally
performed between these steps or involving multiple coordinate or texel
values.
Texel input validation operations inspect instruction/image/sampler state
or coordinates, and in certain circumstances cause the texel value to be
replaced or become undefined.
There are a series of validations that the texel undergoes.
Instruction/Sampler/Image View Validation
There are a number of cases where a SPIR-V instruction can mismatch with
the sampler, the image view, or both, and a number of further cases where
the sampler can mismatch with the image view.
In such cases the value of the texel returned is undefined.
These cases include:
The sampler borderColor is an integer type and the image view
format is not one of the VkFormat integer types or a stencil
component of a depth/stencil format.
The sampler borderColor is a float type and the image view
format is not one of the VkFormat float types or a depth
component of a depth/stencil format.
VkSamplerBorderColorComponentMappingCreateInfoEXT::components,
if specified, has a component swizzle that does not match the component
swizzle of the image view, and either component swizzle is not a form of
identity swizzle.
The sampler borderColor is a custom color
(VK_BORDER_COLOR_FLOAT_CUSTOM_EXT or
VK_BORDER_COLOR_INT_CUSTOM_EXT) and the supplied
VkSamplerCustomBorderColorCreateInfoEXT::customBorderColor
is outside the bounds of the values representable in the image view’s
format.
The sampler was created with flags containing
VK_SAMPLER_CREATE_SUBSAMPLED_BIT_EXT and the image was not created
with flags containing VK_IMAGE_CREATE_SUBSAMPLED_BIT_EXT.
The sampler was not created with flags containing
VK_SAMPLER_CREATE_SUBSAMPLED_BIT_EXT and the image was created
with flags containing VK_IMAGE_CREATE_SUBSAMPLED_BIT_EXT.
The sampler was created with flags containing
VK_SAMPLER_CREATE_SUBSAMPLED_BIT_EXT and is used with a function
that is not OpImageSampleImplicitLod or
OpImageSampleExplicitLod, or is used with operands Offset or
ConstOffsets.
The SPIR-V instruction is one of the OpImage*Dref* instructions and
the sampler compareEnable is VK_FALSE
The SPIR-V instruction is not one of the OpImage*Dref* instructions
and the sampler compareEnable is VK_TRUE
The SPIR-V instruction is one of the OpImage*Dref* instructions,
the image view format is one of the depth/stencil formats, and the
image view aspect is not VK_IMAGE_ASPECT_DEPTH_BIT.
The SPIR-V instruction’s image variable’s properties are not compatible
with the image view:
If the image view’s viewType is one of
VK_IMAGE_VIEW_TYPE_1D_ARRAY, VK_IMAGE_VIEW_TYPE_2D_ARRAY,
or VK_IMAGE_VIEW_TYPE_CUBE_ARRAY then the instruction must have
Arrayed = 1.
Otherwise the instruction must have Arrayed = 0.
If the image was created with VkImageCreateInfo::samples
equal to VK_SAMPLE_COUNT_1_BIT, the instruction must have
MS = 0.
If the image was created with VkImageCreateInfo::samples
not equal to VK_SAMPLE_COUNT_1_BIT, the instruction must have
MS = 1.
If the SampledType of the OpTypeImage does not match
the SPIR-V Type.
If the image was created with VkImageCreateInfo::flags
containing VK_IMAGE_CREATE_CORNER_SAMPLED_BIT_NV, the sampler
addressing modes must only use a VkSamplerAddressMode of
VK_SAMPLER_ADDRESS_MODE_CLAMP_TO_EDGE.
The SPIR-V instruction is OpImageSampleFootprintNV with Dim =
2D and addressModeU or addressModeV in the sampler is not
VK_SAMPLER_ADDRESS_MODE_CLAMP_TO_EDGE.
The SPIR-V instruction is OpImageSampleFootprintNV with Dim =
3D and addressModeU, addressModeV, or addressModeW in
the sampler is not VK_SAMPLER_ADDRESS_MODE_CLAMP_TO_EDGE.
The sampler is sampling an image view of
VK_FORMAT_B4G4R4A4_UNORM_PACK16,
VK_FORMAT_B5G6R5_UNORM_PACK16, or
VK_FORMAT_B5G5R5A1_UNORM_PACK16 format without a specified
VkSamplerCustomBorderColorCreateInfoEXT::format.
Only OpImageSample* and OpImageSparseSample*can be used with a
sampler or image view that enables sampler Y′CBCR conversion.
OpImageFetch, OpImageSparseFetch, OpImage*Gather, and
OpImageSparse*Gathermust not be used with a sampler or image view
that enables sampler Y′CBCR conversion.
The ConstOffset and Offset operands must not be used with a
sampler or image view that enables sampler Y′CBCR conversion.
If the underlying VkImage format has an X component in its format
description, undefined values are read from those bits.
If the VkImage format and VkImageView format are the same, these
bits will be unused by format conversion and this will have no effect.
However, if the VkImageView format is different, then some bits of the
result may be undefined.
For example, when a VK_FORMAT_R10X6_UNORM_PACK16VkImage is
sampled via a VK_FORMAT_R16_UNORMVkImageView, the low 6 bits of
the value before format conversion are undefined and format conversion may
return a range of different values.
Some implementations will return undefined values in the case where a
sampler uses a VkSamplerAddressMode of
VK_SAMPLER_ADDRESS_MODE_MIRRORED_REPEAT, the sampler is used with
operands Offset, ConstOffset, or ConstOffsets, and the value
of the offset is larger than or equal to the corresponding width, height, or
depth of any accessed image level.
This behavior was not tested prior to Vulkan conformance test suite version
1.3.8.0.
Affected implementations will have a conformance test waiver for this issue.
Integer Texel Coordinate Validation
Integer texel coordinates are validated against the size of the image level,
and the number of layers and number of samples in the image.
For SPIR-V instructions that use integer texel coordinates, this is
performed directly on the integer coordinates.
For instructions that use normalized or unnormalized texel coordinates, this
is performed on the coordinates that result after
conversion to integer texel
coordinates.
If the integer texel coordinates do not satisfy all of the conditions
0 ≤ i < ws
0 ≤ j < hs
0 ≤ k < ds
0 ≤ l < layers
0 ≤ n < samples
where:
ws = width of the image level
hs = height of the image level
ds = depth of the image level
layers = number of layers in the image
samples = number of samples per texel in the image
then the texel fails integer texel coordinate validation.
There are four cases to consider:
Valid Texel Coordinates
If the texel coordinates pass validation (that is, the coordinates lie
within the image),
then the texel value comes from the value in image memory.
Border Texel
If the texel coordinates fail validation, and
If the read is the result of an image sample instruction or image gather
instruction, and
If the image is not a cube image,
or if a sampler created with
VK_SAMPLER_CREATE_NON_SEAMLESS_CUBE_MAP_BIT_EXT is used,
then the texel is a border texel and texel replacement is performed.
Invalid Texel
If the texel coordinates fail validation, and
If the read is the result of an image fetch instruction, image read
instruction, or atomic instruction,
then the texel is an invalid texel and texel replacement is performed.
Cube Map Edge or Corner
Otherwise the texel coordinates lie beyond the edges or corners of the
selected cube map face, and Cube map edge handling
is performed.
Cube Map Edge Handling
If the texel coordinates lie beyond the edges or corners of the selected
cube map face (as described in the prior section), the following steps are
performed.
Note that this does not occur when using VK_FILTER_NEAREST filtering
within a mip level, since VK_FILTER_NEAREST is treated as using
VK_SAMPLER_ADDRESS_MODE_CLAMP_TO_EDGE.
Cube Map Edge Texel
If the texel lies beyond the selected cube map face in either only
i or only j, then the coordinates (i,j) and the array
layer l are transformed to select the adjacent texel from the
appropriate neighboring face.
Cube Map Corner Texel
If the texel lies beyond the selected cube map face in both i and
j, then there is no unique neighboring face from which to read
that texel.
The texel should be replaced by the average of the three values of the
adjacent texels in each incident face.
However, implementations may replace the cube map corner texel by
other methods.
The methods are subject to the constraint that for linear filtering if the
three available texels have the same value, the resulting filtered texel
must have that value, and for cubic filtering if the twelve available
samples have the same value, the resulting filtered texel must have that
value.
Sparse Validation
If the texel reads from an unbound region of a sparse image, the texel is a
sparse unbound texel, and processing continues with
texel replacement.
Layout Validation
If all planes of a disjointmulti-planar image are not in the same
image layout, the image must not be sampled
with sampler Y′CBCR conversion enabled.
Format Conversion
Texels undergo a format conversion from the VkFormat of the image view
to a vector of either floating-point or signed or unsigned integer
components, with the number of components based on the number of components
present in the format.
Color formats have one, two, three, or four components, according to the
format.
Depth/stencil formats are one component.
The depth or stencil component is selected by the aspectMask of
the image view.
If the image view format is sRGB, the color components are first converted
as if they are UNORM, and then sRGB to linear conversion is applied to the
R, G, and B components as described in the “sRGB EOTF” section of the
Khronos Data Format Specification.
The A component, if present, is unchanged.
If the image view format is block-compressed, then the texel value is first
decoded, then converted based on the type and number of components defined
by the compressed format.
Texel Replacement
A texel is replaced if it is one (and only one) of:
a border texel,
an invalid texel, or
a sparse unbound texel.
Border texels are replaced with a value based on the image format and the
borderColor of the sampler.
The border color is:
The custom border color (U) may be rounded by implementations prior
to texel replacement, but the error introduced by such a rounding must not
exceed one ULP of the image’s format.
The names VK_BORDER_COLOR_*_TRANSPARENT_BLACK,
VK_BORDER_COLOR_*_OPAQUE_BLACK, and
VK_BORDER_COLOR_*_OPAQUE_WHITE are meant to describe which components
are zeros and ones in the vocabulary of compositing, and are not meant to
imply that the numerical value of VK_BORDER_COLOR_INT_OPAQUE_WHITE is
a saturating value for integers.
This is substituted for the texel value by replacing the number of
components in the image format
Table 2. Border Texel Components After Replacement
Texel Aspect or Format
Component Assignment
Depth aspect
D = Br
Stencil aspect
S = Br†
One component color format
Colorr = Br
Two component color format
[Colorr,Colorg] = [Br,Bg]
Three component color format
[Colorr,Colorg,Colorb] = [Br,Bg,Bb]
Four component color format
[Colorr,Colorg,Colorb,Colora] = [Br,Bg,Bb,Ba]
Single component alpha format
[Colorr,Colorg,Colorb, Colora] = [0,0,0,Ba]
† S = Bgmay be substituted as the replacement method by the
implementation when VkSamplerCreateInfo::borderColor is
VK_BORDER_COLOR_INT_CUSTOM_EXT and
VkSamplerCustomBorderColorCreateInfoEXT::format is
VK_FORMAT_UNDEFINED.
Implementations should use S = Br as the replacement method.
The value returned by a read of an invalid texel is undefined, unless that
read operation is from a buffer resource and the
robustBufferAccess feature is
enabled.
In that case, an invalid texel is replaced as described by the
robustBufferAccess feature.
If the access is to an image resource and the x, y, z, or layer coordinate
validation fails and
the robustImageAccess feature is
enabled, then zero must be returned for the R, G, and B components, if
present.
Either zero or one must be returned for the A component, if present.
If
If the robustImageAccess2 feature is
enabled, zero values must be returned.
If only the sample index was invalid, the values returned are undefined.
Additionally, if the robustImageAccess
feature is enabled,
but the robustImageAccess2 feature is
not,
any invalid texels may be expanded to four components prior to texel
replacement.
This means that components not present in the image format may be replaced
with 0 or may undergo conversion to RGBA as
normal.
Loads from a null descriptor return a four component color value of all
zeros.
However, for storage images and storage texel buffers using an explicit
SPIR-V Image Format, loads from a null descriptor may return an alpha value
of 1 (float or integer, depending on format) if the format does not include
alpha.
If the
VkPhysicalDeviceSparseProperties::residencyNonResidentStrict
property is VK_TRUE, a sparse unbound texel is replaced with 0 or 0.0
values for integer and floating-point components of the image format,
respectively.
If residencyNonResidentStrict is VK_FALSE, the value of the
sparse unbound texel is undefined.
Depth Compare Operation
If the image view has a depth/stencil format, the depth component is
selected by the aspectMask, and the operation is an OpImage*Dref*
instruction, a depth comparison is performed.
The result is 1.0 if the comparison evaluates to true, and
0.0 otherwise.
This value replaces the depth component D.
The compare operation is selected by the VkCompareOp value set by
VkSamplerCreateInfo::compareOp.
The reference value from the SPIR-V operand Dref and the texel depth
value Dtex are used as the reference and test values,
respectively, in that operation.
If the image being sampled has an unsigned normalized fixed-point format,
then Dref is clamped to [0,1] before the compare operation.
Conversion to RGBA
The texel is expanded from one, two, or three components to four components
based on the image base color:
The swizzle can rearrange the components of the texel, or substitute zero
or one for any components.
It is defined as follows for each color component:
where:
If the border color is one of the VK_BORDER_COLOR_*_OPAQUE_BLACK enums
and the VkComponentSwizzle is not the
identity swizzle for all
components, the value of the texel after swizzle is undefined.
If the image view has a depth/stencil format and the
VkComponentSwizzle is VK_COMPONENT_SWIZZLE_ONE, and
VkPhysicalDeviceMaintenance5Properties::depthStencilSwizzleOneSupport
is not VK_TRUE, the value of the texel after swizzle is undefined.
Sparse Residency
OpImageSparse* instructions return a structure which includes a
residency code indicating whether any texels accessed by the instruction
are sparse unbound texels.
This code can be interpreted by the OpImageSparseTexelsResident
instruction which converts the residency code to a boolean value.
Chroma Reconstruction
In some color models, the color representation is defined in terms of
monochromatic light intensity (often called “luma”) and color differences
relative to this intensity, often called “chroma”.
It is common for color models other than RGB to represent the chroma
components at lower spatial resolution than the luma component.
This approach is used to take advantage of the eye’s lower spatial
sensitivity to color compared with its sensitivity to brightness.
Less commonly, the same approach is used with additive color, since the
green component dominates the eye’s sensitivity to light intensity and the
spatial sensitivity to color introduced by red and blue is lower.
Lower-resolution components are “downsampled” by resizing them to a lower
spatial resolution than the component representing luminance.
This process is also commonly known as “chroma subsampling”.
There is one luminance sample in each texture texel, but each chrominance
sample may be shared among several texels in one or both texture dimensions.
“_444” formats do not spatially downsample chroma values
compared with luma: there are unique chroma samples for each texel.
“_422” formats have downsampling in the x dimension
(corresponding to u or s coordinates): they are sampled at half the
resolution of luma in that dimension.
“_420” formats have downsampling in the x dimension
(corresponding to u or s coordinates) and the y dimension
(corresponding to v or t coordinates): they are sampled at half the
resolution of luma in both dimensions.
The process of reconstructing a full color value for texture access involves
accessing both chroma and luma values at the same location.
To generate the color accurately, the values of the lower-resolution
components at the location of the luma samples are reconstructed from the
lower-resolution sample locations, an operation known here as “chroma
reconstruction” irrespective of the actual color model.
The location of the chroma samples relative to the luma coordinates is
determined by the xChromaOffset and yChromaOffset members of the
VkSamplerYcbcrConversionCreateInfo structure used to create the
sampler Y′CBCR conversion.
The following diagrams show the relationship between unnormalized (u,v)
coordinates and (i,j) integer texel positions in the luma component
(shown in black, with circles showing integer sample positions) and the
texel coordinates of reduced-resolution chroma components, shown as crosses
in red.
If the chroma values are reconstructed at the locations of the luma samples
by means of interpolation, chroma samples from outside the image bounds are
needed; these are determined according to Wrapping Operation.
These diagrams represent this by showing the bounds of the “chroma texel”
extending beyond the image bounds, and including additional chroma sample
positions where required for interpolation.
The limits of a sample for NEAREST sampling is shown as a grid.
If the format of the image that is to be sampled sets
VK_FORMAT_FEATURE_SAMPLED_IMAGE_YCBCR_CONVERSION_CHROMA_RECONSTRUCTION_EXPLICIT_BIT,
or the VkSamplerYcbcrConversionCreateInfo’s
forceExplicitReconstruction is VK_TRUE, reconstruction is
performed as an explicit step independent of filtering, described in the
Explicit Reconstruction section.
If the format of the image that is to be sampled does not set
VK_FORMAT_FEATURE_SAMPLED_IMAGE_YCBCR_CONVERSION_CHROMA_RECONSTRUCTION_EXPLICIT_BIT
and if the VkSamplerYcbcrConversionCreateInfo’s
forceExplicitReconstruction is VK_FALSE, reconstruction is
performed as an implicit part of filtering prior to color model conversion,
with no separate post-conversion texel filtering step, as described in the
Implicit Reconstruction section.
If the format’s R and B components are reduced in resolution in just
width by a factor of two relative to the G component (i.e. this is a
“_422” format), the values
accessed by texel filtering are
reconstructed as follows:
If the format’s R and B components are reduced in resolution in width
and height by a factor of two relative to the G component (i.e. this is
a “_420” format), the values
accessed by texel filtering are
reconstructed as follows:
xChromaOffset and yChromaOffset have no effect if
chromaFilter is VK_FILTER_NEAREST for explicit reconstruction.
If the format’s R and B components are reduced in resolution in just
width by a factor of two relative to the G component (i.e. this is a
“_422” format):
If xChromaOffset is VK_CHROMA_LOCATION_COSITED_EVEN:
If xChromaOffset is VK_CHROMA_LOCATION_MIDPOINT:
If the format’s R and B components are reduced in resolution in width
and height by a factor of two relative to the G component (i.e. this is
a “_420” format), a similar relationship applies.
Due to the number of options, these formulae are expressed more
concisely as follows:
In the case where the texture itself is bilinearly interpolated as described
in Texel Filtering, thus requiring four
full-color samples for the filtering operation, and where the reconstruction
of these samples uses bilinear interpolation in the chroma components due to
chromaFilter=VK_FILTER_LINEAR, up to nine chroma samples may be
required, depending on the sample location.
Implicit Reconstruction
Implicit reconstruction takes place by the samples being interpolated, as
required by the filter settings of the sampler, except that
chromaFilter takes precedence for the chroma samples.
If chromaFilter is VK_FILTER_NEAREST, an implementation may
behave as if xChromaOffset and yChromaOffset were both
VK_CHROMA_LOCATION_MIDPOINT, irrespective of the values set.
This will not have any visible effect if the locations of the luma samples
coincide with the location of the samples used for rasterization.
The sample coordinates are adjusted by the downsample factor of the
component (such that, for example, the sample coordinates are divided by two
if the component has a downsample factor of two relative to the luma
component):
Sampler Y′CBCR Conversion
Sampler Y′CBCR conversion performs the following operations, which an
implementation may combine into a single mathematical operation:
Sampler Y′CBCR range expansion is applied to color component values after
all texel input operations which are not specific to sampler Y′CBCR
conversion.
For example, the input values to this stage have been converted using the
normal format conversion rules.
The input values to this stage may have been converted using sRGB to linear
conversion if the ycbcrDegamma feature is
enabled.
Sampler Y′CBCR range expansion is not applied if ycbcrModel is
VK_SAMPLER_YCBCR_MODEL_CONVERSION_RGB_IDENTITY.
That is, the shader receives the vector C'rgba as output by the Component
Swizzle stage without further modification.
For other values of ycbcrModel, range expansion is applied to the
texel component values output by the Component Swizzle defined by the components member of
VkSamplerYcbcrConversionCreateInfo.
Range expansion applies independently to each component of the image.
For the purposes of range expansion and Y′CBCR model conversion, the R and
B components contain color difference (chroma) values and the G component
contains luma.
The A component is not modified by sampler Y′CBCR range expansion.
If ycbcrRange is VK_SAMPLER_YCBCR_RANGE_ITU_FULL, the
following transformations are applied:
These formulae correspond to the “full range” encoding in the
“Quantization schemes” chapter of the Khronos Data Format Specification.
Should any future amendments be made to the ITU specifications from which
these equations are derived, the formulae used by Vulkan may also be
updated to maintain parity.
If ycbcrRange is VK_SAMPLER_YCBCR_RANGE_ITU_NARROW, the
following transformations are applied:
These formulae correspond to the “narrow range” encoding in the
“Quantization schemes” chapter of the Khronos Data Format Specification.
n is the bit-depth of the components in the format.
The precision of the operations performed during range expansion must be at
least that of the source format.
An implementation may clamp the results of these range expansion operations
such that Y′ falls in the range [0,1], and/or such that CB and CR
fall in the range [-0.5,0.5].
Sampler Y′CBCR Model Conversion
The range-expanded values are converted between color models, according to
the color model conversion specified in the ycbcrModel member:
VK_SAMPLER_YCBCR_MODEL_CONVERSION_RGB_IDENTITY
The color components are not modified by the color model conversion
since they are assumed already to represent the desired color model in
which the shader is operating; Y′CBCR range expansion is also ignored.
VK_SAMPLER_YCBCR_MODEL_CONVERSION_YCBCR_IDENTITY
The color components are not modified by the color model conversion and
are assumed to be treated as though in Y′CBCR form both in memory and
in the shader; Y′CBCR range expansion is applied to the components as
for other Y′CBCR models, with the vector (CR,Y′,CB,A)
provided to the shader.
VK_SAMPLER_YCBCR_MODEL_CONVERSION_YCBCR_709
The color components are transformed from a Y′CBCR representation to an
R′G′B′ representation as described in the “BT.709 Y′CBCR
conversion” section of the Khronos Data Format Specification.
VK_SAMPLER_YCBCR_MODEL_CONVERSION_YCBCR_601
The color components are transformed from a Y′CBCR representation to an
R′G′B′ representation as described in the “BT.601 Y′CBCR
conversion” section of the Khronos Data Format Specification.
VK_SAMPLER_YCBCR_MODEL_CONVERSION_YCBCR_2020
The color components are transformed from a Y′CBCR representation to an
R′G′B′ representation as described in the “BT.2020 Y′CBCR
conversion” section of the Khronos Data Format Specification.
In this operation, each output component is dependent on each input
component.
An implementation may clamp the R′G′B′ results of these conversions to
the range [0,1].
The precision of the operations performed during model conversion must be
at least that of the source format.
The alpha component is not modified by these model conversions.
Sampling operations in a non-linear color space can introduce color and
intensity shifts at sharp transition boundaries.
To avoid this issue, the technically precise color correction sequence
described in the “Introduction to Color Conversions” chapter of the
Khronos Data Format Specification may be performed as
follows:
Convert the non-linear A′R′G′B′ outputs of the Y′CBCR
conversions to linear ARGB values as described in the “Transfer
Functions” chapter of the Khronos Data Format Specification.
The additional calculations and, especially, additional number of sampling
operations in the VK_FILTER_LINEAR case can be expected to have a
performance impact compared with using the outputs directly.
Since the variations from “correct” results are subtle for most content,
the application author should determine whether a more costly implementation
is strictly necessary.
If chromaFilter, and minFilter or magFilter are both
VK_FILTER_NEAREST, these operations are redundant and sampling using
sampler Y′CBCR conversion at the desired
sample coordinates will produce the “correct” results without further
processing.
Texel Output Operations
Texel output instructions are SPIR-V image instructions that write to an
image.
Texel output operations are a set of steps that are performed on state,
coordinates, and texel values while processing a texel output instruction,
and which are common to some or all texel output instructions.
They include the following steps, which are performed in the listed order:
Texel output validation operations inspect instruction/image state or
coordinates, and in certain circumstances cause the write to have no effect.
There are a series of validations that the texel undergoes.
Texel Format Validation
If the image format of the OpTypeImage is not
compatible with the VkImageView’s
format, the write causes the contents of the image’s memory to become
undefined.
Texel Type Validation
If the SampledType of the OpTypeImage does not match the
SPIR-V Type, the write causes the value of the texel to
become undefined.
For integer types, if the signedness of the access does not match the signedness of the accessed resource, the write
causes the value of the texel to become undefined.
Integer Texel Coordinate Validation
The integer texel coordinates are validated according to the same rules as
for texel input coordinate validation.
If the texel fails integer texel coordinate validation, then the write has
no effect.
Sparse Texel Operation
If the texel attempts to write to an unbound region of a sparse image, the
texel is a sparse unbound texel.
In such a case, if the
VkPhysicalDeviceSparseProperties::residencyNonResidentStrict
property is VK_TRUE, the sparse unbound texel write has no effect.
If residencyNonResidentStrict is VK_FALSE, the write may have a
side effect that becomes visible to other accesses to unbound texels in any
resource, but will not be visible to any device memory allocated by the
application.
Texel Output Format Conversion
If the image format is sRGB, a linear to sRGB conversion is applied to the
R, G, and B components as described in the “sRGB EOTF” section of the
Khronos Data Format Specification.
The A component, if present, is unchanged.
Texels then undergo a format conversion from the floating-point, signed, or
unsigned integer type of the texel data to the VkFormat of the image
view.
If the number of components in the texel data is larger than the number of
components in the format, additional components are discarded.
Each component is converted based on its type and size (as defined in the
Format Definition section for each VkFormat).
Floating-point outputs are converted as described in
Floating-Point Format Conversions and
Fixed-Point Data Conversion.
Integer outputs are converted such that their value is preserved.
The converted value of any integer that cannot be represented in the target
format is undefined.
If the VkImageView format has an X component in its format
description, undefined values are written to those bits.
If the underlying VkImage format has an X component in its format
description, undefined values are also written to those bits, even if
result format conversion produces a valid value for those bits because the
VkImageView format is different.
Normalized Texel Coordinate Operations
If the image sampler instruction provides normalized texel coordinates, some
of the following operations are performed.
Projection Operation
For Proj image operations, the normalized texel coordinates
(s,t,r,q,a) and (if present) the Dref coordinate are
transformed as follows:
Derivative Image Operations
Derivatives are used for LOD selection.
These derivatives are either implicit (in an ImplicitLod image
instruction in a
mesh, task,
compute, or
fragment shader) or explicit (provided explicitly by shader to the image
instruction in any shader).
For implicit derivatives image instructions, the derivatives of texel
coordinates are calculated in the same manner as
derivative operations.
That is:
Partial derivatives not defined above for certain image dimensionalities are
set to zero.
For explicit LOD image instructions, if the optional SPIR-V operand
Grad is provided, then the operand values are used for the derivatives.
The number of components present in each derivative for a given image
dimensionality matches the number of partial derivatives computed above.
If the optional SPIR-V operand Lod is provided, then derivatives are
set to zero, the cube map derivative transformation is skipped, and the
scale factor operation is skipped.
Instead, the floating-point scalar coordinate is directly assigned to
λbase as described in LOD Operation.
If the image or sampler object used by an implicit derivative image
instruction is not uniform across the quad and
quadDivergentImplicitLod is not
supported, then the derivative and LOD values are undefined.
Implicit derivatives are well-defined when the image and sampler and control
flow are uniform across the quad, even if they diverge between different
quads.
If quadDivergentImplicitLod is
supported, then derivatives and implicit LOD values are well-defined even if
the image or sampler object are not uniform within a quad.
The derivatives are computed as specified above, and the implicit LOD
calculation proceeds for each shader invocation using its respective image
and sampler object.
Cube Map Face Selection and Transformations
For cube map image instructions, the (s,t,r) coordinates are treated
as a direction vector (rx,ry,rz).
The direction vector is used to select a cube map face.
The direction vector is transformed to a per-face texel coordinate system
(sface,tface), The direction vector is also used to transform the
derivatives to per-face derivatives.
Cube Map Face Selection
The direction vector selects one of the cube map’s faces based on the
largest magnitude coordinate direction (the major axis direction).
Since two or more coordinates can have identical magnitude, the
implementation must have rules to disambiguate this situation.
The rules should have as the first rule that rz wins over
ry and rx, and the second rule that ry wins over
rx.
An implementation may choose other rules, but the rules must be
deterministic and depend only on (rx,ry,rz).
The layer number (corresponding to a cube map face), the coordinate
selections for sc, tc, rc, and the selection of
derivatives, are determined by the major axis direction as specified in the
following two tables.
The other derivatives are simplified similarly, resulting in
Scale Factor Operation, LOD Operation and Image Level(s) Selection
LOD selection can be either explicit (provided explicitly by the image
instruction) or implicit (determined from a scale factor calculated from the
derivatives).
The LOD must be computed with mipmapPrecisionBits of accuracy.
Scale Factor Operation
The magnitude of the derivatives are calculated by:
mux = |∂s/∂x| × wbase
mvx = |∂t/∂x| × hbase
mwx = |∂r/∂x| × dbase
muy = |∂s/∂y| × wbase
mvy = |∂t/∂y| × hbase
mwy = |∂r/∂y| × dbase
where:
∂t/∂x = ∂t/∂y = 0 (for 1D
images)
∂r/∂x = ∂r/∂y = 0 (for 1D,
2D or Cube images)
and:
wbase = image.w
hbase = image.h
dbase = image.d
(for the baseMipLevel, from the image descriptor).
For corner-sampled images, the wbase, hbase, and
dbase are instead:
wbase = image.w - 1
hbase = image.h - 1
dbase = image.d - 1
A point sampled in screen space has an elliptical footprint in texture
space.
The minimum and maximum scale factors (ρmin, ρmax)should
be the minor and major axes of this ellipse.
The scale factorsρx and ρy, calculated from the
magnitude of the derivatives in x and y, are used to compute the minimum and
maximum scale factors.
ρx and ρymay be approximated with functions
fx and fy, subject to the following constraints:
The minimum and maximum scale factors (ρmin,ρmax) are
determined by:
If ρmax = ρmin = 0, then all the partial derivatives are
zero, the fragment’s footprint in texel space is a point, and ηshould be treated as 1.
If ρmax ≠ 0 and ρmin = 0 then all partial
derivatives along one axis are zero, the fragment’s footprint in texel space
is a line segment, and ηshould be treated as maxAniso.
However, anytime the footprint is small in texel space the implementation
may use a smaller value of η, even when ρmin is zero
or close to zero.
If either VkPhysicalDeviceFeatures::samplerAnisotropy or
VkSamplerCreateInfo::anisotropyEnable are VK_FALSE,
maxAniso is set to 1.
If η = 1, sampling is isotropic.
If η > 1, sampling is anisotropic.
The sampling rate (N) is derived as:
N = ⌈η⌉
An implementation may round N up to the nearest supported sampling
rate.
An implementation may use the value of N as an approximation of
η.
The image level(s) d, dhi, and dlo which texels are
read from are determined by an image-level parameter dl, which is
computed based on the LOD parameter, as follows:
where:
and:
baseMipLevel and levelCount are taken from the
subresourceRange of the image view.
minLodimageViewmust be less or equal to levelbase + q.
If the sampler’s mipmapMode is VK_SAMPLER_MIPMAP_MODE_NEAREST,
then the level selected is d = dl.
If the sampler’s mipmapMode is VK_SAMPLER_MIPMAP_MODE_LINEAR,
two neighboring levels are selected:
The normalized texel coordinates are scaled by the image level dimensions
and the array layer is selected.
This transformation is performed once for each level used in
filtering (either d, or dhi and
dlo).
where:
widthscale = widthlevel
heightscale = heightlevel
depthscale = depthlevel
for conventional images, and:
widthscale = widthlevel - 1
heightscale = heightlevel - 1
depthscale = depthlevel - 1
for corner-sampled images.
and where (Δi, Δj, Δk) are
taken from the image instruction if it includes a ConstOffset or
Offset operand, otherwise they are taken to be zero.
Operations then proceed to Unnormalized Texel Coordinate Operations.
Unnormalized Texel Coordinate Operations
(u,v,w,a) to (i,j,k,l,n) Transformation and Array Layer Selection
The unnormalized texel coordinates are transformed to integer texel
coordinates relative to the selected mipmap level.
The layer index l is computed as:
l = clamp(RNE(a), 0, layerCount - 1) +
baseArrayLayer
where layerCount is the number of layers in the image subresource
range of the image view, baseArrayLayer is the first layer from the
subresource range, and where:
The sample index n is assigned the value 0.
Nearest filtering (VK_FILTER_NEAREST) computes the integer texel
coordinates that the unnormalized coordinates lie within:
where:
shift = 0.0
for conventional images, and:
shift = 0.5
for corner-sampled images.
Linear filtering (VK_FILTER_LINEAR) computes a set of neighboring
coordinates which bound the unnormalized coordinates.
The integer texel coordinates are combinations of i0 or i1,
j0 or j1, k0 or k1, as well as weights
α, β, and γ.
where:
shift = 0.5
for conventional images, and:
shift = 0.0
for corner-sampled images,
and where:
where the number of fraction bits retained is specified by
VkPhysicalDeviceLimits::subTexelPrecisionBits.
Cubic filtering (VK_FILTER_CUBIC_EXT) computes a set of neighboring
coordinates which bound the unnormalized coordinates.
The integer texel coordinates are combinations of i0, i1,
i2 or i3, j0, j1, j2 or j3,
k0, k1, k2 or k3, as well as weights
α, β, and γ.
where:
where the number of fraction bits retained is specified by
VkPhysicalDeviceLimits::subTexelPrecisionBits.
Integer Texel Coordinate Operations
Integer texel coordinate operations may supply a LOD which texels are to be
read from or written to using the optional SPIR-V operand Lod.
If the Lod is provided then it must be an integer.
The image level selected is:
If d does not lie in the range [baseMipLevel,
baseMipLevel + levelCount)
or d is less than minLodIntegerimageView,
then any values fetched are
zero if the robustImageAccess2
feature is enabled, otherwise are
undefined, and any writes (if supported) are discarded.
Image Sample Operations
Wrapping Operation
If the used sampler was created without
VK_SAMPLER_CREATE_NON_SEAMLESS_CUBE_MAP_BIT_EXT,
Cube images ignore the wrap modes specified in the sampler.
Instead, if VK_FILTER_NEAREST is used within a mip level then
VK_SAMPLER_ADDRESS_MODE_CLAMP_TO_EDGE is used, and if
VK_FILTER_LINEAR is used within a mip level then sampling at the edges
is performed as described earlier in the Cube map edge handling section.
The first integer texel coordinate i is transformed based on the
addressModeU parameter of the sampler.
where:
j (for 2D and Cube image) and k (for 3D image) are similarly
transformed based on the addressModeV and addressModeW
parameters of the sampler, respectively.
Texel Gathering
SPIR-V instructions with Gather in the name return a vector derived
from 4 texels in the base level of the image view.
The rules for the VK_FILTER_LINEAR minification filter are applied to
identify the four selected texels.
Each texel is then converted to an RGBA value according to
conversion to RGBA and then
swizzled.
A four-component vector is then assembled by taking the component indicated
by the Component value in the instruction from the swizzled color value
of the four texels.
If the operation does not use the ConstOffsets image operand then the
four texels form the 2 × 2 rectangle used for texture filtering:
If the operation does use the ConstOffsets image operand then the
offsets allow a custom filter to be defined:
If levelbase < minLodIntegerimageView, then any values fetched are
zero if the robustImageAccess2
feature is enabled.
Otherwise values are
undefined.
Texel Filtering
Texel filtering is first performed for each level (either d or
dhi and dlo).
If λ is less than or equal to zero, the texture is said to be
magnified, and the filter mode within a mip level is selected by the
magFilter in the sampler.
If λ is greater than zero, the texture is said to be
minified, and the filter mode within a mip level is selected by the
minFilter in the sampler.
Texel Nearest Filtering
Within a mip level, VK_FILTER_NEAREST filtering selects a single value
using the (i, j, k) texel coordinates, with all texels taken from
layer l.
Texel Linear Filtering
Within a mip level, VK_FILTER_LINEAR filtering combines 8 (for 3D), 4
(for 2D or Cube), or 2 (for 1D) texel values, together with their linear
weights.
The linear weights are derived from the fractions computed earlier:
The values of multiple texels, together with their weights, are combined to
produce a filtered value.
The VkSamplerReductionModeCreateInfo::reductionModecan control
the process by which multiple texels, together with their weights, are
combined to produce a filtered texture value.
When the reductionMode is set (explicitly or implicitly) to
VK_SAMPLER_REDUCTION_MODE_WEIGHTED_AVERAGE, a weighted average is
computed:
However, if the reduction mode is VK_SAMPLER_REDUCTION_MODE_MIN or
VK_SAMPLER_REDUCTION_MODE_MAX, the process operates on the above set
of multiple texels, together with their weights, computing a component-wise
minimum or maximum, respectively, of the components of the set of texels
with non-zero weights.
Texel Cubic Filtering
Within a mip level, VK_FILTER_CUBIC_EXT, filtering computes a weighted
average of
64 (for 3D),
16 (for 2D), or 4 (for 1D) texel values, together with their
Catmull-Rom, Zero Tangent Cardinal, B-Spline, or Mitchell-Netravali weights
as specified by VkSamplerCubicWeightsCreateInfoQCOM.
Catmull-Rom weights
specified by VK_CUBIC_FILTER_WEIGHTS_CATMULL_ROM_QCOM
are derived from the fractions computed earlier.
Zero Tangent Cardinal weights specified by
VK_CUBIC_FILTER_WEIGHTS_ZERO_TANGENT_CARDINAL_QCOM are derived from
the fractions computed earlier.
B-Spline weights specified by VK_CUBIC_FILTER_WEIGHTS_B_SPLINE_QCOM
are derived from the fractions computed earlier.
Mitchell-Netravali weights specified by
VK_CUBIC_FILTER_WEIGHTS_MITCHELL_NETRAVALI_QCOM are derived from the
fractions computed earlier.
The values of multiple texels, together with their weights, are combined to
produce a filtered value.
The VkSamplerReductionModeCreateInfo::reductionModecan control
the process by which multiple texels, together with their weights, are
combined to produce a filtered texture value.
When the reductionMode is set (explicitly or implicitly) to
VK_SAMPLER_REDUCTION_MODE_WEIGHTED_AVERAGE
or VK_SAMPLER_REDUCTION_MODE_WEIGHTED_AVERAGE_RANGECLAMP_QCOM
, a weighted average is computed:
However, if the reduction mode is VK_SAMPLER_REDUCTION_MODE_MIN or
VK_SAMPLER_REDUCTION_MODE_MAX, the process operates on the above set
of multiple texels, together with their weights, computing a component-wise
minimum or maximum, respectively, of the components of the set of texels
with non-zero weights.
Texel Range Clamp
When reductionMode is
VK_SAMPLER_REDUCTION_MODE_WEIGHTED_AVERAGE_RANGECLAMP_QCOM, the
weighted average is clamped to be within the component-wise minimum and
maximum of the set of texels with non-zero weights.
Texel Mipmap Filtering
VK_SAMPLER_MIPMAP_MODE_NEAREST filtering returns the value of a single
mipmap level,
τ = τ[d].
VK_SAMPLER_MIPMAP_MODE_LINEAR filtering combines the values of
multiple mipmap levels (τ[hi] and τ[lo]), together with their linear
weights.
The linear weights are derived from the fraction computed earlier:
The values of multiple mipmap levels, together with their weights, are
combined to produce a final filtered value.
The VkSamplerReductionModeCreateInfo::reductionModecan control
the process by which multiple texels, together with their weights, are
combined to produce a filtered texture value.
When the reductionMode is set (explicitly or implicitly) to
VK_SAMPLER_REDUCTION_MODE_WEIGHTED_AVERAGE, a weighted average is
computed:
However, if the reduction mode is VK_SAMPLER_REDUCTION_MODE_MIN or
VK_SAMPLER_REDUCTION_MODE_MAX, the process operates on the above
values, together with their weights, computing a component-wise minimum or
maximum, respectively, of the components of the values with non-zero
weights.
Texel Anisotropic Filtering
Anisotropic filtering is enabled by the anisotropyEnable in the
sampler.
When enabled, the image filtering scheme accounts for a degree of
anisotropy.
The particular scheme for anisotropic texture filtering is
implementation-dependent.
Implementations should consider the magFilter, minFilter and
mipmapMode of the sampler to control the specifics of the anisotropic
filtering scheme used.
In addition, implementations should consider minLod and maxLod
of the sampler.
For historical reasons, vendor implementations of anisotropic filtering
interpret these sampler parameters in different ways, particularly in corner
cases such as magFilter, minFilter of NEAREST or
maxAnisotropy equal to 1.0.
Applications should not expect consistent behavior in such cases, and should
use anisotropic filtering only with parameters which are expected to give a
quality improvement relative to LINEAR filtering.
The following describes one particular approach to implementing anisotropic
filtering for the 2D Image case; implementations may choose other methods:
Given a magFilter, minFilter of VK_FILTER_LINEAR and a
mipmapMode of VK_SAMPLER_MIPMAP_MODE_NEAREST:
Instead of a single isotropic sample, N isotropic samples are sampled within
the image footprint of the image level d to approximate an anisotropic
filter.
The sum τ2Daniso is defined using the single isotropic
τ2D(u,v) at level d.
When VkSamplerReductionModeCreateInfo::reductionMode is
VK_SAMPLER_REDUCTION_MODE_WEIGHTED_AVERAGE, the above summation is
used.
However, if the reduction mode is VK_SAMPLER_REDUCTION_MODE_MIN or
VK_SAMPLER_REDUCTION_MODE_MAX, the process operates on the above
values, together with their weights, computing a component-wise minimum or
maximum, respectively, of the components of the values with non-zero
weights.
Texel Footprint Evaluation
The SPIR-V instruction OpImageSampleFootprintNV evaluates the set of
texels from a single mip level that would be accessed during a
texel filtering operation.
In addition to the inputs that would be accepted by an equivalent
OpImageSample* instruction, OpImageSampleFootprintNV accepts two
additional inputs.
The Granularity input is an integer identifying the size of texel
groups used to evaluate the footprint.
Each bit in the returned footprint mask corresponds to an aligned block of
texels whose size is given by the following table:
Table 6. Texel Footprint Granularity Values
Granularity
Dim = 2D
Dim = 3D
0
unsupported
unsupported
1
2x2
2x2x2
2
4x2
unsupported
3
4x4
4x4x2
4
8x4
unsupported
5
8x8
unsupported
6
16x8
unsupported
7
16x16
unsupported
8
unsupported
unsupported
9
unsupported
unsupported
10
unsupported
16x16x16
11
64x64
32x16x16
12
128x64
32x32x16
13
128x128
32x32x32
14
256x128
64x32x32
15
256x256
unsupported
The Coarse input is used to select between the two mip levels that may
be accessed during texel filtering when using a mipmapMode of
VK_SAMPLER_MIPMAP_MODE_LINEAR.
When filtering between two mip levels, a Coarse value of true
requests the footprint in the lower-resolution mip level (higher level
number), while false requests the footprint in the higher-resolution
mip level.
If texel filtering would access only a single mip level, the footprint in
that level would be returned when Coarse is false; an empty
footprint would be returned when Coarse is true.
The footprint for OpImageSampleFootprintNV is returned in a structure
with six members:
The first member is a boolean value that is true if the texel filtering
operation would access only a single mip level.
The second member is a two- or three-component integer vector holding
the footprint anchor location.
For two-dimensional images, the returned components are in units of
eight texel groups.
For three-dimensional images, the returned components are in units of
four texel groups.
The third member is a two- or three-component integer vector holding a
footprint offset relative to the anchor.
All returned components are in units of texel groups.
The fourth member is a two-component integer vector mask, which holds a
bitfield identifying the set of texel groups in an 8x8 or 4x4x4
neighborhood relative to the anchor and offset.
The fifth member is an integer identifying the mip level containing the
footprint identified by the anchor, offset, and mask.
The sixth member is an integer identifying the granularity of the
returned footprint.
For footprints in two-dimensional images (Dim2D), the mask returned by
OpImageSampleFootprintNV indicates whether each texel group in a 8x8
local neighborhood of texel groups would have one or more texels accessed
during texel filtering.
In the mask, the texel group with local group coordinates
is considered covered if and only if
where:
and ; and
is the returned two-component mask.
The local group with coordinates in the mask is
considered covered if and only if the texel filtering operation would access
one or more texels in the returned mip level where:
and
and ;
is a two-component vector holding the width and height
of the texel group identified by the granularity;
is the returned two-component anchor vector; and
is the returned two-component offset vector.
For footprints in three-dimensional images (Dim3D), the mask returned
by OpImageSampleFootprintNV indicates whether each texel group in a
4x4x4 local neighborhood of texel groups would have one or more texels
accessed during texel filtering.
In the mask, the texel group with local group coordinates
, is considered covered if and only if:
where:
, , and \(0
\leq lgz < 4\); and
is the returned two-component mask.
The local group with coordinates in the mask is
considered covered if and only if the texel filtering operation would access
one or more texels in the returned mip level where:
and
, ,
;
is a three-component vector holding the width, height,
and depth of the texel group identified by the granularity;
is the returned three-component anchor vector; and
is the returned three-component offset vector.
If the sampler used by OpImageSampleFootprintNV enables anisotropic
texel filtering via anisotropyEnable, it is possible that the set of
texel groups accessed in a mip level may be too large to be expressed using
an 8x8 or 4x4x4 mask using the granularity requested in the instruction.
In this case, the implementation uses a texel group larger than the
requested granularity.
When a larger texel group size is used, OpImageSampleFootprintNV
returns an integer granularity value that can be interpreted in the same
manner as the granularity value provided to the instruction to determine the
texel group size used.
If anisotropic texel filtering is disabled in the sampler, or if an
anisotropic footprint can be represented as an 8x8 or 4x4x4 mask with the
requested granularity, OpImageSampleFootprintNV will use the requested
granularity as-is and return a granularity value of zero.
OpImageSampleFootprintNV supports only two- and three-dimensional image
accesses (Dim2D and Dim3D), and the footprint returned is
undefined if a sampler uses an addressing mode other than
VK_SAMPLER_ADDRESS_MODE_CLAMP_TO_EDGE.
Weight Image Sampling
The SPIR-V instruction OpImageWeightedSampleQCOM specifies a texture
sampling operation involving two images: the sampled image and the weight
image.
It is similar to bilinear filtering except more than 2x2 texels may
participate in the filter and the filter weights are application-specified
rather than computed by fixed-function hardware.
The weight image view defines a 2D kernel weights used during sampling.
The OpImageWeightedSampleQCOM support normalized or unnormalized texel
coordinates.
In addition to the inputs that would be accepted by an equivalent
OpImageSample* instruction, OpImageWeightedSampleQCOM accepts a
weight input that specifies the view of a sample weight image
The input weightmust be a view of a 2D or 1D image with
miplevels equal to 1, samples equal to
VK_SAMPLE_COUNT_1_BIT, created with an identity swizzle, and created
with usage that includes VK_IMAGE_USAGE_SAMPLE_WEIGHT_BIT_QCOM.
The VkImageViewSampleWeightCreateInfoQCOM specifies additional
parameters of the view: filterCenter, filterSize, and
numPhases.
described in more detail below.
The weight input must be bound using a
sample weight image descriptor type.
The weight view defines a filtering kernel that is a region of view’s
subresource range.
The kernel spans a region from integer texel coordinate (0,0) to
(filterSize.x-1, filterSize.y-1).
It is valid for the view’s subresource to have dimensions larger than the
kernel but the texels with integer coordinates greater than
(filterSize.width-1, filterSize.height-1) are ignored by
weight sampling.
The value returned by queries OpImageQuerySize,
OpImageQuerySizeLod, OpImageQueryLevels, and
OpImageQuerySamples return for a weight image is undefined.
filterCenter designates an integer texel coordinate within the filter
kernel as being the 'center' of the kernel.
The center must be in the range (0,0) to (filterSize.x-1,
filterSize.y-1).
numPhases describes the number of filter phases used to provide
sub-pixel filtering.
Both are described in more detail below.
Weight Image Layout
The weight image specifies filtering kernel weight values.
A 2D image view can be used to specify a 2D matrix of filter weights.
For separable filers, a 1D image view can be used to specity the horizontal
and vertical weights.
2D Non-Separable Weight Filters
A 2D image view defined with VkImageViewSampleWeightCreateInfoQCOM
describes a 2D matrix (filterSize.width ×
filterSize.height) of weight elements with filter’s center point at
filterCenter.
Note that filterSize can be smaller than the view’s subresource, but
the filter will always be located starting at integer texel coordinate
(0,0).
The following figure illustrates a 2D convolution filter having
filterSize of (4,3) and filterCenter at (1, 1).
Figure 10. 2D Convolution Filter
For a 2D weight filter, the phases are stored as layers of a 2D array image.
The width and height of the view’s subresource range must be less than or
equal to
VkPhysicalDeviceImageProcessingPropertiesQCOM::maxWeightFilterDimension.
The layers are stored in horizontal phase major order.
Expressed as a formula, the layer index for each filter phase is computed
as:
A separable weight filter is a 2D filter that can be specified by two 1D
filters in the x and y directions such that their product yields
the 2D filter.
The following example shows a 2D filter and its associated separable 1D
horizontal and vertical filters.
Figure 11. Separable 2D Convolution Filter
A 1D array image view defined with
VkImageViewSampleWeightCreateInfoQCOM and with layerCount equal
to '2' describes a separable weight filter.
The horizontal weights are specified in slice '0' and the vertical weights
in slice '1'.
The filterSize and filterCenter specify the size and origin of
the of the horizontal and vertical filters.
For many use cases, 1D separable filters can offer a performance advantage
over 2D filters.
For a 1D separable weight filter, the phases are arranged into a 1D array
image with two layers.
The horizontal weights are stored in layer 0 and the vertical weights in
layer 1.
Within each layer of the 1D array image, the weights are arranged into
groups of 4, and then arranged by phase.
Expressed as a formula, the 1D texel offset for each weight within each
layer is computed as:
// Let horizontal weights have a weightIndex of [0, filterSize.width - 1]
// Let vertical weights have a weightIndex of [0, filterSize.height - 1]
// Let phaseCount be the number of phases in either the vertical or horizontal direction.
texelOffset(phaseIndex,weightIndex,phaseCount) = (phaseCount * 4 * (weightIndex / 4)) + (phaseIndex * 4) + (weightIndex % 4)
Weight Sampling Phases
When using weight image sampling, the texture coordinates may not align with
a texel center in the sampled image.
In this case, the filter weights can be adjusted based on the subpixel
location.
This is termed “subpixel filtering” to indicate that the origin of the
filter lies at a subpixel location other than the texel center.
Conceptually, this means that the weight filter is positioned such that
filter taps do not align with sampled texels exactly.
In such a case, modified filter weights may be needed to adjust for the
off-center filter taps.
Unlike bilinear filtering where the subpixel weights are computed by the
implementation, subpixel weight image sampling requires that the per-phase
filter weights are pre-computed by the application and stored in an array
where each slice of the array is a “filter phase”.
The array is indexed by the implementation based on subpixel positioning.
Rather than a single 2D kernel of filter weights, the application provides
an array of kernels, one set of filter weights per phase.
The number of phases are restricted by following requirements, which apply
to both separable and non-separable filters:
The number of phases in the vertical direction, phaseCountvert,
must be a power of two (i.e., 1, 2, 4, etc.).
The number of phases in the horizontal direction
phaseCounthoriz, must equal phaseCountvert.
Weight sampling requires VkSamplerCreateInfoaddressModeU and
addressModeVmust be VK_SAMPLER_ADDRESS_MODE_CLAMP_TO_EDGE or
VK_SAMPLER_ADDRESS_MODE_CLAMP_TO_BORDER.
If VK_SAMPLER_ADDRESS_MODE_CLAMP_TO_BORDER is used, then the border
color must be VK_BORDER_COLOR_FLOAT_TRANSPARENT_BLACK.
Weight Sampling Operation
The 2D unnormalized texel coordinates are transformed by
to specify coordinates .
Two sets of neighboring integer 2D texel coordinates are generated.
The first set is used for selecting texels from the sampled image
and the second set used for selecting texels from the
weight image .
The first set of neighboring coordinates are combinations of
to and to
.
The second set of neighboring coordinates are combinations of
to and to
.
The first and second sets each contain \((filterWidth \times
filterHeight)\) of pairs of and
coordinates respectively.
Each pair of texel coordinates in the first set selects a
single texel value from the sampled image.
Each pair of texel coordinates in the second set, combined
with phaseIndex , selects a single weight from the weight
image .
If is a 2D array view, then non-separable filtering is
specified, and integer coordinates are used to select
texels from layer of .
If is a 1D array view, then separable filtering is specified
and integer coordinates are transformed to
, and used to select horizontal weight
and vertical weight texels
from layer 0 and layer 1 of respectively.
Where refers to the integer modulo operator.
The values of multiple texels, together with their weights, are combined to
produce a filtered value.
When VkSamplerReductionModeCreateInfo::reductionMode is
VK_SAMPLER_REDUCTION_MODE_WEIGHTED_AVERAGE, the above summation is
used.
However, if the reduction mode is VK_SAMPLER_REDUCTION_MODE_MIN or
VK_SAMPLER_REDUCTION_MODE_MAX, the process operates on the above
values, computing a component-wise minimum or maximum of the texels with
non-zero weights.
If the reduction mode is VK_SAMPLER_REDUCTION_MODE_MIN or
VK_SAMPLER_REDUCTION_MODE_MAX, each weight
must be equal to 0.0 or 1.0, otherwise the undefined values are returned.
The SPIR-V instruction opImageBlockMatchSAD and
opImageBlockMatchSSD specify texture block matching operations where a
block or region of texels within a target image is compared with a
same-sized region a reference image.
The instructions make use of two image views: the target view and the
reference view.
The target view and reference view can be the same view, allowing block
matching of two blocks within a single image.
Similar to an equivalent OpImageFetch instruction,
opImageBlockMatchSAD and opImageBlockMatchSAD specify an
image and an integer texel coordinate which describes the
bottom-left texel of the target block.
There are three additional inputs.
The reference and refCoodinate specifies bottom-left texel of the
reference block.
The blockSize specifies the integer width and height of the target and
reference blocks to be compared, and must not be greater than
VkPhysicalDeviceImageProcessingPropertiesQCOM.maxBlockMatchRegion.
opImageBlockMatchWindowSAD and opImageBlockMatchWindowSAD take the
same input parameters as the corresponding non-window instructions.
The block matching comparison is performed for all pixel values within a 2D
window whose dimensions are specified in the sampler.
Block Matching Sampler Parameters
For opImageBlockMatchSAD and opImageBlockMatchSSD, the input
samplermust be created with addressModeU and addressModeV,
equal to VK_SAMPLER_ADDRESS_MODE_CLAMP_TO_EDGE, or
VK_SAMPLER_ADDRESS_MODE_CLAMP_TO_BORDER with
VK_BORDER_COLOR_FLOAT_TRANSPARENT_BLACK.
The input samplermust be created with unnormalizedCoordinates
equal to VK_TRUE.
The input samplermust be created with components equal to
VK_COMPONENT_SWIZZLE_IDENTITY.
For opImageBlockMatchWindowSAD and opImageBlockMatchWindowSSD
instructions, the target sampler must have been created with
VkSamplerBlockMatchWindowCreateInfoQCOM in the pNext chain.
For opImageBlockMatchWindowSAD, opImageBlockMatchWindowSSD,
opImageBlockMatchGatherSAD, or
opImageBlockMatchGatherSSDinstructions, the input samplermust be
created with addressModeU and addressModeV, equal to
VK_SAMPLER_ADDRESS_MODE_CLAMP_TO_BORDER with
VK_BORDER_COLOR_FLOAT_TRANSPARENT_BLACK.
Other sampler states are ignored.
Block Matching Operation
Block matching SPIR-V instructions opImageBlockMatchSAD and
opImageBlockMatchSSD specify two sets of 2D integer texel coordinates:
target coordinates and reference coordinates
.
The coordinates define the bottom-left texel of the target block
and the reference block \((k_{0},
l_{0})\).
For the target block, a set of neighboring integer texel coordinates are
generated.
The neighboring coordinates are combinations of to
and to
.
The set is of size .
Similarly for the reference block, a set of neighboring integer texel
coordinates are generated.
Each reference texel coordinate in the set must not fail
integer texel coordinate validation.
To avoid undefined behavior, application shader should guarantee that the
reference block is fully within the bounds of the reference image.
Each pair of texel coordinates in the set selects a single
texel value from the target image .
Each pair of texel coordinates in the set selects a single
texel value from the reference image .
The difference between target and reference texel values is summed to
compute a difference metric.
The opTextureBlockMatchSAD computes the sum of absolute differences.
The opImageBlockMatchSSD computes the sum of the squared differences.
When VkSamplerReductionModeCreateInfo::reductionMode is
VK_SAMPLER_REDUCTION_MODE_WEIGHTED_AVERAGE, the above summation is
used.
However, if the reduction mode is VK_SAMPLER_REDUCTION_MODE_MIN or
VK_SAMPLER_REDUCTION_MODE_MAX, the process operates on the above
values, computing a component-wise minimum or maximum of
, respectively.
For , the minimum or maximum difference is computed
and for , the square of the minimum or maximum is
computed.
Finally, the operations described in
Conversion to RGBA and
Component swizzle are performed and the final
result is returned to the shader.
The component swizzle is specified by the target image descriptor; any
swizzle specified by the reference image descriptor is ignored.
Block Matching Window Operation
Window block matching SPIR-V instructions opImageBlockMatchWindowSAD
and opImageBlockMatchWindowSSD specify two sets of 2D integer texel
coordinates: target coordinates and reference coordinates
.
The block matching operation is
performed repeatedly, for multiple sets of target integer coordinates within
the specified window.
These instructions effectively search a region or “window” within the
target texture and identify the window coordinates where the minimum or
maximum error metric is found.
These instructions only support single component image formats.
The target coordinates are combinations of coordinates from
to
where and are specified by
VkSamplerBlockMatchWindowCreateInfoQCOM::windowExtent.
At each target coordinate, a block matching operation is performed, resulting in a difference metric.
The reference coordinate is fixed.
The block matching operation is repeated \(windowWidth \times
windowHeight\) times.
The resulting minimum or maximum error is returned in the R component of the
output.
The integer window coordinates are returned in the G and B
components of the output.
The A component is 0.
The minimum or maximum behavior is selected by
VkSamplerBlockMatchWindowCreateInfoQCOM::windowCompareMode.
The following pseudocode describes the operation
opImageBlockMatchWindowSAD.
The pseudocode for opImageBlockMatchWindowSSD follows an identical
pattern.
vec4 opImageBlockMatchGatherSAD( sampler2D target,
uvec2 targetCoord,
samler2D reference,
uvec2 refCoord,
uvec2 blocksize) {
// Two parameters are sourced from the VkSampler associated with
// `target`:
// compareMode (which can be either `MIN` or `MAX`)
// uvec2 window (which defines the search window)
minSAD = INF;
maxSAD = -INF;
uvec2 minCoord;
uvec2 maxCoord;
for (uint x=0, x<window.width; x++) {
for (uint y=0; y<window.height; y++) {
float SAD = textureBlockMatchSAD(target,
targetCoord + uvec2(x, y),
reference,
refCoord,
blocksize).x;
if (SAD < minSAD) {
minSAD = SAD;
minCoord = uvec2(x,y);
}
if (SAD > maxSAD) {
maxSAD = SAD;
maxCoord = uvec2(x,y);
}
}
}
if (compareMode==MIN) {
return vec4(minSAD, minCoord.x, minCoord.y, 0.0);
} else {
return vec4(maxSAD, maxCoord.x, maxCoord.y, 0.0);
}
}
Block Matching Gather Operation
Block matching Gather SPIR-V instructions opImageBlockMatchGatherSAD
and opImageBlockMatchGatherSSD specify two sets of 2D integer texel
coordinates: target coordinates and reference coordinates
.
These instructions perform the block matching operation 4 times, using integer target coordinates
, , , and
.
The R component from each of those 4 operations is gathered and returned in
the R, G, B, and A components of the output respectively.
For each block match operation, the reference coordinate is
.
For each block match operation, only the R component of the target and
reference images are compared.
The following pseudocode describes the operation opImageBlockMatchGatherSAD.
The pseudocode for opImageBlockMatchGatherSSD follows an identical pattern.
The SPIR-V instruction OpImageBoxFilterQCOM specifies texture box
filtering operation where a weighted average of a region of texels is
computed, with the weights proportional to the coverage of each of the
texels.
In addition to the inputs that would be accepted by an equivalent
OpImageSample* instruction, OpImageBoxFilterQCOM accepts one
additional input, boxSize which specifies the width and height in
texels of the region to be averaged.
The figure below shows an example of using OpImageBoxFilterQCOM to
sample from a 8 × 4 texel two-dimensional image, with
unnormalized texture coordinates (4.125, 2.625) and boxSize of
(2.75, 2.25).
The filter will read 12 texel values and compute a weights based portion of
each texel covered by the box.
Figure 12. Box Filter Sampling Example
If boxSize has height and width both equal to 1.0, then this
instruction will behave as traditional bilinear filtering.
The boxSize parameter must be greater than or equal to 1.0 and must
not be greater than
VkPhysicalDeviceImageProcessingPropertiesQCOM.maxBoxFilterBlockSize.
Box Filter Sampler Parameters
The input samplermust be created with addressModeU and
addressModeV, equal to VK_SAMPLER_ADDRESS_MODE_CLAMP_TO_EDGE, or
VK_SAMPLER_ADDRESS_MODE_CLAMP_TO_BORDER with
VK_BORDER_COLOR_FLOAT_TRANSPARENT_BLACK.
Box Filter Operation
The 2D unnormalized texel coordinates are transformed by
to specify integer texel coordinates \((i_{0},
j_{0})\) of the bottom left texel for the filter.
where and are specified by the
code:(x,y) components of the boxSize operand.
The filter dimensions are
computed from the fractional portion of the coordinates
and the .
where the number of fraction bits retained by is
specified by VkPhysicalDeviceLimits::subTexelPrecisionBits.
A set of neighboring integer texel coordinates are generated.
The neighboring coordinates are combinations of to
and to
, with being the
top-left coordinate of this set.
The set is of size .
Horizontal weights to
and vertical weights
to are
computed.
Texels that are fully covered by the box will have a horizontal and vertical
weight of 1.
Texels partially covered by the box will have will have a reduced weights
proportional to the coverage.
The values of multiple texels, together with their horizontal and vertical
weights, are combined to produce a box filtered value.
When VkSamplerReductionModeCreateInfo::reductionMode is
VK_SAMPLER_REDUCTION_MODE_WEIGHTED_AVERAGE, the above summation is
used.
However, if the reduction mode is VK_SAMPLER_REDUCTION_MODE_MIN or
VK_SAMPLER_REDUCTION_MODE_MAX, the process operates on the above
values, computing a component-wise minimum or maximum of the texels.
Image Operation Steps
Each step described in this chapter is performed by a subset of the image
instructions:
Texel Input Validation Operations, Format Conversion, Texel Replacement,
Conversion to RGBA, and Component Swizzle: Performed by all instructions
except OpImageWrite.
Depth Comparison: Performed by OpImage*Dref instructions.
All Texel output operations: Performed by OpImageWrite.
Projection: Performed by all OpImage*Proj instructions.
Derivative Image Operations, Cube Map Operations, Scale Factor
Operation, LOD Operation and Image Level(s) Selection, and Texel
Anisotropic Filtering: Performed by all OpImageSample* and
OpImageSparseSample* instructions.
(s,t,r,q,a) to (u,v,w,a) Transformation, Wrapping, and (u,v,w,a) to
(i,j,k,l,n) Transformation And Array Layer Selection: Performed by all
OpImageSample, OpImageSparseSample, and OpImage*Gather
instructions.
Texel Gathering: Performed by OpImage*Gather instructions.
Texel Footprint Evaluation: Performed by OpImageSampleFootprint
instructions.
Texel Filtering: Performed by all OpImageSample* and
OpImageSparseSample* instructions.
Sparse Residency: Performed by all OpImageSparse* instructions.
(s,t,r,q,a) to (u,v,w,a) Transformation, Wrapping, and Weight Image
Sampling: Performed by OpImageWeightedSample* instructions.
(s,t,r,q,a) to (u,v,w,a) Transformation, Wrapping, and Block Matching:
Performed by opImageBlockMatch* instructions.
(s,t,r,q,a) to (u,v,w,a) Transformation, Wrapping, and Box Filter
Sampling: Performed by OpImageBoxFilter* instructions.
Image Query Instructions
Image Property Queries
OpImageQuerySize, OpImageQuerySizeLod, OpImageQueryLevels,
and OpImageQuerySamples query properties of the image descriptor that
would be accessed by a shader image operation.
They return 0 if the bound descriptor is a null descriptor.
OpImageQuerySizeLod returns the size of the image level identified by
the LevelofDetail operand.
If that level does not exist in the image,
and the descriptor is not null,
then the value returned is undefined.
LOD Query
OpImageQueryLod returns the LOD parameters that would be used in an
image operation with the given image and coordinates.
If the descriptor that would be accessed is a null descriptor then
(0,0) is returned.
Otherwise, the
steps described in this chapter are performed as if for
OpImageSampleImplicitLod, up to Scale Factor Operation, LOD Operation and Image Level(s) Selection.
The return value is the vector (λ', dl - levelbase).
These values may be subject to implementation-specific maxima and minima
for very large, out-of-range values.