A deferred rendering pipeline provides great opportunities to run post processing shaders on the results from your G-buffer (A G-buffer is the target for the first pass in a deferred renderer, usually consisting of colour, normal, depth ± material textures, which stores the information needed for subsequent lighting and effects passes). There are a couple of issues, however:
1) OpenGL can’t both read from and write to a texture at the same time; attempting to do so will give an undefined result (i.e. per OpenGL tradition it will look like the result you wanted, except when it doesn’t)
2) What, therefore, do you do about transparency in a deferred renderer? You need the information about what’s behind the transparent object, and how far away it is. That’s in your G-buffer, which you’re already writing to (I’m assuming here that you’re rendering transparent materials last, which is really the only sane way to do it).
What you’re going to need is a copy of a subset of your G-buffer to provide the information you need to draw the stuff behind your transparent material. There’s an expensive way and a cheap way to do this; the expensive way is to do all of your deferred lighting before you render any transparent materials (which does make life easier in some ways, but means you need to do two lighting passes, one for opaque and one for transparent materials), the cheap way is to decide that you’re not going to bother to light the stuff behind the transparency because you’re already going to be throwing a bunch of refraction effects on top anyway and all you really need is a bit of detail to sell the effect.
Here’s where I tripped myself up, in the usual manner for a novice learning OpenGL from 10-year-old tutorials on the internet; I naïvely thought that the most logical thing to do at this point would be to blit (i.e. copy the pixels directly) from my G-buffer to another set of textures which I would then use as the source for rendering transparency. Duplicating a chunk of memory seemed like it was going to be a much faster operation than actually drawing anything. Because I’m not a total idiot, I did at least avoid using glCopyTexImage2D and went straight for the faster glCopyTexSubImage2D operation instead. The typical way you would use this is:
//bind the framebuffer you’re going to read from glBindFrameBuffer(GL_FRAMEBUFFER, myGBuffer); glViewport(0.0, 0.0, my_gBuffer_width, my_gBuffer_height); //specify which framebuffer attachment you’re going to read glReadBuffer(GL_COLOR_ATTACHMENT0); //or whichever //bind the texture you’re going to copy to glActiveTexture(GL_TEXTURE0); glBindTexture(GL_TEXTURE_2D, myDuplicateTexture); //do the copy operation glCopyTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, 0, 0, my_gBuffer_width, my_gBuffer_height);
The problem with this is, the performance is terrible, especially if you’re copying across a PCI bus. I was copying a colour and a depth texture for a water effect, and a quick root around in the driver monitor led to the discovery that the GL was spending at least 70% of its time on those two copy operations alone. I’d imagine that this is likely a combination of copying into system memory for some reason and stalling the pipeline; to make matters worse, in this sort of situation you can only really copy the textures immediately before you need to use them, so unless you’re going to rig up some complicated double-buffer solution copying the last frame, asynchronous pixel buffer transfers aren’t going to save you. Using driver hints to keep the texture data in GPU memory might work, or it might not.
Here’s one solution:
Continue reading How I Learned to Stop Blitting and Love the Framebuffer