// Copyright (c) 2016-2018 Khronos Group. This work is licensed under a
// Creative Commons Attribution 4.0 International License; see
// http://creativecommons.org/licenses/by/4.0/

include::meta/VK_KHR_shader_draw_parameters.txt[]

*Last Modified Date*::
    2016-10-05
*IP Status*::
    No known IP claims.
*Interactions and External Dependencies*::
  - Requires the
    https://www.khronos.org/registry/spir-v/extensions/KHR/SPV_KHR_shader_draw_parameters.html[+SPV_KHR_shader_draw_parameters+]
    SPIR-V extension.
  - Requires
    https://www.khronos.org/registry/OpenGL/extensions/ARB/ARB_shader_draw_parameters.txt[+GL_ARB_shader_draw_parameters+]
    for GLSL source languages.
*Contributors*::
  - Daniel Koch, NVIDIA Corporation
  - Jeff Bolz, NVIDIA
  - Daniel Rakos, AMD
  - Jan-Harald Fredriksen, ARM
  - John Kessenich, Google
  - Stuart Smith, IMG

This extension adds support for the following SPIR-V extension in Vulkan:

  * +SPV_KHR_shader_draw_parameters+

The extension provides access to three additional built-in shader variables
in Vulkan:

  * code:BaseInstance, which contains the pname:firstInstance parameter
    passed to draw commands,
  * code:BaseVertex, which contains the pname:firstVertex/pname:vertexOffset
    parameter passed to draw commands, and
  * code:DrawIndex, which contains the index of the draw call currently
    being processed from an indirect draw call.

When using GLSL source-based shader languages, the following variables from
+GL_ARB_shader_draw_parameters+ can map to these SPIR-V built-in
decorations:

  * `in int gl_BaseInstanceARB;` -> code:BaseInstance,
  * `in int gl_BaseVertexARB;` -> code:BaseVertex, and
  * `in int gl_DrawIDARB;` -> code:DrawIndex.

=== New Object Types

None.

=== New Enum Constants

None.

=== New Enums

None.

=== New Structures

None.

=== New Functions

None.

=== New Built-In Variables

  * <<interfaces-builtin-variables-baseinstance,code:BaseInstance>>
  * <<interfaces-builtin-variables-basevertex,code:BaseVertex>>
  * <<interfaces-builtin-variables-drawindex,code:DrawIndex>>

=== New SPIR-V Capabilities

  * <<spirvenv-capabilities-table-drawparameters,DrawParameters>>

=== Issues

1) Is this the same functionality as +GL_ARB_shader_draw_parameters+?

*RESOLVED*: It's actually a superset as it also adds in support for arrayed
drawing commands.

In GL for +GL_ARB_shader_draw_parameters+, code:gl_BaseVertexARB holds the
integer value passed to the parameter to the command that resulted in the
current shader invocation.
In the case where the command has no code:baseVertex parameter, the value of
code:gl_BaseVertexARB is zero.
This means that code:gl_BaseVertexARB = code:baseVertex (for
code:glDrawElements commands with code:baseVertex) or 0.
In particular there are no code:glDrawArrays commands that take a
code:baseVertex parameter.

Now in Vulkan, we have code:BaseVertex = pname:vertexOffset (for indexed
drawing commands) or pname:firstVertex (for arrayed drawing commands), and
so Vulkan's version is really a superset of GL functionality.

=== Version History

  * Revision 1, 2016-10-05 (Daniel Koch)
    - Internal revisions