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: