vkCmdBeginTransformFeedbackEXT

Transform feedback for specific transform feedback buffers is made active by calling:

void vkCmdBeginTransformFeedbackEXT(
    VkCommandBuffer commandBuffer,
    uint32_t firstCounterBuffer,
    uint32_t counterBufferCount,
    const VkBuffer* pCounterBuffers,
    const VkDeviceSize* pCounterBufferOffsets);

• commandBuffer is the command buffer into which the command is recorded.
• firstCounterBuffer is the index of the first transform feedback buffer corresponding to pCounterBuffers[0] and pCounterBufferOffsets[0].
• counterBufferCount is the size of the pCounterBuffers and pCounterBufferOffsets arrays.
• pCounterBuffers is NULL or a pointer to an array of VkBuffer handles to counter buffers. Each buffer contains a 4 byte integer value representing the byte offset from the start of the corresponding transform feedback buffer from where to start capturing vertex data. If the byte offset stored to the counter buffer location was done using vkCmdEndTransformFeedbackEXT it can be used to resume transform feedback from the previous location. In that case, a pipeline barrier is required between the calls to vkCmdEndTransformFeedbackEXT and vkCmdBeginTransformFeedbackEXT, with VK_PIPELINE_STAGE_TRANSFORM_FEEDBACK_BIT_EXT as the source and destination stages, VK_ACCESS_TRANSFORM_FEEDBACK_COUNTER_WRITE_BIT_EXT as the source access and VK_ACCESS_TRANSFORM_FEEDBACK_COUNTER_READ_BIT_EXT as the destination access. If pCounterBuffers is NULL, then transform feedback will start capturing vertex data to byte offset zero in all bound transform feedback buffers. For each element of pCounterBuffers that is VK_NULL_HANDLE, transform feedback will start capturing vertex data to byte zero in the corresponding bound transform feedback buffer.
• pCounterBufferOffsets is NULL or a pointer to an array of VkDeviceSize values specifying offsets within each of the pCounterBuffers where the counter values were previously written. The location in each counter buffer at these offsets must be large enough to contain 4 bytes of data. This data is the number of bytes captured by the previous transform feedback to this buffer. If pCounterBufferOffsets is NULL, then it is assumed the offsets are zero.

The active transform feedback buffers will capture primitives emitted from the corresponding XfbBuffer in the bound graphics pipeline. Any XfbBuffer emitted that does not output to an active transform feedback buffer will not be captured.