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, 28 insertions, 1 deletions

diff --git a/source/blender/draw/intern/draw_manager_exec.c b/source/blender/draw/intern/draw_manager_exec.c
index 033168680b8..78ac96e41b2 100644
--- a/source/blender/draw/intern/draw_manager_exec.c
+++ b/source/blender/draw/intern/draw_manager_exec.c
@@ -102,6 +102,21 @@ void drw_state_set(DRWState state)
 		}
 	}
 
+	/* Raster Discard */
+	{
+		if (CHANGED_ANY(DRW_STATE_WRITE_DEPTH | DRW_STATE_WRITE_COLOR |
+		                DRW_STATE_WRITE_STENCIL | DRW_STATE_WRITE_STENCIL_SHADOW))
+		{
+			if ((state & (DRW_STATE_WRITE_DEPTH | DRW_STATE_WRITE_COLOR |
+			              DRW_STATE_WRITE_STENCIL | DRW_STATE_WRITE_STENCIL_SHADOW)) != 0) {
+				glDisable(GL_RASTERIZER_DISCARD);
+			}
+			else {
+				glEnable(GL_RASTERIZER_DISCARD);
+			}
+		}
+	}
+
 	/* Cull */
 	{
 		DRWState test;
@@ -900,6 +915,7 @@ static void draw_shgroup(DRWShadingGroup *shgroup, DRWState pass_state)
 	int val;
 	float fval;
 	const bool shader_changed = (DST.shader != shgroup->shader);
+	bool use_tfeedback = false;
 
 	if (shader_changed) {
 		if (DST.shader) GPU_shader_unbind();
@@ -907,6 +923,13 @@ static void draw_shgroup(DRWShadingGroup *shgroup, DRWState pass_state)
 		DST.shader = shgroup->shader;
 	}
 
+	if ((pass_state & DRW_STATE_TRANS_FEEDBACK) != 0 &&
+	    (shgroup->type == DRW_SHG_FEEDBACK_TRANSFORM))
+	{
+		use_tfeedback = GPU_shader_transform_feedback_enable(shgroup->shader,
+		                                                     shgroup->tfeedback_target->vbo_id);
+	}
+
 	release_ubo_slots(shader_changed);
 	release_texture_slots(shader_changed);
 
@@ -1023,7 +1046,7 @@ static void draw_shgroup(DRWShadingGroup *shgroup, DRWState pass_state)
 #endif
 
 	/* Rendering Calls */
-	if (!ELEM(shgroup->type, DRW_SHG_NORMAL)) {
+	if (!ELEM(shgroup->type, DRW_SHG_NORMAL, DRW_SHG_FEEDBACK_TRANSFORM)) {
 		/* Replacing multiple calls with only one */
 		if (ELEM(shgroup->type, DRW_SHG_INSTANCE, DRW_SHG_INSTANCE_EXTERNAL)) {
 			if (shgroup->type == DRW_SHG_INSTANCE_EXTERNAL) {
@@ -1104,6 +1127,10 @@ static void draw_shgroup(DRWShadingGroup *shgroup, DRWState pass_state)
 		glFrontFace(DST.frontface);
 	}
 
+	if (use_tfeedback) {
+		GPU_shader_transform_feedback_disable(shgroup->shader);
+	}
+
 	/* TODO: remove, (currently causes alpha issue with sculpt, need to investigate) */
 	DRW_state_reset();
 }