Image Operations

Image Operations Overview

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:

normalized texel coordinates [0.0, 1.0]
unnormalized texel coordinates [0.0, width / height / depth)
integer texel coordinates [0, width / height / depth)

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 Δ_i and Δ_j, the four texels selected by the offset are i₀j'₀, i₁j'₀, i₀j'₁, and i₁j'₁.

For formats with reduced-resolution components, Δ_i and Δ_j 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.

Figure 2. Texel Coordinate Systems, Nearest Filtering

The Texel Coordinate Systems - For the example shown of an 8×4 texel two dimensional image.

Texel coordinates as above. Also shown for nearest filtering:
- Given the unnormalized coordinates (u,v), the texel selected is ij.
- Given the offset Δ_i and Δ_j, the texel selected by the offset is ij'.

For corner-sampled images, the texel samples are located at the grid intersections instead of the texel centers.

Figure 3. Texel Coordinate Systems, Corner Sampling

Conversion Formulas

RGB to Shared Exponent Conversion

An RGB color (red, green, blue) is transformed to a shared exponent color (red_shared, green_shared, blue_shared, exp_shared) as follows:

First, the components (red, green, blue) are clamped to (red_clamped, green_clamped, blue_clamped) as:

: red_clamped = max(0, min(sharedexp_max, red))
: green_clamped = max(0, min(sharedexp_max, green))
: blue_clamped = max(0, min(sharedexp_max, blue))

where:

NaN, if supported, is handled as in
IEEE 754-2008 minNum() and maxNum(). This results in any NaN being mapped to zero.

The largest clamped component, max_clamped is determined:

: max_clamped = max(red_clamped, green_clamped, blue_clamped)

A preliminary shared exponent exp' is computed:

The shared exponent exp_shared is computed:

Finally, three integer values in the range 0 to 2^N are computed:

Shared Exponent to RGB

A shared exponent color (red_shared, green_shared, blue_shared, exp_shared) is transformed to an RGB color (red, green, blue) as follows:


: \(green = green_{shared} \times {2^{(exp_{shared}-B-N)}}\)

where:

: N = 9 (number of mantissa bits per component)
: B = 15 (exponent bias)

Texel Input Operations

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:

Validation operations
Format conversion
Texel replacement
Depth comparison
Conversion to RGBA
Component swizzle
Chroma reconstruction
Y′C_BC_R conversion

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.

If Chroma Reconstruction is implicit, Texel Filtering instead takes place during chroma reconstruction, before sampler Y′C_BC_R conversion occurs.

The operations described in block matching and weight image sampling are performed before Conversion to RGBA and Component swizzle.

Texel Input Validation Operations

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.
The sampler borderColor is one of the opaque black colors (VK_BORDER_COLOR_FLOAT_OPAQUE_BLACK or VK_BORDER_COLOR_INT_OPAQUE_BLACK) and the image view VkComponentSwizzle for any of the VkComponentMapping components is not the identity swizzle, and VkPhysicalDeviceBorderColorSwizzleFeaturesEXT::borderColorSwizzleFromImage feature is not enabled, and VkSamplerBorderColorComponentMappingCreateInfoEXT is not specified.
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.
VkSamplerBorderColorComponentMappingCreateInfoEXT::srgb, if specified, does not match the sRGB encoding of the image view.
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 borderColor is a custom color (VK_BORDER_COLOR_FLOAT_CUSTOM_EXT or VK_BORDER_COLOR_INT_CUSTOM_EXT) and the image view VkComponentSwizzle for any of the VkComponentMapping components is not the identity swizzle, and VkPhysicalDeviceBorderColorSwizzleFeaturesEXT::borderColorSwizzleFromImage feature is not enabled, and VkSamplerBorderColorComponentMappingCreateInfoEXT is not specified.
The VkImageLayout of any subresource in the image view does not match the VkDescriptorImageInfo::imageLayout used to write the image descriptor.
The SPIR-V Image Format is not compatible with the image view’s format.
The sampler unnormalizedCoordinates is VK_TRUE and any of the limitations of unnormalized coordinates are violated.
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 Sampled Type of the OpTypeImage does not match the SPIR-V Type.
- If the signedness of any read or sample operation does not match the signedness of the image’s format.
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 was created with a specified VkSamplerCustomBorderColorCreateInfoEXT::format which does not match the VkFormat of the image view(s) it is sampling.
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′C_BC_R conversion.

OpImageFetch, OpImageSparseFetch, OpImage*Gather, and OpImageSparse*Gather must not be used with a sampler or image view that enables sampler Y′C_BC_R conversion.

The ConstOffset and Offset operands must not be used with a sampler or image view that enables sampler Y′C_BC_R 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_PACK16 VkImage is sampled via a VK_FORMAT_R16_UNORM VkImageView, 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 < w_s
: 0 ≤ j < h_s
: 0 ≤ k < d_s
: 0 ≤ l < layers
: 0 ≤ n < samples

where:

: w_s = width of the image level
: h_s = height of the image level
: d_s = 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 disjoint multi-planar image are not in the same image layout, the image must not be sampled with sampler Y′C_BC_R 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.

Each component is converted based on its type and size (as defined in the Format Definition section for each VkFormat), using the appropriate equations in 16-Bit Floating-Point Numbers, Unsigned 11-Bit Floating-Point Numbers, Unsigned 10-Bit Floating-Point Numbers, Fixed-Point Data Conversion, and Shared Exponent to RGB. Signed integer components smaller than 32 bits are sign-extended.

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 VkSamplerYcbcrConversionYcbcrDegammaCreateInfoQCOM::enableYDegamma is equal to VK_TRUE, then sRGB to linear conversion is applied to the G component as described in the “sRGB EOTF” section of the Khronos Data Format Specification. If VkSamplerYcbcrConversionYcbcrDegammaCreateInfoQCOM::enableCbCrDegamma is equal to VK_TRUE, then sRGB to linear conversion is applied to the R 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:

Table 1. Border Color B, Custom Border Color VkSamplerCustomBorderColorCreateInfoEXT::`customBorderColor` U
Sampler `borderColor`	Corresponding Border Color
`VK_BORDER_COLOR_FLOAT_TRANSPARENT_BLACK`	[B_r, B_g, B_b, B_a] = [0.0, 0.0, 0.0, 0.0]
`VK_BORDER_COLOR_FLOAT_OPAQUE_BLACK`	[B_r, B_g, B_b, B_a] = [0.0, 0.0, 0.0, 1.0]
`VK_BORDER_COLOR_FLOAT_OPAQUE_WHITE`	[B_r, B_g, B_b, B_a] = [1.0, 1.0, 1.0, 1.0]
`VK_BORDER_COLOR_INT_TRANSPARENT_BLACK`	[B_r, B_g, B_b, B_a] = [0, 0, 0, 0]
`VK_BORDER_COLOR_INT_OPAQUE_BLACK`	[B_r, B_g, B_b, B_a] = [0, 0, 0, 1]
`VK_BORDER_COLOR_INT_OPAQUE_WHITE`	[B_r, B_g, B_b, B_a] = [1, 1, 1, 1]
`VK_BORDER_COLOR_FLOAT_CUSTOM_EXT`	[B_r, B_g, B_b, B_a] = [U_r, U_g, U_b, U_a]
`VK_BORDER_COLOR_INT_CUSTOM_EXT`	[B_r, B_g, B_b, B_a] = [U_r, U_g, U_b, U_a]

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 = B_r
Stencil aspect	S = B_r†
One component color format	Color_r = B_r
Two component color format	[Color_r,Color_g] = [B_r,B_g]
Three component color format	[Color_r,Color_g,Color_b] = [B_r,B_g,B_b]
Four component color format	[Color_r,Color_g,Color_b,Color_a] = [B_r,B_g,B_b,B_a]
Single component alpha format	[Color_r,Color_g,Color_b, Color_a] = [0,0,0,B_a]

† S = B_g may 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 = B_r 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 D_ref and the texel depth value D_tex 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 D_ref 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:

Table 3. Texel Color After Conversion To RGBA
Texel Aspect or Format	RGBA Color
Depth aspect	[Color_r,Color_g,Color_b, Color_a] = [D,0,0,one]
Stencil aspect	[Color_r,Color_g,Color_b, Color_a] = [S,0,0,one]
One component color format	[Color_r,Color_g,Color_b, Color_a] = [Color_r,0,0,one]
Two component color format	[Color_r,Color_g,Color_b, Color_a] = [Color_r,Color_g,0,one]
Three component color format	[Color_r,Color_g,Color_b, Color_a] = [Color_r,Color_g,Color_b,one]
Four component color format	[Color_r,Color_g,Color_b, Color_a] = [Color_r,Color_g,Color_b,Color_a]
One alpha component color format	[Color_r,Color_g,Color_b, Color_a] = [0,0,0,Color_a]

where one = 1.0f for floating-point formats and depth aspects, and one = 1 for integer formats and stencil aspects.

Component Swizzle

All texel input instructions apply a swizzle based on:

the VkComponentSwizzle enums in the components member of the VkImageViewCreateInfo structure for the image being read if sampler Y′C_BC_R conversion is not enabled, and
the VkComponentSwizzle enums in the components member of the VkSamplerYcbcrConversionCreateInfo structure for the sampler Y′C_BC_R conversion if sampler Y′C_BC_R conversion is enabled.

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′C_BC_R 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.

Figure 4. 422 downsampling, xChromaOffset=COSITED_EVEN

Figure 5. 422 downsampling, xChromaOffset=MIDPOINT

Figure 6. 420 downsampling, xChromaOffset=COSITED_EVEN, yChromaOffset=COSITED_EVEN

Figure 7. 420 downsampling, xChromaOffset=MIDPOINT, yChromaOffset=COSITED_EVEN

Figure 8. 420 downsampling, xChromaOffset=COSITED_EVEN, yChromaOffset=MIDPOINT

Figure 9. 420 downsampling, xChromaOffset=MIDPOINT, yChromaOffset=MIDPOINT

Reconstruction is implemented in one of two ways:

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.

Explicit Reconstruction

If the chromaFilter member of the VkSamplerYcbcrConversionCreateInfo structure is VK_FILTER_NEAREST:
- 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 chromaFilter member of the VkSamplerYcbcrConversionCreateInfo structure is VK_FILTER_LINEAR:
- 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′C_BC_R Conversion

Sampler Y′C_BC_R conversion performs the following operations, which an implementation may combine into a single mathematical operation:

Sampler Y′C_BC_R Range Expansion
Sampler Y′C_BC_R Model Conversion

Sampler Y′C_BC_R Range Expansion

Sampler Y′C_BC_R range expansion is applied to color component values after all texel input operations which are not specific to sampler Y′C_BC_R 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′C_BC_R 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′C_BC_R 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′C_BC_R range expansion.

The range expansion to be applied is defined by the ycbcrRange member of the VkSamplerYcbcrConversionCreateInfo structure:

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:

Image Operations