Vulkan-Docs/doc/specs/vulkan/chapters/fxvertex.txt

// Copyright (c) 2015-2017 The Khronos Group Inc.
// Copyright notice at https://www.khronos.org/registry/speccopyright.html

[[fxvertex]]
= Fixed-Function Vertex Processing

Some implementations have specialized fixed-function hardware for fetching
and format-converting vertex input data from buffers, rather than performing
the fetch as part of the vertex shader.
Vulkan includes a vertex attribute fetch stage in the graphics pipeline in
order to take advantage of this.


[[fxvertex-attrib]]
== Vertex Attributes

Vertex shaders can: define input variables, which receive _vertex attribute_
data transferred from one or more sname: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
fname: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 sname:VkPhysicalDeviceLimits::pname:maxVertexInputAttributes
number of vertex input attributes and
sname:VkPhysicalDeviceLimits::pname: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 code:location layout qualifier.
The code:component layout qualifier associates components of a vertex shader
input variable with components of a vertex input attribute.

.GLSL example
[source,glsl]
---------------------------------------------------
// 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 code:Location decoration.
The code:Component decoration associates components of a vertex shader input
variable with components of a vertex input attribute.
The code:Location and code:Component decorations are specified via the
code:OpDecorate instruction.

.SPIR-V example
[source,spirv]
---------------------------------------------------
               ...
          %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
               ...
---------------------------------------------------


[[fxvertex-attrib-location]]
=== Attribute Location and Component Assignment

Vertex shaders allow code:Location and code:Component decorations on input
variable declarations.
The code:Location decoration specifies which vertex input attribute is used
to read and interpret the data that a variable will consume.
The code: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
sname:VkVertexInputAttributeDescription::pname:location.
The components used depend on the type of variable and the code:Component
decoration specified in the variable declaration, as identified in
<<fxvertex-attrib-components>>.
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 <<fxvertex-input-extraction,below>>.


[[fxvertex-attrib-components]]
.Input attribute components accessed by 32-bit input variables
[width="65%",cols="<5,<3,<3",options="header"]
|====
| 32-bit data type      | code: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
sname:VkVertexInputAttributeDescription::pname:location.
Such matrices are treated as an array of column vectors with values taken
from the input attributes identified in <<fxvertex-attrib-matrix>>.
The sname:VkVertexInputAttributeDescription::pname:format must: be specified
with a elink:VkFormat that corresponds to the appropriate type of column
vector.
The code:Component decoration must: not be used with matrix types.

[[fxvertex-attrib-matrix]]
.Input attributes accessed by 32-bit input matrix variables
[width="100%",cols="<10%,<24%,<21%,<45%",options="header"]
|====
| 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
sname:VkVertexInputAttributeDescription::pname:location.
The locations and components used depend on the type of variable and the
code:Component decoration specified in the variable declaration, as
identified in <<fxvertex-attrib-double>>.
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.

[[fxvertex-attrib-double]]
.Input attribute locations and components accessed by 64-bit input variables
[width="100%",cols="<18%,^12%,<25%,^14%,^18%,<13%",options="header"]
|====
^.^| Input format | Locations consumed
        ^.^| 64-bit data type   |code:Location decoration |code:Component decoration ^| 32-bit components consumed
| R64          | i
        | scalar                  | i                         | 0 or unspecified           | (x, y, -, -)
.3+<.^| R64G64 .3+^.^| 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)
.5+<.^| R64G64B64 .5+^.^| 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, -, -)
.8+<.^| R64G64B64A64 .8+^.^| 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 <<fxvertex-attrib-double>>.
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
sname:VkVertexInputAttributeDescription::pname:location.
The number of attributes and components assigned to each element are
determined according to the data type of the array elements and
code: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: all
have the same SPIR-V floating-point component type or all have the same
width integer-type components.


[[fxvertex-input]]
== Vertex Input Description

Applications specify vertex input attribute and vertex input binding
descriptions as part of graphics pipeline creation.
The slink:VkGraphicsPipelineCreateInfo::pname:pVertexInputState points to a
structure of type sname:VkPipelineVertexInputStateCreateInfo.

// refBegin VkPipelineVertexInputStateCreateInfo Structure specifying parameters of a newly created pipeline vertex input state

The sname:VkPipelineVertexInputStateCreateInfo structure is defined as:

include::../api/structs/VkPipelineVertexInputStateCreateInfo.txt[]

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:flags is reserved for future use.
  * pname:vertexBindingDescriptionCount is the number of vertex binding
    descriptions provided in pname:pVertexBindingDescriptions.
  * pname:pVertexBindingDescriptions is a pointer to an array of
    sname:VkVertexInputBindingDescription structures.
  * pname:vertexAttributeDescriptionCount is the number of vertex attribute
    descriptions provided in pname:pVertexAttributeDescriptions.
  * pname:pVertexAttributeDescriptions is a pointer to an array of
    sname:VkVertexInputAttributeDescription structures.

.Valid Usage
****
  * pname:vertexBindingDescriptionCount must: be less than or equal to
    sname:VkPhysicalDeviceLimits::pname:maxVertexInputBindings
  * pname:vertexAttributeDescriptionCount must: be less than or equal to
    sname:VkPhysicalDeviceLimits::pname:maxVertexInputAttributes
  * For every pname:binding specified by any given element of
    pname:pVertexAttributeDescriptions, a
    sname:VkVertexInputBindingDescription must: exist in
    pname:pVertexBindingDescriptions with the same value of pname:binding
  * All elements of pname:pVertexBindingDescriptions must: describe distinct
    binding numbers
  * All elements of pname:pVertexAttributeDescriptions must: describe
    distinct attribute locations
****

include::../validity/structs/VkPipelineVertexInputStateCreateInfo.txt[]

Each vertex input binding is specified by an instance of the
sname:VkVertexInputBindingDescription structure.

// refBegin VkVertexInputBindingDescription Structure specifying vertex input binding description

The sname:VkVertexInputBindingDescription structure is defined as:

include::../api/structs/VkVertexInputBindingDescription.txt[]

  * pname:binding is the binding number that this structure describes.
  * pname:stride is the distance in bytes between two consecutive elements
    within the buffer.
  * pname:inputRate specifies whether vertex attribute addressing is a
    function of the vertex index or of the instance index.
    Possible values include:
+
--
// refBegin VkVertexInputRate Specify rate at which vertex attributes are pulled from buffers
include::../api/enums/VkVertexInputRate.txt[]
--
  ** ename:VK_VERTEX_INPUT_RATE_VERTEX indicates that vertex attribute
     addressing is a function of the vertex index.
  ** ename:VK_VERTEX_INPUT_RATE_INSTANCE indicates that vertex attribute
     addressing is a function of the instance index.

.Valid Usage
****
  * pname:binding must: be less than
    sname:VkPhysicalDeviceLimits::pname:maxVertexInputBindings
  * pname:stride must: be less than or equal to
    sname:VkPhysicalDeviceLimits::pname:maxVertexInputBindingStride
****

include::../validity/structs/VkVertexInputBindingDescription.txt[]

Each vertex input attribute is specified by an instance of the
sname:VkVertexInputAttributeDescription structure.

// refBegin VkVertexInputAttributeDescription Structure specifying vertex input attribute description

The sname:VkVertexInputAttributeDescription structure is defined as:

include::../api/structs/VkVertexInputAttributeDescription.txt[]

  * pname:location is the shader binding location number for this attribute.
  * pname:binding is the binding number which this attribute takes its data
    from.
  * pname:format is the size and type of the vertex attribute data.
  * pname:offset is a byte offset of this attribute relative to the start of
    an element in the vertex input binding.

.Valid Usage
****
  * pname:location must: be less than
    sname:VkPhysicalDeviceLimits::pname:maxVertexInputAttributes
  * pname:binding must: be less than
    sname:VkPhysicalDeviceLimits::pname:maxVertexInputBindings
  * pname:offset must: be less than or equal to
    sname:VkPhysicalDeviceLimits::pname:maxVertexInputAttributeOffset
  * pname:format must: be allowed as a vertex buffer format, as specified by
    the ename:VK_FORMAT_FEATURE_VERTEX_BUFFER_BIT flag in
    sname:VkFormatProperties::pname:bufferFeatures returned by
    fname:vkGetPhysicalDeviceFormatProperties
****

include::../validity/structs/VkVertexInputAttributeDescription.txt[]

// refBegin vkCmdBindVertexBuffers Bind vertex buffers to a command buffer

To bind vertex buffers to a command buffer for use in subsequent draw
commands, call:

include::../api/protos/vkCmdBindVertexBuffers.txt[]

  * pname:commandBuffer is the command buffer into which the command is
    recorded.
  * pname:firstBinding is the index of the first vertex input binding whose
    state is updated by the command.
  * pname:bindingCount is the number of vertex input bindings whose state is
    updated by the command.
  * pname:pBuffers is a pointer to an array of buffer handles.
  * pname:pOffsets is a pointer to an array of buffer offsets.

The values taken from elements [eq]#i# of pname:pBuffers and pname:pOffsets
replace the current state for the vertex input binding
[eq]#pname:firstBinding + i#, for [eq]#i# in [eq]#[0, pname:bindingCount)#.
The vertex input binding is updated to start at the offset indicated by
pname:pOffsets[i] from the start of the buffer pname:pBuffers[i].
All vertex input attributes that use each of these bindings will use these
updated addresses in their address calculations for subsequent draw
commands.

.Valid Usage
****
  * pname:firstBinding must: be less than
    sname:VkPhysicalDeviceLimits::pname:maxVertexInputBindings
  * The sum of pname:firstBinding and pname:bindingCount must: be less than
    or equal to sname:VkPhysicalDeviceLimits::pname:maxVertexInputBindings
  * All elements of pname:pOffsets must: be less than the size of the
    corresponding element in pname:pBuffers
  * All elements of pname:pBuffers must: have been created with the
    ename:VK_BUFFER_USAGE_VERTEX_BUFFER_BIT flag
  * Each element of pname:pBuffers that is non-sparse must: be bound
    completely and contiguously to a single sname:VkDeviceMemory object
****

include::../validity/protos/vkCmdBindVertexBuffers.txt[]

The address of each attribute for each code:vertexIndex and
code:instanceIndex is calculated as follows:

  * Let attribDesc be the member of
    sname:VkPipelineVertexInputStateCreateInfo::pname:pVertexAttributeDescriptions
    with sname:VkVertexInputAttributeDescription::pname:location equal to
    the vertex input attribute number.
  * Let bindingDesc be the member of
    sname:VkPipelineVertexInputStateCreateInfo::pname:pVertexBindingDescriptions
    with sname:VkVertexInputAttributeDescription::pname:binding equal to
    attribDesc.binding.
  * Let code:vertexIndex be the index of the vertex within the draw (a value
    between pname:firstVertex and pname:firstVertex+pname:vertexCount for
    fname:vkCmdDraw, or a value taken from the index buffer for
    fname:vkCmdDrawIndexed), and let code:instanceIndex be the instance
    number of the draw (a value between pname:firstInstance and
    pname:firstInstance+pname:instanceCount).

[source,c]
---------------------------------------------------
bufferBindingAddress = buffer[binding].baseAddress + offset[binding];

if (bindingDesc.inputRate == VK_VERTEX_INPUT_RATE_VERTEX)
    vertexOffset = vertexIndex * bindingDesc.stride;
else
    vertexOffset = instanceIndex * bindingDesc.stride;

attribAddress = bufferBindingAddress + vertexOffset + attribDesc.offset;
---------------------------------------------------

[[fxvertex-input-extraction]]
For each attribute, raw data is extracted starting at `attribAddress` and is
converted from the sname:VkVertexInputAttributeDescription's pname:format to
either to floating-point, unsigned integer, or signed integer based on the
base type of the format; the base type of the format must: match the base
type of the input variable in the shader.
If pname:format is a packed format, `attribAddress` must: be a multiple of
the size in bytes of the whole attribute data type as described in
<<features-formats-packed,Packed Formats>>.
Otherwise, `attribAddress` must: be a multiple of the size in bytes of the
component type indicated by pname:format (see <<features-formats,Formats>>).
If the format does not include G, B, or A components, then those are filled
with [eq]#(0,0,1)# as needed (using either 1.0f or integer 1 based on the
format) for attributes that are not 64-bit data types.
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.


[[fxvertex-example]]
== Example

To create a graphics pipeline that uses the following vertex description:

[source,c++]
---------------------------------------------------
struct Vertex
{
    float   x, y, z, w;
    uint8_t u, v;
};
---------------------------------------------------

The application could use the following set of structures:

[source,c++]
---------------------------------------------------
const VkVertexInputBindingDescription binding =
{
    0,                                          // binding
    sizeof(Vertex),                             // stride
    VK_VERTEX_INPUT_RATE_VERTEX                 // inputRate
};

const VkVertexInputAttributeDescription attributes[] =
{
    {
        0,                                      // location
        binding.binding,                        // binding
        VK_FORMAT_R32G32B32A32_SFLOAT,          // format
        0                                       // offset
    },
    {
        1,                                      // location
        binding.binding,                        // binding
        VK_FORMAT_R8G8_UNORM,                   // format
        4 * sizeof(float)                       // offset
    }
};

const VkPipelineVertexInputStateCreateInfo viInfo =
{
    VK_STRUCTURE_TYPE_PIPELINE_VERTEX_INPUT_CREATE_INFO,    // sType
    NULL,                         // pNext
    0,                            // flags
    1,                            // vertexBindingDescriptionCount
    &binding,                     // pVertexBindingDescriptions
    2,                            // vertexAttributeDescriptionCount
    &attributes[0]                // pVertexAttributeDescriptions
};
---------------------------------------------------