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kRogue

OpenGL FBO - question

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As I read through the FBO spec and Gamedev's tutorial, I did not see how to do this with FBO: Draw to Multiple Color Buffers but use the "default" depth and or stencil buffers, i.e. I want the render to texture only the color stuff, but I want the depth and stencil values to be the "usual" openGL depth and stencil buffers... to do this, do I just not attach a depth and stencil render object, or do I have to do something to make sure that "the standard openGL" depth and stencil buffers are the one drawn to, or is this not possible?

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You can't do that. You still need to attach a depth/stencil buffer for the FBO. Using the framebuffers depth/stencil is not possible.

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I started to figure that since if it was possible to use the stencil and framebuffer from the window system, then the FBO would have to be the same resolution as the window you asked for, not to mention all the mess associated to "pixel ownership test", since it always is supposed to pass for FBO, but the windowing system decides in the case of non-FBO, so atleast it makes sense... sigh...

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Some of us requested this while the spec was being written and the ARB was requesting feedback. If the framebuffer is large and your FBO is small, why not use the framebuffer depth-stencil?

So, it's just simpler to allocate some depth/stencil for the FBO and perhaps they will make a new extension for being able to do that.... case closed :)

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It would be nice if there was an extension to the FBO spec where one can specify a sub region of the framebuffer to be used for rendering, but this can run badly afoul with the pixel ownership test which happens before anything (the windowing system decides this test), so the danger is that if you used a portion of the framebuffer that was obscured, then the results at that portion are undefined, which would be a bad thing for the whole "render to texture" mentality (this is the main reason why glCopy is not good enough).. though that in itself for color buffers is not as a big deal, but for stencil and depth buffers... it can create very strange hard to track bugs in windowed GL apps.... oh well... no real big deal at the end of the day...except the possibly wasted memory: one depth and stencil buffer per different resolution of FBO's (because a fixed depth or stencil buffer can be shared between FBO's)...

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