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, 15 insertions, 1 deletions

diff --git a/source/blender/gpu/GPU_shader.h b/source/blender/gpu/GPU_shader.h
index 2a672873d86..baaa23c2398 100644
--- a/source/blender/gpu/GPU_shader.h
+++ b/source/blender/gpu/GPU_shader.h
@@ -50,6 +50,13 @@ enum {
 	GPU_SHADER_FLAGS_NEW_SHADING        = (1 << 1),
 };
 
+typedef enum GPUShaderTFBType {
+	GPU_SHADER_TFB_NONE         = 0, /* Transform feedback unsupported. */
+	GPU_SHADER_TFB_POINTS       = 1,
+	GPU_SHADER_TFB_LINES        = 2,
+	GPU_SHADER_TFB_TRIANGLES    = 3,
+} GPUShaderTFBType;
+
 GPUShader *GPU_shader_create(
         const char *vertexcode,
         const char *fragcode,
@@ -62,12 +69,19 @@ GPUShader *GPU_shader_create_ex(
         const char *geocode,
         const char *libcode,
         const char *defines,
-        const int flags);
+        const int flags,
+        const GPUShaderTFBType tf_type,
+        const char **tf_names,
+        const int tf_count);
 void GPU_shader_free(GPUShader *shader);
 
 void GPU_shader_bind(GPUShader *shader);
 void GPU_shader_unbind(void);
 
+/* Returns true if transform feedback was succesfully enabled. */
+bool GPU_shader_transform_feedback_enable(GPUShader *shader, unsigned int vbo_id);
+void GPU_shader_transform_feedback_disable(GPUShader *shader);
+
 int GPU_shader_get_program(GPUShader *shader);
 
 void *GPU_shader_get_interface(GPUShader *shader);