Transform feedback question
I try to use feedback buffer to transform vertices that would be sent afterwards for rendering. I want to render points in the feedback buffer than get the transformed vertices back and render triangles with an empty vertex shader.
The reason for this is that I have a pretty intensive vertex shader and a lot of triangles. My GPU seems to have a very poor caching system, as I did the following test: sent a triangle STRIP indexed primitive buffer with 4225 vertices, 8448 indexes (in those 4225 vertices) and noticed 24000+ calls to the vertex shader ( about 2.88 invocations per triangle ). This is not my current geometry, but only a test one.
I intend to send the vertices as points to the transform feedback, invoke the vertex shader and store the outputs in the feedback buffer. Then, set the output feedback buffer to my triangle list renderer with an empty vertex shader just for primitive assembly.
For this to work, I need the vertices, sent to transform feedback via
glDrawArrays(GL_POINTS, nFirst, nCount);
to write the transformed vertexes exactly in the same order they where received.
Or, in other words, if I'd write the gl_VertexID in the output buffer and send GL_POINTS to the transform feedback, when examining the output buffer, I would see a consecutive sequence of 0, 1, 2, ... nCount - 1
I want to know whether the OpenGL standard specifically guarantees the order written to the feedback buffer as being exactly the order in which the vertexes are sent when drawing GL_POINTS in feedback mode.
"The attributes of the first vertex received after
BeginTransformFeedback are written at the starting offsets of the bound
buffer objects set by BindBufferRange, and subsequent vertex attributes
are appended to the buffer object. "
This was confusing. I need an explicit confirmation for my problem.