Safe Haskell	Safe-Inferred
Language	Haskell2010

Vulkan.Extensions.VK_KHR_shader_draw_parameters

Description

Name

VK_KHR_shader_draw_parameters - device extension

VK_KHR_shader_draw_parameters

Name String: VK_KHR_shader_draw_parameters

Extension Type

Device extension

Registered Extension Number

64

Revision

1

Ratification Status

Ratified

Extension and Version Dependencies; Deprecation State

Promoted to Vulkan 1.1

Contact

Daniel Koch @dgkoch%0A*Here describe the issue or question you have about the VK_KHR_shader_draw_parameters extension*

Other Extension Metadata

Last Modified Date: 2017-09-05

IP Status

No known IP claims.

Interactions and External Dependencies

This extension requires SPV_KHR_shader_draw_parameters
This extension provides API support for GL_ARB_shader_draw_parameters
Promoted to Vulkan 1.1 Core

Contributors

Daniel Koch, NVIDIA Corporation
Jeff Bolz, NVIDIA
Daniel Rakos, AMD
Jan-Harald Fredriksen, ARM
John Kessenich, Google
Stuart Smith, IMG

Description

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:

BaseInstance, containing the firstInstance parameter passed to drawing commands,
BaseVertex, containing the firstVertex or vertexOffset parameter passed to drawing commands, and
DrawIndex, containing the index of the draw call currently being processed from an indirect drawing 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; → BaseInstance,
in int gl_BaseVertexARB; → BaseVertex, and
in int gl_DrawIDARB; → DrawIndex.

Promotion to Vulkan 1.1

All functionality in this extension is included in core Vulkan 1.1. However, the shaderDrawParameters feature bit was added to distinguish whether it is actually available or not.

New Enum Constants

KHR_SHADER_DRAW_PARAMETERS_EXTENSION_NAME

KHR_SHADER_DRAW_PARAMETERS_SPEC_VERSION

New Built-In Variables

BaseInstance

New SPIR-V Capabilities

DrawParameters

Issues

1) Is this the same functionality as GL_ARB_shader_draw_parameters?

RESOLVED: It is actually a superset, as it also adds in support for arrayed drawing commands.

In GL for GL_ARB_shader_draw_parameters, 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 baseVertex parameter, the value of gl_BaseVertexARB is zero. This means that gl_BaseVertexARB = baseVertex (for glDrawElements commands with baseVertex) or 0. In particular there are no glDrawArrays commands that take a baseVertex parameter.

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