14.6. Built-In Variables

Built-in variables are accessed in shaders by declaring a variable decorated with a BuiltIn decoration. The meaning of each BuiltIn decoration is as follows. In the remainder of this section, the name of a built-in is used interchangeably with a term equivalent to a variable decorated with that particular built-in. Built-ins that represent integer values can be declared as either signed or unsigned 32-bit integers.

ClipDistance

Decorating a variable with the ClipDistance built-in decoration will make that variable contain the mechanism for controlling user clipping. ClipDistance is an array such that the ith element of the array specifies the clip distance for plane i. A clip distance of 0 means the vertex is on the plane, a positive distance means the vertex is inside the clip half-space, and a negative distance means the point is outside the clip half-space.

The ClipDistance decoration must be used only within vertex, fragment, tessellation control, tessellation evaluation, and geometry shaders.

In vertex shaders, any variable decorated with ClipDistance must be declared using the output storage class.

In fragment shaders, any variable decorated with ClipDistance must be declared using the input storage class.

In tessellation control, tessellation evaluation, or geometry shaders, any variable decorated with ClipDistance must not be in a storage class other than input or output.

Any variable decorated with ClipDistance must be declared as an array of 32-bit floating-point values.

[Note]Note

The array variable decorated with ClipDistance is explicitly sized by the shader.

[Note]Note

In the last vertex processing stage, these values will be linearly interpolated across the primitive and the portion of the primitive with interpolated distances less than 0 will be considered outside the clip volume. If ClipDistance is then used by a fragment shader, ClipDistance contains these linearly interpolated values.

CullDistance

Decorating a variable with the CullDistance built-in decoration will make that variable contain the mechanism for controlling user culling. If any member of this array is assigned a negative value for all vertices belonging to a primitive, then the primitive is discarded before rasterization.

The CullDistance decoration must be used only within vertex, fragment, tessellation control, tessellation evaluation, and geometry shaders.

In vertex shaders, any variable decorated with CullDistance must be declared using the output storage class.

In fragment shaders, any variable decorated with CullDistance must be declared using the input storage class.

In tessellation control, tessellation evaluation, or geometry shaders, any variable decorated with CullDistance must not be declared in a storage class other than input or output.

Any variable decorated with CullDistance must be declared as an array of 32-bit floating-point values.

[Note]Note

In fragment shaders, the values of the CullDistance array are linearly interpolated across each primitive.

[Note]Note

If CullDistance decorates an input variable, that variable will contain the corresponding value from the CullDistance decorated output variable from the previous shader stage.

FragCoord

Decorating a variable with the FragCoord built-in decoration will make that variable contain the framebuffer coordinate $(x,y,z,\frac{1}{w})$ of the fragment being processed. The $(x,y)$ coordinate $(0,0)$ is the upper left corner of the upper left pixel in the framebuffer.

When sample shading is enabled, the $x$ and $y$ components of FragCoord reflect the location of the sample corresponding to the shader invocation.

When sample shading is not enabled, the x and y components of FragCoord reflect the location of the center of the pixel, $(0.5,0.5)$ .

The $z$ component of FragCoord is the interpolated depth value of the primitive.

The $w$ component is the interpolated $\frac{1}{w}$ .

The FragCoord decoration must be used only within fragment shaders.

The variable decorated with FragCoord must be declared using the input storage class.

The Centroid interpolation decoration is ignored on FragCoord.

The variable decorated with FragCoord must be declared as a four-component vector of 32-bit floating-point values.

FragDepth

Decorating a variable with the FragDepth built-in decoration will make that variable contain the new depth value for all samples covered by the fragment. This value will be used for depth testing and, if the depth test passes, any subsequent write to the depth/stencil attachment.

To write to FragDepth, a shader must declare the DepthReplacing execution mode. If a shader declares the DepthReplacing execution mode and there is an execution path through the shader that does not set FragDepth, then the fragment’s depth value is undefined for executions of the shader that take that path.

The FragDepth decoration must be used only within fragment shaders.

The variable decorated with FragDepth must be declared using the output storage class.

The variable decorated with FragDepth must be declared as a scalar 32-bit floating-point value.

FrontFacing

Decorating a variable with the FrontFacing built-in decoration will make that variable contain whether a primitive is front or back facing. This variable is non-zero if the current fragment is considered to be part of a front-facing primitive and is zero if the fragment is considered to be part of a back-facing primitive.

The FrontFacing decoration must be used only within fragment shaders.

The variable decorated with FrontFacing must be declared using the input storage class.

The variable decorated with FrontFacing must be declared as a boolean.

GlobalInvocationId

Decorating a variable with the GlobalInvocationId built-in decoration will make that variable contain the location of the current invocation within the global workgroup. Each component is equal to the index of the local workgroup multiplied by the size of the local workgroup plus LocalInvocationId.

The GlobalInvocationId decoration must be used only within compute shaders.

The variable decorated with GlobalInvocationId must be declared using the input storage class.

The variable decorated with GlobalInvocationId must be declared as a three-component vector of 32-bit integers.

HelperInvocation

Decorating a variable with the HelperInvocation built-in decoration will make that variable contain whether the current invocation is a helper invocation. This variable is non-zero if the current fragment being shaded is a helper invocation and zero otherwise. A helper invocation is an invocation of the shader that is produced to satisfy internal requirements such as the generation of derivatives.

The HelperInvocation decoration must be used only within fragment shaders.

The variable decorated with HelperInvocation must be declared using the input storage class.

The variable decorated with HelperInvocation must be declared as a boolean.

[Note]Note

It is very likely that a helper invocation will have a value of SampleMask fragment shader input value that is zero.

InvocationId

Decorating a variable with the InvocationId built-in decoration will make that variable contain the index of the current shader invocation in a geometry shader, or the index of the output patch vertex in a tessellation control shader.

In a geometry shader, the index of the current shader invocation ranges from zero to the number of instances declared in the shader minus one. If the instance count of the geometry shader is one or is not specified, then InvocationId will be zero.

The InvocationId decoration must be used only within tessellation control and geometry shaders.

The variable decorated with InvocationId must be declared using the input storage class.

The variable decorated with InvocationId must be declared as a scalar 32-bit integer.

InstanceIndex

Decorating a variable with the InstanceIndex built-in decoration will make that variable contain the index of the instance that is being processed by the current vertex shader invocation. InstanceIndex begins at the firstInstance parameter to vkCmdDraw or vkCmdDrawIndexed or at the firstInstance member of a structure consumed by vkCmdDrawIndirect or vkCmdDrawIndexedIndirect.

The InstanceIndex decoration must be used only within vertex shaders.

The variable decorated with InstanceIndex must be declared using the input storage class.

The variable decorated with InstanceIndex must be declared as a scalar 32-bit integer.

Layer

Decorating a variable with the Layer built-in decoration will make that variable contain the select layer of a multi-layer framebuffer attachment.

In a geometry shader, any variable decorated with Layer can be written with the framebuffer layer index to which the primitive produced by the geometry shader will be directed. If a geometry shader entry-point’s interface does not include a variable decorated with Layer, then the first layer is used. If a geometry shader entry-point’s interface includes a variable decorated with Layer, it must write the same value to Layer for all output vertices of a given primitive.

In a fragment shader, a variable decorated with Layer contains the layer index of the primitive that the fragment invocation belongs to.

The Layer decoration must be used only within geometry and fragment shaders.

In a geometry shader, any variable decorated with Layer must be declared using the output storage class.

In a fragment shader, any variable decorated with Layer must be declared using the input storage class.

Any variable decorated with Layer must be declared as a scalar 32-bit integer.

LocalInvocationId

Decorating a variable with the LocalInvocationId built-in decoration will make that variable contain the location of the current compute shader invocation within the local workgroup. Each component ranges from zero through to the size of the workgroup in that dimension minus one.

The LocalInvocationId decoration must be used only within compute shaders.

The variable decorated with LocalInvocationId must be declared using the input storage class.

The variable decorated with LocalInvocationId must be declared as a three-component vector of 32-bit integers.

[Note]Note

If the size of the workgroup in a particular dimension is one, then the LocalInvocationId in that dimension will be zero. If the workgroup is effectively two-dimensional, then LocalInvocationId.z will be zero. If the workgroup is effectively one-dimensional, then both LocalInvocationId.y and LocalInvocationId.z will be zero.

NumWorkgroups

Decorating a variable with the NumWorkgroups built-in decoration will make that variable contain the number of local workgroups that are part of the dispatch that the invocation belongs to. Each component is equal to the values of the parameters passed into vkCmdDispatch or read from the VkDispatchIndirectCommand structure read through a call to vkCmdDispatchIndirect.

The NumWorkgroups decoration must be used only within compute shaders.

The variable decorated with NumWorkgroups must be declared using the input storage class.

The variable decorated with NumWorkgroups must be declared as a three-component vector of 32-bit integers.

PatchVertices

Decorating a variable with the PatchVertices built-in decoration will make that variable contain the number of vertices in the input patch being processed by the shader. A single tessellation control or tessellation evaluation shader can read patches of differing sizes, so the value of the PatchVertices variable may differ between patches.

The PatchVertices decoration must be used only within tessellation control and tessellation evaluation shaders.

The variable decorated with PatchVertices must be declared using the input storage class.

The variable decorated with PatchVertices must be declared as scalar 32-bit integer.

PointCoord

Decorating a variable with the PointCoord built-in decoration will make that variable contain the coordinate of the current fragment within the point being rasterized, normalized to the size of the point with origin in the upper left corner of the point, as described in Basic Point Rasterization. If the primitive the fragment shader invocation belongs to is not a point, then the variable decorated with PointCoord contains an undefined value.

The PointCoord decoration must be used only within fragment shaders.

The variable decorated with PointCoord must be declared using the input storage class.

The variable decorated with PointCoord must be declared as two-component vector of 32-bit floating-point values.

[Note]Note

Depending on how the point is rasterized, PointCoord may never reach $(0,0)$ or $(1,1)$ .

PointSize

Decorating a variable with the PointSize built-in decoration will make that variable contain the size of point primitives. The value written to the variable decorated with PointSize by the last vertex processing stage in the pipeline is used as the framebuffer-space size of points produced by rasterization.

The PointSize decoration must be used only within vertex, tessellation control, tessellation evaluation, and geometry shaders.

In a vertex shader, any variable decorated with PointSize must be declared using the output storage class.

In a tessellation control, tessellation evaluation, or geometry shader, any variable decorated with PointSize must be declared using either the input or output storage class.

Any variable decorated with PointSize must be declared as a scalar 32-bit floating-point value.

[Note]Note

When PointSize decorates a variable in the input storage class, it contains the data written to the output variable decorated with PointSize from the previous shader stage.

Position

Decorating a variable with the Position built-in decoration will make that variable contain the position of the current vertex. In the last vertex processing stage, the value of the variable decorated with Position is used in subsequent primitive assembly, clipping, and rasterization operations.

The Position decoration must be used only within vertex, tessellation control, tessellation evaluation, and geometry shaders.

In a vertex shader, any variable decorated with Position must be declared using the output storage class.

In a tessellation control, tessellation evaluation, or geometry shader, any variable decorated with Position must not be declared in a storage class other than input or output.

Any variable decorated with Position must be declared as a four-component vector of 32-bit floating-point values.

[Note]Note

When Position decorates a variable in the input storage class, it contains the data written to the output variable decorated with Position from the previous shader stage.

PrimitiveId

Decorating a variable with the PrimitiveId built-in decoration will make that variable contain the index of the current primitive.

In tessellation control and tessellation evaluation shaders, it will contain the index of the patch within the current set of rendering primitives that correspond to the shader invocation.

In a geometry shader, it will contain the number of primitives presented as input to the shader since the current set of rendering primitives was started.

In a fragment shader, it will contain the primitive index written by the geometry shader if a geometry shader is present, or with the value that would have been presented as input to the geometry shader had it been present.

If a geometry shader is present and the fragment shader reads from an input variable decorated with PrimitiveId, then the geometry shader must write to an output variable decorated with PrimitiveId in all execution paths.

The PrimitiveId decoration must be used only within fragment, tessellation control, tessellation evaluation, and geometry shaders.

In a fragment, tessellation control or tessellation evaluation shader, any variable decorated with PrimitiveId must be declared using the output storage class.

In a geometry shader, any variable decorated with PrimitiveId must be declared using either the input or output storage class.

Any variable decorated with PrimitiveId must be declared as scalar 32-bit integer.

[Note]Note

When the PrimitiveId decoration is applied to an output variable in the geometry shader, the resulting value is seen through the PrimitiveId decorated input variable in the fragment shader.

SampleId

Decorating a variable with the SampleId built-in decoration will make that variable contain the zero-based index of the sample the invocation corresponds to. SampleId ranges from zero to the number of samples in the framebuffer minus one. If a fragment shader entry-point’s interface includes an input variable decorated with SampleId, per-sample shading is enabled for draws that use that fragment shader.

The SampleId decoration must be used only within fragment shaders.

The variable decorated with SampleId must be declared using the input storage class.

The variable decorated with SampleId must be declared as a scalar 32-bit integer.

SampleMask

Decorating a variable with the SampleMask built-in decoration will make any variable contain the sample coverage mask for the current fragment shader invocation.

A variable in the input storage class decorated with SampleMask will contain a bitmask of the set of samples covered by the primitive generating the fragment during rasterization. It has a sample bit set if and only if the sample is considered covered for this fragment shader invocation. SampleMask[] is an array of integers. Bits are mapped to samples in a manner where bit B of mask M (SampleMask[M]) corresponds to sample $32 \times M + B$ .

When state specifies multiple fragment shader invocations for a given fragment, the sample mask for any single fragment shader invocation specifies the subset of the covered samples for the fragment that correspond to the invocation. In this case, the bit corresponding to each covered sample will be set in exactly one fragment shader invocation.

A variable in the output storage class decorated with SampleMask is an array of integers forming a bit array in a manner similar an input variable decorated with SampleMask, but where each bit represents coverage as computed by the shader. Modifying the sample mask by writing zero to a bit of SampleMask causes the sample to be considered uncovered. However, setting sample mask bits to one will never enable samples not covered by the original primitive. If the fragment shader is being evaluated at any frequency other than per-fragment, bits of the sample mask not corresponding to the current fragment shader invocation are ignored. This array must be sized in the fragment shader either implicitly or explicitly, to be no larger than the implementation-dependent maximum sample-mask (as an array of 32-bit elements), determined by the maximum number of samples. If a fragment shader entry-point’s interface includes an output variable decorated with SampleMask, the sample mask will be undefined for any array elements of any fragment shader invocations that fail to assign a value. If a fragment shader entry-point’s interface does not include an output variable decorated with SampleMask, the sample mask has no effect on the processing of a fragment.

The SampleMask decoration must be used only within fragment shaders.

Any variable decorated with SampleMask must be declared using either the input or output storage class.

Any variable decorated with SampleMask must be declared as an array of 32-bit integers.

SamplePosition

Decorating a variable with the SamplePosition built-in decoration will make that variable contain the sub-pixel position of the sample being shaded. The top left of the pixel is considered to be at coordinate $(0,0)$ and the bottom right of the pixel is considered to be at coordinate $(1,1)$ . If a fragment shader entry-point’s interface includes an input variable decorated with SamplePosition, per-sample shading is enabled for draws that use that fragment shader.

The SamplePosition decoration must be used only within fragment shaders.

The variable decorated with SamplePosition must be declared using the input storage class.

The variable decorated with SamplePosition must be declared as a two-component vector of 32-bit floating-point values.

TessCoord

Decorating a variable with the TessCoord built-in decoration will make that variable contain the three-dimensional $(u,v,w)$ barycentric coordinate of the tessellated vertex within the patch. $u$ , $v$ , and $w$ are in the range $[0,1]$ and vary linearly across the primitive being subdivided. For the tessellation modes of Quads or IsoLines, the third component is always zero.

The TessCoord decoration must be used only within tessellation evaluation shaders.

The variable decorated with TessCoord must be declared using the input storage class.

The variable decorated with TessCoord must be declared as three-component vector of 32-bit floating-point values.

TessLevelOuter

Decorating a variable with the TessLevelOuter built-in decoration will make that variable contain the outer tessellation levels for the current patch.

In tessellation control shaders, the variable decorated with TessLevelOuter can be written to which controls the tessellation factors for the resulting patch. These values are used by the tessellator to control primitive tessellation and can be read by tessellation evaluation shaders.

In tessellation evaluation shaders, the variable decorated with TessLevelOuter can read the values written by the tessellation control shader.

The TessLevelOuter decoration must be used only within tessellation control and tessellation evaluation shaders.

In a tessellation control shader, any variable decorated with TessLevelOuter must be declared using the output storage class.

In a tessellation evaluation shader, any variable decorated with TessLevelOuter must be declared using the input storage class.

Any variable decorated with TessLevelOuter must be declared as an array of size four, containing 32-bit floating-point values.

TessLevelInner

Decorating a variable with the TessLevelInner built-in decoration will make that variable contain the inner tessellation levels for the current patch.

In tessellation control shaders, the variable decorated with TessLevelInner can be written to, which controls the tessellation factors for the resulting patch. These values are used by the tessellator to control primitive tessellation and can be read by tessellation evaluation shaders.

In tessellation evaluation shaders, the variable decorated with TessLevelInner can read the values written by the tessellation control shader.

The TessLevelInner decoration must be used only within tessellation control and tessellation evaluation shaders.

In a tessellation control shader, any variable decorated with TessLevelInner must be declared using the output storage class.

In a tessellation evaluation shader, any variable decorated with TessLevelInner must be declared using the input storage class.

Any variable decorated with TessLevelInner must be declared as an array of size two, containing 32-bit floating-point values.

VertexIndex

Decorating a variable with the VertexIndex built-in decoration will make that variable contain the index of the vertex that is being processed by the current vertex shader invocation. For non-indexed draws, this variable begins at the firstVertex parameter to vkCmdDraw or the firstVertex member of a structure consumed by vkCmdDrawIndirect and increments by one for each vertex in the draw. For indexed draws, its value is the content of the index buffer for the vertex plus the vertexOffset parameter to vkCmdDrawIndexed or the vertexOffset member of the structure consumed by vkCmdDrawIndexedIndirect.

The VertexIndex decoration must be used only within vertex shaders.

The variable decorated with VertexIndex must be declared using the input storage class.

The variable decorated with VertexIndex must be declared as a scalar 32-bit integer.

[Note]Note

VertexIndex starts at the same starting value for each instance.

ViewportIndex

Decorating a variable with the ViewportIndex built-in decoration will make that variable contain the index of the viewport.

In a geometry shader, the variable decorated with ViewportIndex can be written to with the viewport index to which the primitive produced by the geometry shader will be directed. The selected viewport index is used to select the viewport transform and scissor rectangle. If a geometry shader entry-point’s interface does not include a variable decorated with ViewportIndex, then the first viewport is used. If a geometry shader entry-point’s interface includes a variable decorated with ViewportIndex, it must write the same value to ViewportIndex for all output vertices of a given primitive.

In a fragment shader, the variable decorated with ViewportIndex contains the viewport index of the primitive that the fragment invocation belongs to.

The ViewportIndex decoration must be used only within geometry and fragment shaders.

In a geometry shader, any variable decorated with ViewportIndex must be declared using the output storage class.

In a fragment shader, any variable decorated with ViewportIndex must be declared using the input storage class.

Any variable decorated with ViewportIndex must be declared as a scalar 32-bit integer.

WorkgroupId

Decorating a variable with the WorkgroupId built-in decoration will make that variable contain the global workgroup that the current invocation is a member of. Each component ranges from zero to the values of the parameters passed into vkCmdDispatch or read from the VkDispatchIndirectCommand structure read through a call to vkCmdDispatchIndirect.

The WorkgroupId decoration must be used only within compute shaders.

The variable decorated with WorkgroupId must be declared using the input storage class.

The variable decorated with WorkgroupId must be declared as a three-component vector of 32-bit integers.

WorkgroupSize

Decorating a variable with the WorkgroupSize built-in decoration will make that variable contain the dimensions of a local workgroup. If an object is decorated with the WorkgroupSize decoration, this must take precedence over any execution mode set for LocalSize.

The WorkgroupSize decoration must be used only within compute shaders.

The object decorated with WorkgroupSize must be a specialization constant or a constant.

The object decorated with WorkgroupSize must be declared as a three-component vector of 32-bit integers.