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.

3 files changed, 53 insertions, 3 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);
diff --git a/source/blender/gpu/intern/gpu_shader.c b/source/blender/gpu/intern/gpu_shader.c
index 02baa2e58cb..2068c5a6a75 100644
--- a/source/blender/gpu/intern/gpu_shader.c
+++ b/source/blender/gpu/intern/gpu_shader.c
@@ -268,7 +268,10 @@ GPUShader *GPU_shader_create(const char *vertexcode,
 	                            geocode,
 	                            libcode,
 	                            defines,
-	                            GPU_SHADER_FLAGS_NONE);
+	                            GPU_SHADER_FLAGS_NONE,
+	                            GPU_SHADER_TFB_NONE,
+	                            NULL,
+	                            0);
 }
 
 #define DEBUG_SHADER_NONE ""
@@ -323,7 +326,10 @@ GPUShader *GPU_shader_create_ex(const char *vertexcode,
                                 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)
 {
 #ifdef WITH_OPENSUBDIV
 	bool use_opensubdiv = (flags & GPU_SHADER_FLAGS_SPECIAL_OPENSUBDIV) != 0;
@@ -469,6 +475,13 @@ GPUShader *GPU_shader_create_ex(const char *vertexcode,
 	}
 #endif
 
+	if (tf_names != NULL) {
+		glTransformFeedbackVaryings(shader->program, tf_count, tf_names, GL_INTERLEAVED_ATTRIBS);
+		/* Primitive type must be setup */
+		BLI_assert(tf_type != GPU_SHADER_TFB_NONE);
+		shader->feedback_transform_type = tf_type;
+	}
+
 	glLinkProgram(shader->program);
 	glGetProgramiv(shader->program, GL_LINK_STATUS, &status);
 	if (!status) {
@@ -527,6 +540,27 @@ void GPU_shader_unbind(void)
 	glUseProgram(0);
 }
 
+bool GPU_shader_transform_feedback_enable(GPUShader *shader, unsigned int vbo_id)
+{
+	if (shader->feedback_transform_type == GPU_SHADER_TFB_NONE) {
+		return false;
+	}
+
+	glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 0, vbo_id);
+
+	switch (shader->feedback_transform_type) {
+		case GPU_SHADER_TFB_POINTS:    glBeginTransformFeedback(GL_POINTS);    return true;
+		case GPU_SHADER_TFB_LINES:     glBeginTransformFeedback(GL_LINES);     return true;
+		case GPU_SHADER_TFB_TRIANGLES: glBeginTransformFeedback(GL_TRIANGLES); return true;
+		default: return false;
+	}
+}
+
+void GPU_shader_transform_feedback_disable(GPUShader *UNUSED(shader))
+{
+	glEndTransformFeedback();
+}
+
 void GPU_shader_free(GPUShader *shader)
 {
 	BLI_assert(shader);
diff --git a/source/blender/gpu/intern/gpu_shader_private.h b/source/blender/gpu/intern/gpu_shader_private.h
index 67d8c6e6213..d8ec6b5d6d1 100644
--- a/source/blender/gpu/intern/gpu_shader_private.h
+++ b/source/blender/gpu/intern/gpu_shader_private.h
@@ -36,6 +36,8 @@ struct GPUShader {
 	GLuint fragment; /* handle for fragment shader */
 
 	Gwn_ShaderInterface *interface; /* cached uniform & attrib interface for shader */
+
+	int feedback_transform_type;
 };
 
 #endif  /* __GPU_SHADER_PRIVATE_H__ */