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Gumgo

OpenGL Can't seem to render to FBO

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I've gotten render-to-texture working multiple times in the past fairly easily but for some reason I can't seem to get it working now. I'm attempting to simply clear a texture attached to a framebuffer to the color green and then blit it to the screen (which has been cleared to red). Here is my code (besides window creation and glew, these are the only OpenGL calls in the entire program):

(note: I actually reconstructed this code from a glIntercept log since I don't have the code on me at the moment, but the actual code is essentially identical)
[source]

// generate framebuffer
GLuint fbo;
glGenFramebuffers(1, &fbo);
// bind it
glBindFramebuffer(GL_FRAMEBUFFER, fbo);

// generate a texture
GLuint tex;
glGenTextures(1, &tex);
glBindTexture(GL_TEXTURE_2D, tex);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, 512, 512, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);
glBindTexture(GL_TEXTURE_2D, 0);

// attach the texture to the framebuffer
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, tex, 0);
// check the framebuffer's status
Log::info() << (glCheckFramebufferStatus(GL_FRAMEBUFFER) == GL_FRAMEBUFFER_COMPLETE);

// bind the frame buffer for drawing
glBindFramebuffer(GL_FRAMEBUFFER, fbo);
// draw to color attachment 0
GLuint buf = GL_COLOR_ATTACHMENT0;
glDrawBuffers(1, &buf);
// set the drawing viewport to the size of the texture
glViewport(0,0,512,512);
// clear the framebuffer to green
glClearColor(0.0f, 1.0f, 0.0f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT);

// bind the default framebuffer for drawing
glBindFramebuffer(GL_FRAMEBUFFER, 0);
// bind our framebuffer for reading
glBindFramebuffer(GL_READ_FRAMEBUFFER, fbo);
// read from color attachment 0
glReadBuffer(GL_COLOR_ATTACHMENT0);
// draw to the back buffer
glDrawBuffer(GL_BACK);
// set the viewport to the window size
glViewport(0,0,800,600);
// clear the back buffer to red
glClearColor(1.000000,0.000000,0.000000,1.000000);
glClear(GL_COLOR_BUFFER_BIT);
// blit our framebuffer to the screen
glBlitFramebuffer(0, 0, 512, 512, 0, 0, 512, 512, GL_COLOR_BUFFER_BIT, GL_NEAREST);

// clean up resources
glBindFramebuffer(GL_READ_FRAMEBUFFER,0);
glDeleteTextures(1, &tex);
glDeleteFramebuffers(1, &fbo);
[/source]
Obviously I wouldn't actually want to create and destroy a texture/framebuffer each frame but that's just for testing purposes. The desired result would be a green portion of the screen (the framebuffer) drawn over a red portion of the screen (the backbuffer). However, instead of green, the blitted framebuffer is appearing black, which means it hasn't been cleared. Here is what I have so far determined:

- There are no OpenGL errors being generated - I have checked glGetError() after every call, and also used glIntercept, and no errors are being detected.
- I'm getting GL_FRAMEBUFFER_COMPLETE from glCheckFramebufferStatus().
- If I initialize the texture with some data (e.g. a solid color), that color appears when blitting, indicating that the texture is being set up correctly (at least for being displayed) and the blitting works
- Since blitting works, this means that I am successfully: binding the FBO to GL_READ_FRAMEBUFFER, binding the texture to GL_COLOR_ATTACHMENT0, setting the read buffer to GL_COLOR_ATTACHMENT0, and setting the draw buffer to GL_BACK.
- If I [i]manually[/i] update the texture using glTexSubImage2D() each frame, I can see the results of that when blitting.
- I'm successfully clearing the [i]backbuffer[/i] to red - this means that glClear() is working properly.

So the problem must be with either: binding the FBO for drawing, attaching the texture for drawing, or setting the draw buffers (glDrawBuffers()).

Can anyone see anything I'm doing wrong?

EDIT: or even let me know if there's nothing wrong with it, as then I could be more sure that the problem lies elsewhere (though I'm not sure how that could be since these are the only OpenGL calls).

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I'm sorry to bump, but I've literally been trying to solve this for weeks (something as simple as rendering to an FBO? really?) with no luck whatsoever. I've isolated this code in a separate project to ensure no other code was interfering but that didn't help. I also tried copying other FBO tutorials and those don't work either! However, the difference is that I'm using glBlitFramebuffer, whereas the other tutorials are just rendering quads. (I haven't tried quads yet but I'd prefer blitting as it seems much cleaner.)

Now here's something interesting. If I insert a glReadPixels call after clearing the FBO [i]while the FBO is still bound[/i] it works! What this suggests is that the framebuffer blit operation doesn't wait until previous GL calls have finished - it is immediately called before glClear() has actually been executed on the GPU. However, inserting glFinish() and glFlush() after clearing does [i]not[/i] work - I don't see the result of the clear operation. What could this indicate? Does anyone have any information on glBlitFramebuffer that might be related?

EDIT: Hmm... inserting Sleep(1000) in place of glReadPixels also does not work. So it seems that the problem isn't that glBlitFramebuffer is being called too [i]soon.[/i] It seems that the glClear is only actually executed if I call glReadPixels on the buffer. This has me baffled... I'll mess around some more with it but if anyone has any suggestions it would be really helpful.

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