20.1. Vertex Attributes

Vertex shaders can define input variables, which receive vertex attribute data transferred from one or more VkBuffer(s) by drawing commands. Vertex shader input variables are bound to buffers via an indirect binding where the vertex shader associates a vertex input attribute number with each variable, vertex input attributes are associated to vertex input bindings on a per-pipeline basis, and vertex input bindings are associated with specific buffers on a per-draw basis via the vkCmdBindVertexBuffers command. Vertex input attribute and vertex input binding descriptions also contain format information controlling how data is extracted from buffer memory and converted to the format expected by the vertex shader.

There are VkPhysicalDeviceLimits::maxVertexInputAttributes number of vertex input attributes and VkPhysicalDeviceLimits::maxVertexInputBindings number of vertex input bindings (each referred to by zero-based indices), where there are at least as many vertex input attributes as there are vertex input bindings. Applications can store multiple vertex input attributes interleaved in a single buffer, and use a single vertex input binding to access those attributes.

In GLSL, vertex shaders associate input variables with a vertex input attribute number using the location layout qualifier. The component layout qualifier associates components of a vertex shader input variable with components of a vertex input attribute.

GLSL example. 

// Assign location M to variableName
layout (location=M, component=2) in vec2 variableName;

// Assign locations [N,N+L) to the array elements of variableNameArray
layout (location=N) in vec4 variableNameArray[L];

In SPIR-V, vertex shaders associate input variables with a vertex input attribute number using the Location decoration. The Component decoration associates components of a vertex shader input variable with components of a vertex input attribute. The Location and Component decorations are specified via the OpDecorate instruction.

SPIR-V example. 

               ...
          %1 = OpExtInstImport "GLSL.std.450"
               ...
               OpName %9 "variableName"
               OpName %15 "variableNameArray"
               OpDecorate %18 Builtin VertexIndex
               OpDecorate %19 Builtin InstanceIndex
               OpDecorate %9 Location M
               OpDecorate %9 Component 2
               OpDecorate %15 Location N
               ...
          %2 = OpTypeVoid
          %3 = OpTypeFunction %2
          %6 = OpTypeFloat 32
          %7 = OpTypeVector %6 2
          %8 = OpTypePointer Input %7
          %9 = OpVariable %8 Input
         %10 = OpTypeVector %6 4
         %11 = OpTypeInt 32 0
         %12 = OpConstant %11 L
         %13 = OpTypeArray %10 %12
         %14 = OpTypePointer Input %13
         %15 = OpVariable %14 Input
               ...

20.1.1. Attribute Location and Component Assignment

Vertex shaders allow Location and Component decorations on input variable declarations. The Location decoration specifies which vertex input attribute is used to read and interpret the data that a variable will consume. The Component decoration allows the location to be more finely specified for scalars and vectors, down to the individual components within a location that are consumed. The components within a location are 0, 1, 2, and 3. A variable starting at component N will consume components N, N+1, N+2, … up through its size. For single precision types, it is invalid if the sequence of components gets larger than 3.

When a vertex shader input variable declared using a scalar or vector 32-bit data type is assigned a location, its value(s) are taken from the components of the input attribute specified with the corresponding VkVertexInputAttributeDescription::location. The components used depend on the type of variable and the Component decoration specified in the variable declaration, as identified in Table 20.1, “Input attribute components accessed by 32-bit input variables”. Any 32-bit scalar or vector input will consume a single location. For 32-bit data types, missing components are filled in with default values as described below.

Table 20.1. Input attribute components accessed by 32-bit input variables

32-bit data type Component decoration Components consumed

scalar

0 or unspecified

(x, o, o, o)

scalar

1

(o, y, o, o)

scalar

2

(o, o, z, o)

scalar

3

(o, o, o, w)

two-component vector

0 or unspecified

(x, y, o, o)

two-component vector

1

(o, y, z, o)

two-component vector

2

(o, o, z, w)

three-component vector

0 or unspecified

(x, y, z, o)

three-component vector

1

(o, y, z, w)

four-component vector

0 or unspecified

(x, y, z, w)


Components indicated by ‘o’ are available for use by other input variables which are sourced from the same attribute, and if used, are either filled with the corresponding component from the input format (if present), or the default value.

When a vertex shader input variable declared using a 32-bit floating point matrix type is assigned a location i, its values are taken from consecutive input attributes starting with the corresponding VkVertexInputAttributeDescription::location. Such matrices are treated as an array of column vectors with values taken from the input attributes identified in Table 20.2, “Input attributes accessed by 32-bit input matrix variables”. The VkVertexInputAttributeDescription::format must be specified with a VkFormat that corresponds to the appropriate type of column vector. The Component decoration must not be used with matrix types.

Table 20.2. Input attributes accessed by 32-bit input matrix variables

Data type Column vector type Locations consumed Components consumed

mat2

two-component vector

i, i+1

(x, y, o, o), (x, y, o, o)

mat2x3

three-component vector

i, i+1

(x, y, z, o), (x, y, z, o)

mat2x4

four-component vector

i, i+1

(x, y, z, w), (x, y, z, w)

mat3x2

two-component vector

i, i+1, i+2

(x, y, o, o), (x, y, o, o), (x, y, o, o)

mat3

three-component vector

i, i+1, i+2

(x, y, z, o), (x, y, z, o), (x, y, z, o)

mat3x4

four-component vector

i, i+1, i+2

(x, y, z, w), (x, y, z, w), (x, y, z, w)

mat4x2

two-component vector

i, i+1, i+2, i+3

(x, y, o, o), (x, y, o, o), (x, y, o, o), (x, y, o, o)

mat4x3

three-component vector

i, i+1, i+2, i+3

(x, y, z, o), (x, y, z, o), (x, y, z, o), (x, y, z, o)

mat4

four-component vector

i, i+1, i+2, i+3

(x, y, z, w), (x, y, z, w), (x, y, z, w), (x, y, z, w)


Components indicated by ‘o’ are available for use by other input variables which are sourced from the same attribute, and if used, are either filled with the corresponding component from the input (if present), or the default value.

When a vertex shader input variable declared using a scalar or vector 64-bit data type is assigned a location i, its values are taken from consecutive input attributes starting with the corresponding VkVertexInputAttributeDescription::location. The locations and components used depend on the type of variable and the Component decoration specified in the variable declaration, as identified in Table 20.3, “Input attribute locations and components accessed by 64-bit input variables”. For 64-bit data types, no default attribute values are provided. Input variables must not use more components than provided by the attribute. Input attributes which have one- or two-component 64-bit formats will consume a single location. Input attributes which have three- or four-component 64-bit formats will consume two consecutive locations. A 64-bit scalar data type will consume two components, and a 64-bit two-component vector data type will consume all four components available within a location. A three- or four-component 64-bit data type must not specify a component. A three-component 64-bit data type will consume all four components of the first location and components 0 and 1 of the second location. This leaves components 2 and 3 available for other component-qualified declarations. A four-component 64-bit data type will consume all four components of the first location and all four components of the second location. It is invalid for a scalar or two-component 64-bit data type to specify a component of 1 or 3.

Table 20.3. Input attribute locations and components accessed by 64-bit input variables

Input format Locations consumed 64-bit data type Location decoration Component decoration 32-bit components consumed

R64

i

scalar

i

0 or unspecified

(x, y, -, -)

R64G64

i

scalar

i

0 or unspecified

(x, y, o, o)

scalar

i

2

(o, o, z, w)

two-component vector

i

0 or unspecified

(x, y, z, w)

R64G64B64

i, i+1

scalar

i

0 or unspecified

(x, y, o, o), (o, o, -, -)

scalar

i

2

(o, o, z, w), (o, o, -, -)

scalar

i+1

0 or unspecified

(o, o, o, o), (x, y, -, -)

two-component vector

i

0 or unspecified

(x, y, z, w), (o, o, -, -)

three-component vector

i

unspecified

(x, y, z, w), (x, y, -, -)

R64G64B64A64

i, i+1

scalar

i

0 or unspecified

(x, y, o, o), (o, o, o, o)

scalar

i

2

(o, o, z, w), (o, o, o, o)

scalar

i+1

0 or unspecified

(o, o, o, o), (x, y, o, o)

scalar

i+1

2

(o, o, o, o), (o, o, z, w)

two-component vector

i

0 or unspecified

(x, y, z, w), (o, o, o, o)

two-component vector

i+1

0 or unspecified

(o, o, o, o), (x, y, z, w)

three-component vector

i

unspecified

(x, y, z, w), (x, y, o, o)

four-component vector

i

unspecified

(x, y, z, w), (x, y, z, w)


Components indicated by ‘o’ are available for use by other input variables which are sourced from the same attribute. Components indicated by ‘-’ are not available for input variables as there are no default values provided for 64-bit data types, and there is no data provided by the input format.

When a vertex shader input variable declared using a 64-bit floating-point matrix type is assigned a location i, its values are taken from consecutive input attribute locations. Such matrices are treated as an array of column vectors with values taken from the input attributes as shown in Table 20.3, “Input attribute locations and components accessed by 64-bit input variables”. Each column vector starts at the location immediately following the last location of the previous column vector. The number of attributes and components assigned to each matrix is determined by the matrix dimensions and ranges from two to eight locations.

When a vertex shader input variable declared using an array type is assigned a location, its values are taken from consecutive input attributes starting with the corresponding VkVertexInputAttributeDescription::location. The number of attributes and components assigned to each element are determined according to the data type of the array elements and Component decoration (if any) specified in the declaration of the array, as described above. Each element of the array, in order, is assigned to consecutive locations, but all at the same specified component within each location.

Only input variables declared with the data types and component decorations as specified above are supported. Location aliasing is causing two variables to have the same location number. Component aliasing is assigning the same (or overlapping) component number for two location aliases. Location aliasing is allowed only if it does not cause component aliasing. Further, when location aliasing, the aliases sharing the location must have the same underlying numerical type (floating-point or integer). Failure to meet these requirements will result in an invalid pipeline.