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mrbig

OpenGL Real-Time Rendered Objects in Pre-Rendered Scenes?

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Hi! As the title says, I want to use OpenGL to render objects in real-time, and have them correctly interact with a pre-rendered scene, for now only visually. By that I mean, to at least give the resulting hybrid scene a correct sense of depth, that is, to have the real-time rendered objects drawn behind or in front of the pre-rendered objects correctly. I think a simple way of achieving this effect would be to render the pre-rendered scene (with OpenGL, of course) and keep its Z-buffer, then, when drawing the hybrid scene, to initialize the Z-buffer to that of the pre-rendered scene, and only then to draw the real-time objects. (The "camera settings" for both scenes would have to be the same, of course.) There's just one problem - I have no idea how to do that using OpenGL. That is, I don't really know what extensions I'm going to need or how to use them. How do I send the Z-buffer content from the GPU to the computer RAM so I could save it in a file? (Now that I think of it, I don't know how to do that for the color buffer either!) How do I initialize the Z-buffer of an OpenGL scene to whatever I want? Any help will be appreciated! Thanks.

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Sounds like you want nailboards, but I have no idea how you actually implement that in OpenGL.

Alternatively, you could just create a simplified version of the geometry and render that to the zbuffer.

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Look up glDepthMask, GL_DEPTH_TEST, glColorMask.

These will help you do what you want! :)

Basically,

Disable colorwrites, enable depthwrites
Render 3d "collision" mesh
Enable colorwrites, disable depthwrites
render 2d background
enable depthwrites
render characters etc.

Cheers!

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What about frame buffer objects?
Is there a way to copy one renderbuffer into another?
If both renderbuffers are in the GPU RAM, it should be possible to do that quickly... or am I wrong?
Maybe I could copy the z-buffer of the pre-rendered scene into a renderbuffer using 'glWritePixels', then each frame copy that framebuffer into another framebuffer, which could then be used as the depth buffer for the hybrid scene?

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No need for FBOs; the functionality required has been in GL from the dawn of time :)

glDrawPixels and glReadPixels both accept GL_DEPTH_COMPONENT as their format parameter, instructing them to read to and write from the depth buffer, respectively.

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Yes, but using 'glDrawPixels' would require the application to send the z-buffer to the GPU every frame, which should be slow in high resolutions.
I thought that copying data which is already in the GPU RAM should be a lot faster, if it's possible.
Of course using frame buffer objects means losing compatibility with older hardware, but I doubt my application could run with a reasonable frame-rate on such hardware with all the 'glDrawPixels' stuff.

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That's true, plus the GL 2D pathway pixel transfer functions tend to be slow in today's consumer-level cards, but the read part shouldn't be a problem as it's only done once. To speed up writing, you can just draw a textured screen-sized quad using a simple pixel shader that writes texel values into the depth buffer.

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I'm not sure if I'm getting you right...
You mean I should make a texture from the pre-rendered z-buffer, access it from inside a fragment shader and use it to modify the depth buffer values?
I guess this means that I would have to encode the z-buffer to make it fit in the ordinary 32-bit texture format. It shouldn't be hard, but it sounds somewhat inconvenient... Isn't there another way? (Preferably one that avoids using shaders...)

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No need to modify anything; depth components in textures have been permitted since OpenGL 1.4. See the docs for the format and internalFormat parameters. I don't see why you wouldn't want to use shaders; they are after all more widely supported than FBOs. There are a couple recent extensions that permit quite free copying of data between different buffers (for example, straight from a FBO to the depth buffer), but these are even less widely supported than FBOs.

EDIT: Fixed the link

[Edited by - Sharlin on July 13, 2007 11:49:01 AM]

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