Fixed-Function Vertex Processing
Vertex fetching is controlled via configurable state, as a logically distinct graphics pipeline stage.
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
...
Attribute Location and Component Assignment
The Location
decoration specifies which vertex input attribute is used
to read and interpret the data that a variable will consume.
When a vertex shader input variable declared using a 16- or 32-bit scalar or
vector 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 32. Input Attribute Components Accessed By 16-Bit and 32-Bit Input Variables.
Any 16-bit or 32-bit scalar or vector input will consume a single
Location
.
For 16-bit and 32-bit data types, missing components are filled in with
default values as described below.
If an implementation supports storageInputOutput16
, vertex shader input variables can have a
width of 16 bits.
16-bit or 32-bit data type | 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 33. 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.
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 Location
slots and Component
words used depend on the type of
variable and the Component
decoration specified in the variable
declaration, as identified in Table 34. 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 format | Locations consumed | 64-bit data type | 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 34. 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.
Two variables are allowed to share the same Location
slot only if their
Component
words do not overlap.
If multiple variables share the same Location
slot, they must all have
the same SPIR-V floating-point component type or all have the same width
scalar type components.
Vertex Input Description
Applications specify vertex input attribute and vertex input binding
descriptions as part of graphics pipeline creation by setting the
VkGraphicsPipelineCreateInfo::pVertexInputState
pointer to a
VkPipelineVertexInputStateCreateInfo structure.
Alternatively, if the graphics pipeline is created with the
VK_DYNAMIC_STATE_VERTEX_INPUT_EXT
dynamic state enabled, then the
vertex input attribute and vertex input binding descriptions are specified
dynamically with vkCmdSetVertexInputEXT, and the
VkGraphicsPipelineCreateInfo::pVertexInputState
pointer is
ignored.
Vertex Attribute Divisor in Instanced Rendering
Vertex Input Address Calculation
The address of each attribute for each vertexIndex
and
instanceIndex
is calculated as follows:
- Let
attribDesc
be the member of VkPipelineVertexInputStateCreateInfo::pVertexAttributeDescriptions
with VkVertexInputAttributeDescription::location
equal to the vertex input attribute number. - Let
bindingDesc
be the member of VkPipelineVertexInputStateCreateInfo::pVertexBindingDescriptions
with VkVertexInputAttributeDescription::binding
equal toattribDesc.binding
. - Let
vertexIndex
be the index of the vertex within the draw (a value betweenfirstVertex
andfirstVertex
+vertexCount
forvkCmdDraw
, or a value taken from the index buffer plusvertexOffset
forvkCmdDrawIndexed
), and letinstanceIndex
be the instance number of the draw (a value betweenfirstInstance
andfirstInstance
+instanceCount
). - Let
offset
be an array of offsets into the currently bound vertex buffers specified duringvkCmdBindVertexBuffers
orvkCmdBindVertexBuffers2
withpOffsets
. - Let
divisor
be the member of VkPipelineVertexInputDivisorStateCreateInfoKHR::pVertexBindingDivisors
with VkVertexInputBindingDivisorDescriptionKHR::binding
equal toattribDesc.binding
. If the vertex binding state is dynamically set, instead letdivisor
be the member of thepVertexBindingDescriptions
parameter to the vkCmdSetVertexInputEXT call with VkVertexInputBindingDescription2EXT::binding
equal toattribDesc.binding
. - Let
stride
be the member of VkPipelineVertexInputStateCreateInfo::pVertexBindingDescriptions→stride
unless there is dynamic state causing the value to be ignored. In this case the value is set from the last value from one of the following- vkCmdSetVertexInputEXT::
pVertexBindingDescriptions→stride
- vkCmdBindVertexBuffers2::
pStride
, if notNULL
- vkCmdSetVertexInputEXT::
bufferBindingAddress = buffer[binding].baseAddress + offset[binding];
if (bindingDesc.inputRate == VK_VERTEX_INPUT_RATE_VERTEX)
effectiveVertexOffset = vertexIndex * stride;
else
if (divisor == 0)
effectiveVertexOffset = firstInstance * stride;
else
effectiveVertexOffset = (firstInstance + ((instanceIndex - firstInstance) / divisor)) * stride;
attribAddress = bufferBindingAddress + effectiveVertexOffset + attribDesc.offset;
Vertex Input Extraction
For each attribute, raw data is extracted starting at attribAddress
and is
converted from the VkVertexInputAttributeDescription’s format
to
either floating-point, unsigned integer, or signed integer based on the
numeric type of format
.
The numeric type of format
must match the numeric type of the input
variable in the shader.
The input variable in the shader must be declared as a 64-bit data type if
and only if format
is a 64-bit data type.
If
either format
is a 64-bit format or legacyVertexAttributes
is not enabled, and
format
is a packed format, attribAddress
must be a multiple of the
size in bytes of the whole attribute data type as described in
Packed Formats.
Otherwise,
if either format
is a 64-bit format or
legacyVertexAttributes
is not
enabled,
attribAddress
must be a multiple of the size in bytes of the component
type indicated by format
(see Formats).
For attributes that are not 64-bit data types, each component is converted
to the format of the input variable 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.
Attributes that are not 64-bit data types are expanded to four components in
the same way as described in conversion to
RGBA.
The number of components in the vertex shader input variable need not
exactly match the number of components in the format.
If the vertex shader has fewer components, the extra components are
discarded.