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EthanIowa
01-24-2013, 09:39 AM
Hi,

I do not understand the behavior of ClearBufferData in my OpenGL 4.3 application. Basically, it seems that clearing a buffer with ClearBufferData then writing into it in shaders causes the buffer to always be empty after the shaders execute.

Specifically, my application does the following:

1. ClearBufferData of myBuffer
2. Make myBuffer resident on the GPU and set it as a uniform in shaders
3. a. Enable shaders
3. b. Draw stuff with shaders (each fragment shader writes into myBuffer)
3. c. Disable shaders
4. Make myBuffer non resident

After #4 myBuffer contains all 0's. However, if I do not clear myBuffer (remove #1) then its contents is written by the shaders as I would expect. I've tried putting a Finish() call between the clear and draw, but it has no effect. What is going on here? I'd appreciate any explanation.

Thanks!

Alfonse Reinheart
01-24-2013, 12:21 PM
First, if all you're doing is writing to the buffer, there's no point in clearing it beforehand; you'll just overwrite the cleared data.

Second, I don't see the part where you put a glMemoryBarrierEXT(GL_SHADER_GLOBAL_ACCESS_BARRIER _BIT_NV) between the clear and the part where you write to it. Without that barrier, OpenGL will not synchronize memory accesses. Also, you need a barrier between when you write the data in your shader and when you read it to ensure that it becomes visible.