VK_EXT_transform_feedback(3) Manual Page

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This extension adds transform feedback to the Vulkan API by exposing the SPIR-V TransformFeedback and GeometryStreams capabilities to capture vertex, tessellation or geometry shader outputs to one or more buffers. It adds API functionality to bind transform feedback buffers to capture the primitives emitted by the graphics pipeline from SPIR-V outputs decorated for transform feedback. The transform feedback capture can be paused and resumed by way of storing and retrieving a byte counter. The captured data can be drawn again where the vertex count is derived from the byte counter without CPU intervention. If the implementation is capable, a vertex stream other than zero can be rasterized.

All these features are designed to match the full capabilities of OpenGL core transform feedback functionality and beyond. Many of the features are optional to allow base OpenGL ES GPUs to also implement this extension.

The primary purpose of the functionality exposed by this extension is to support translation layers from other 3D APIs. This functionality is not considered forward looking, and is not expected to be promoted to a KHR extension or to core Vulkan. Unless this is needed for translation, it is recommended that developers use alternative techniques of using the GPU to process and capture vertex data.

1) Should we include pause/resume functionality?

RESOLVED: Yes, this is needed to ease layering other APIs which have this functionality. To pause use vkCmdEndTransformFeedbackEXT and provide valid buffer handles in the pCounterBuffers array and offsets in the pCounterBufferOffsets array for the implementation to save the resume points. Then to resume use vkCmdBeginTransformFeedbackEXT with the previous pCounterBuffers and pCounterBufferOffsets values. Between the pause and resume there needs to be a memory barrier for the counter buffers with a source access of VK_ACCESS_TRANSFORM_FEEDBACK_COUNTER_WRITE_BIT_EXT at pipeline stage VK_PIPELINE_STAGE_TRANSFORM_FEEDBACK_BIT_EXT to a destination access of VK_ACCESS_TRANSFORM_FEEDBACK_COUNTER_READ_BIT_EXT at pipeline stage VK_PIPELINE_STAGE_TRANSFORM_FEEDBACK_BIT_EXT.

2) How does this interact with multiview?

RESOLVED: Transform feedback cannot be made active in a render pass with multiview enabled.

3) How should queries be done?

RESOLVED: There is a new query type VK_QUERY_TYPE_TRANSFORM_FEEDBACK_STREAM_EXT. A query pool created with this type will capture 2 integers - numPrimitivesWritten and numPrimitivesNeeded - for the specified vertex stream output from the last pre-rasterization shader stage. The vertex stream output queried is zero by default, but can be specified with the new vkCmdBeginQueryIndexedEXT and vkCmdEndQueryIndexedEXT commands.

VkPhysicalDeviceTransformFeedbackFeaturesEXT, VkPhysicalDeviceTransformFeedbackPropertiesEXT, VkPipelineRasterizationStateStreamCreateFlagsEXT, VkPipelineRasterizationStateStreamCreateInfoEXT, vkCmdBeginQueryIndexedEXT, vkCmdBeginTransformFeedbackEXT, vkCmdBindTransformFeedbackBuffersEXT, vkCmdDrawIndirectByteCountEXT, vkCmdEndQueryIndexedEXT, vkCmdEndTransformFeedbackEXT

For more information, see the Vulkan Specification

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