Contact
-
Piers Daniell pdaniell-nv
Other Extension Metadata
- Last Modified Date
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2018-10-09
- Contributors
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Baldur Karlsson, Valve
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Boris Zanin, Mobica
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Daniel Rakos, AMD
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Donald Scorgie, Imagination
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Henri Verbeet, CodeWeavers
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Jan-Harald Fredriksen, Arm
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Faith Ekstrand, Intel
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Jeff Bolz, NVIDIA
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Jesse Barker, Unity
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Jesse Hall, Google
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Pierre-Loup Griffais, Valve
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Philip Rebohle, DXVK
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Ruihao Zhang, Qualcomm
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Samuel Pitoiset, Valve
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Slawomir Grajewski, Intel
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Stu Smith, Imagination Technologies
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Description
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.
New Enum Constants
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VK_EXT_TRANSFORM_FEEDBACK_EXTENSION_NAME
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VK_EXT_TRANSFORM_FEEDBACK_SPEC_VERSION
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Extending VkAccessFlagBits:
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VK_ACCESS_TRANSFORM_FEEDBACK_COUNTER_READ_BIT_EXT
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VK_ACCESS_TRANSFORM_FEEDBACK_COUNTER_WRITE_BIT_EXT
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VK_ACCESS_TRANSFORM_FEEDBACK_WRITE_BIT_EXT
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Extending VkBufferUsageFlagBits:
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VK_BUFFER_USAGE_TRANSFORM_FEEDBACK_BUFFER_BIT_EXT
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VK_BUFFER_USAGE_TRANSFORM_FEEDBACK_COUNTER_BUFFER_BIT_EXT
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Extending VkPipelineStageFlagBits:
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VK_PIPELINE_STAGE_TRANSFORM_FEEDBACK_BIT_EXT
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Extending VkQueryType:
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VK_QUERY_TYPE_TRANSFORM_FEEDBACK_STREAM_EXT
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Extending VkStructureType:
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VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_TRANSFORM_FEEDBACK_FEATURES_EXT
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VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_TRANSFORM_FEEDBACK_PROPERTIES_EXT
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VK_STRUCTURE_TYPE_PIPELINE_RASTERIZATION_STATE_STREAM_CREATE_INFO_EXT
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Issues
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.
See Also
VkPhysicalDeviceTransformFeedbackFeaturesEXT, VkPhysicalDeviceTransformFeedbackPropertiesEXT, VkPipelineRasterizationStateStreamCreateFlagsEXT, VkPipelineRasterizationStateStreamCreateInfoEXT, vkCmdBeginQueryIndexedEXT, vkCmdBeginTransformFeedbackEXT, vkCmdBindTransformFeedbackBuffersEXT, vkCmdDrawIndirectByteCountEXT, vkCmdEndQueryIndexedEXT, vkCmdEndTransformFeedbackEXT
Document Notes
For more information, see the Vulkan Specification
This page is a generated document. Fixes and changes should be made to the generator scripts, not directly.