GPUShader/DRW: Add Transform Feedback support.

This is a usefull feature that can be used to do a lot of precomputation on
the GPU instead of the CPU.

Implementation is simple and only covers the most usefull case.

How to use:
- Create shader with transform feedback.
- Create a pass with DRW_STATE_TRANS_FEEDBACK.
- Create a target Gwn_VertBuf (make sure it's big enough).
- Create a shading group with DRW_shgroup_transform_feedback_create().
- Add your draw calls to the shading group.
- Render your pass normaly.

Current limitation:
- Only one output buffer.
- Cannot pause/resume tfb rendering to interleave with normal drawcalls.
- Cannot get the number of verts drawn.

1 files changed, 5 insertions, 0 deletions

diff --git a/source/blender/draw/intern/draw_manager.h b/source/blender/draw/intern/draw_manager.h
index e71da41f118..0f9a68552fe 100644
--- a/source/blender/draw/intern/draw_manager.h
+++ b/source/blender/draw/intern/draw_manager.h
@@ -193,6 +193,7 @@ typedef enum {
 	DRW_SHG_TRIANGLE_BATCH,
 	DRW_SHG_INSTANCE,
 	DRW_SHG_INSTANCE_EXTERNAL,
+	DRW_SHG_FEEDBACK_TRANSFORM,
 } DRWShadingGroupType;
 
 struct DRWShadingGroup {
@@ -206,6 +207,10 @@ struct DRWShadingGroup {
 		struct { /* DRW_SHG_NORMAL */
 			DRWCall *first, *last; /* Linked list of DRWCall or DRWCallDynamic depending of type */
 		} calls;
+		struct { /* DRW_SHG_FEEDBACK_TRANSFORM */
+			DRWCall *first, *last; /* Linked list of DRWCall or DRWCallDynamic depending of type */
+			struct Gwn_VertBuf *tfeedback_target; /* Transform Feedback target. */
+		};
 		struct { /* DRW_SHG_***_BATCH */
 			struct Gwn_Batch *batch_geom;     /* Result of call batching */
 			struct Gwn_VertBuf *batch_vbo;