Sampling

Sampling is performed on images, typically in conjunction with a sampler, to return a value based on a neighborhood of texels in an image.

Sampling instructions 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.
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.
OpImageQueryLod returns the LOD parameters that would be used in a sample operation. The actual operation is not performed.
OpImageSampleWeightedQCOM 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.

Sampling Coordinate Systems

There are three sampling coordinate systems used in this chapter:

Normalized Sampling Coordinates
Unnormalized Sampling Coordinates
Integer Sampling Coordinates

SPIR-V opImageBlockMatchSADQCOM, opImageBlockMatchSSDQCOM, opImageBlockMatchWindowSADQCOM, opImageBlockMatchWindowSSDQCOM, OpImageFetch, and OpImageSparseFetch instructions use integer sampling coordinates.

Other image instructions can use either normalized or unnormalized sampling 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 Sampling Coordinates

Normalized sampling 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 values s,t,r,q,a are all floating-point values. Values of s,t,r in the normalized range [0.0,1.0] and a in the range [0.0, layers - 1] correspond to locations in the image being sampled, where layers is the dimension of the image with the same name. The value q is used as a divisor for each of s,t,r, rather than directly corresponding to a location in an image.

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 Sampling Coordinates

Unnormalized sampling 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.

The values u,v,w,a are all floating-point values, with values in the ranges [0.0,width), [0.0, height), [0.0, depth), and [0,layers-1] corresponding to locations in the image being sampled. The values width, height, depth, and layers are the dimensions of the accessed image.

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 Sampling Coordinates

Integer sampling 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.

The values i,j,k,l,n are all integer values, with values in the ranges [0,width), [0, height), [0, depth), [0,layers), and [0,samples) corresponding to locations in the image being sampled. The values width, height, depth, layers, and samples are the dimensions of the accessed image.

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.

Integer coordinates are used as image coordinates to perform an image read after sampling calculations, directly translating each coordinate as follows:

i → x
j → y
k → z
l → layer
n → sample

level is calculated separately via the Lod image operand if present, or is set to 0 otherwise.

Integer sampling coordinates are used as image coordinates to perform an image read after sampling calculations, directly translating each coordinate as follows:

x = i
y = j
z = k
layer = l + VkImageSubresourceRange::baseArrayLayer
sample = n

Sampling Coordinate Diagrams

The following diagrams illustrate the different sampling coordinate systems for 2D images.

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

Sampling Operations

Sampling instructions are SPIR-V image instructions that read from an image with a sampler. Sampling operations are a set of steps that are performed on state, coordinates, and texel values while processing a sampling instruction, and which are common to some or all sampling instructions. They include the following steps, which are performed in the listed order:

Validation operations
Border Replacement
Texel Reads
Depth comparison
Component swizzle
Y′C_BC_R degamma
Chroma reconstruction
Y′C_BC_R conversion

For sampling 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 sampling 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 are performed before Component Substitution during texel read.

The operations described in weight image sampling are performed before 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 poison.

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.

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.

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.

Coordinate Validation

Once the normalized or unnormalized coordinates have been converted to integer image coordinates, the integer coordinates are validated as image coordinates, as outlined in Image Coordinate Validation, converted as follows:

x = i
y = j
z = k
layer = l
sample = n
level = d

Cube Map Edge Handling

When sampling a cube map, if the image coordinates are out of bounds of the selected cube map face, the following steps are performed.

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 x or only y, then the coordinates (x,y,layer) 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 x and y, 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.

Border Replacement

If the sampler includes a border, out of bounds 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.

If rgba4OpaqueBlackSwizzled is VK_FALSE, the implementation may swap the blue and alpha channels when sampling non-custom border colors with the VK_FORMAT_B4G4R4A4_UNORM_PACK16 format, or the red and alpha channels with the VK_FORMAT_R4G4B4A4_UNORM_PACK16 format.

As VK_FORMAT_B4G4R4A4_UNORM_PACK16 is required by Vulkan, support must be advertised for this format. Some Vulkan implementations on Apple hardware implement these formats through a hardware format with a different channel order, swizzled to match Vulkan’s expectations. Unfortunately the swizzle cannot be readily applied to the fixed border colors - resulting in the apparent channel swap. For most standard border colors this does not result in a modification to the sampled output. However, VK_BORDER_COLOR_*_OPAQUE_BLACK will instead be sampled as transparent red or blue. If the customBorderColorWithoutFormat feature is supported and enabled, this functionality is expected to work without issue, but this feature may come with a performance cost.

When border color replacement occurs, texel reads are skipped, and the replaced color is used for ongoing operations instead.

Texel Reads

A texel is read from an image, performed as outlined in Image Reads, using the converted image coordinates.

The returned components of each texel are then processed by further input operations.

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.

If the value of magFilter is VK_FILTER_LINEAR, or the value of minFilter is VK_FILTER_LINEAR, then D may be computed in an implementation-dependent manner which differs from the normal rules of linear filtering. The resulting value must be in the range [0,1] and should be proportional to, or a weighted average of, the number of comparison passes or failures.

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.

Y′C_BC_R Degamma

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

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 on sampled data, in order:

Sampler Y′CBCR Component Swizzle