DrawTransformFeedback() needs to obtain the number of vertices written
to a particular stream during the last Begin/EndTransformFeedback block.
The new driver hook returns exactly that information.
Gallium drivers already implement this functionality by passing the
transform feedback object to the drawing function. I prefer to avoid
this for two reasons:
1. Complexity:
Normally, the drawing function takes an array of _mesa_prim objects,
each of which specifies a vertex count. If tfb_vertcount != NULL,
however, there will only be one _mesa_prim object with an invalid
vertex count (of 1), so it needs to be ignored.
Since the _mesa_prim pointers are const, you can't even override it to
the proper value; you need to pass around extra "ignore that, here's
the real count" parameters.
The drawing function is already terribly complicated, so I don't want to
make it even more complicated.
2. Primitive restart:
vbo_draw_arrays() performs software primitive restart, splitting a draw
call in two when necessary. vbo_draw_transform_feedback() currently
doesn't because it has no idea how many vertices need to be drawn. The
new driver hook gives it that information, allowing us to reuse the
existing vbo_draw_arrays() code to do everything right.
Signed-off-by: Kenneth Graunke <kenn...@whitecape.org>
---
src/mesa/main/dd.h | 8 ++++++++
src/mesa/vbo/vbo_exec_array.c | 6 ++++++
2 files changed, 14 insertions(+)
diff --git a/src/mesa/main/dd.h b/src/mesa/main/dd.h
index 29469ce..11d5a9e 100644
--- a/src/mesa/main/dd.h
+++ b/src/mesa/main/dd.h
@@ -843,6 +843,14 @@ struct dd_function_table {
struct gl_transform_feedback_object *obj);
/**
+ * Return the number of vertices written to a stream during the last
+ * Begin/EndTransformFeedback block.
+ */
+ GLsizei (*GetTransformFeedbackVertexCount)(struct gl_context *ctx,
+ struct
gl_transform_feedback_object *obj,
+ GLuint stream);
+
+ /**
* \name GL_NV_texture_barrier interface
*/
void (*TextureBarrier)(struct gl_context *ctx);
diff --git a/src/mesa/vbo/vbo_exec_array.c b/src/mesa/vbo/vbo_exec_array.c
index 1670409..11bb76a 100644
--- a/src/mesa/vbo/vbo_exec_array.c
+++ b/src/mesa/vbo/vbo_exec_array.c
@@ -1464,6 +1464,12 @@ vbo_draw_transform_feedback(struct gl_context *ctx,
GLenum mode,
return;
}
+ if (ctx->Driver.GetTransformFeedbackVertexCount) {
+ GLsizei n = ctx->Driver.GetTransformFeedbackVertexCount(ctx, obj,
stream);
+ vbo_draw_arrays(ctx, mode, 0, n, numInstances, 0);
+ return;
+ }
+
vbo_bind_arrays(ctx);
/* init most fields to zero */
--
1.8.3.2
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