VulkanHub

VK_EXT_primitives_generated_query.proposal

This proposal regards layering OpenGL over Vulkan, and provides a convenience query for use by such layers.

Problem Statement

In OpenGL, the GL_PRIMITIVES_GENERATED query can be used independently from whether transform feedback is active or not. There is no direct equivalent in Vulkan.

This extension provides a simple and efficient way to implement this OpenGL query on top of Vulkan.

Solution Space

Emulation Through Other Vulkan Queries

In Vulkan, the second result from the VK_QUERY_TYPE_TRANSFORM_FEEDBACK_STREAM_EXT query produces the same value as OpenGL’s GL_PRIMITIVES_GENERATED query. However, this can only be used when transform feedback is active, and thus is not suitable.

The result of VK_QUERY_PIPELINE_STATISTIC_CLIPPING_INVOCATIONS_BIT from the VK_QUERY_TYPE_PIPELINE_STATISTICS query also produces the same result. Support for this query implies support for a number of other statistics that are not universally available, and so the pipelineStatisticsQuery feature is often not available on Android devices. Furthermore, emulating GL_PRIMITIVES_GENERATED and GL_CLIPPING_INPUT_PRIMITIVES_ARB through the same Vulkan query creates unnecessary complications, given that only one query of each type can be active at a time in Vulkan.

A New Query Type

A new Vulkan query type can be introduced to provide identical results to OpenGL’s GL_PRIMITIVES_GENERATED query. There are a number of limitations to address:

  • Similarly to VK_QUERY_PIPELINE_STATISTIC_CLIPPING_INVOCATIONS_BIT, as stated by the Vulkan spec, if rasterizerDiscardEnable is enabled the query may not produce valid results on some hardware.
  • Some hardware cannot produce a valid value when a non-zero transform feedback stream is used (i.e. VkPipelineRasterizationStateStreamCreateInfoEXT::rasterizationStream is not zero).

This solution is adopted for this problem.

Proposal

A new query type is added, namely VK_QUERY_TYPE_PRIMITIVES_GENERATED_EXT, that behaves identically to the OpenGL GL_PRIMITIVES_GENERATED query.

Features

typedef struct VkPhysicalDevicePrimitivesGeneratedQueryFeaturesEXT {
    VkStructureType    sType;
    void*              pNext;
    VkBool32           primitivesGeneratedQuery;
    VkBool32           primitivesGeneratedQueryWithRasterizerDiscard;
    VkBool32           primitivesGeneratedQueryWithNonZeroStreams;
} VkPhysicalDevicePrimitivesGeneratedQueryFeaturesEXT;
  • primitivesGeneratedQuery specifies if the query is usable.
  • If primitivesGeneratedQueryWithRasterizerDiscard is false, then rasterizer discard (through VkPipelineRasterizationStateCreateInfo::rasterizerDiscardEnable or equivalent dynamic state) must not be enabled.
  • If primitivesGeneratedQueryWithNonZeroStreams is false, the query cannot be used in conjunction with non-zero transform feedback streams.

On hardware where primitivesGeneratedQueryWithRasterizerDiscard is not available, the OpenGL layer can discard the rasterization result by some other means; for example by using an empty scissor.

On hardware where primitivesGeneratedQueryWithNonZeroStreams is not available, the transform feedback query can be used for non-zero streams since transform feedback is necessarily active. This is nonetheless not a concern for OpenGL layers as non-zero transform feedback streams are not supported in OpenGL.