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OpenGL Drawing pixel's depth instead of colour

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Hi To begin with - I'm working on a movie about computer game and I have everything to "postprocess" some realistic and precise camera-like effects excluding depth of each pixel. Unfortunately I have a little knowledge of opengl but I understand in general how does pc, os and libraries like opengl work (used to write in assembler) so you can know that probably I'll understand your explanations :) . My problem is that I need an opengl hook (or other method, I don't have source code) that will make the game I'm working on (jedi knight 2 on q3 engine) to alter each pixel's 24 bit colour with it's depth. Accuracy is very important to me. I've heard that opengl uses non-linear depth buffer but maps that I use aren't very big. However if it's going to be non-linear precision then I need to know how to convert it to real depth. I would be very grateful if someone could write such program for me or help me solving this problem in any other way :) Regards and best Christmas wishes Fau [Edited by - fau on December 20, 2008 6:14:49 AM]

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Here are pointers:
- make and draw on a RGBA32f (4 floats per pixel) FBO (Frame-buffer-object). FBOs are like drawing in a texture, not the limited back-buffer
- use shaders
- put "gl_FragColor = vec(gl_FragCoord.z);" or "gl_FragColor = vec(1.0/gl_FragCoord.w);"
- maybe glReadPixels(...,GL_RGBA,GL_FLOAT);
- done

If you simply copy that FBO's texture onto the backbuffer, to see the color(depth), you'll discard any HDR info and make the RGBA32f rendering useless.

The nonlinearity of the depth-buffer comes only from that 1/depth. (this makes near-to-the-camera pixels get much more precision).

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Also, since you're using quake3's engine, you can compile your shader with nVidia's Cg's cgc.exe into ARB asm, and follow tutorials on quake-net or something about how to apply that shader on everything. (I've seen there's a tute like that there).

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I understand your concept but as I said getting to know how to write it by myself would take me days. I can compile it and make minor changes if needed but could you possibly write it for me? Also I'd like to ask you for link to site you've mentioned since quake-net is rather common sentence.

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Just re-read your first post, mentioning "q3" - which I initially understood as Doom3. This is a problem, Quake3 doesn't use GPU shaders. A quote from a forum: "Quake3's "shaders" are not the same as whats being discussed here (GLSL). In Quake3, a "shader" is a strictly texture only language for compositing (sometimes procedural) textures and rendered utilizing multitexturing."
So, the effect will not work. Unless you suffice with 8-bit color output and just dump the depth-buffer onscreen.

I can write a small app in a framework of mine, to show how to visualize depth via a shader, and how to create a RGBA32f FBO, but it'll be useless as you're limited by the q3 engine. (and manually adding the necessary code requires knowledge in opengl).

The fragment-shader is, as I wrote:

void main(){
gl_FragColor = vec4(gl_FragCoord.z); // vec4(1.0/gl_FragCoord.w);

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Well the code looks simpler than I thought I supposed that there will be some "surrounding" needed. :P
Anyway I still hope that someone might find a solution. Don't you think that my first thought - an opengl hook could make it?

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Hook - no. Adding 350 lines of code to the engine - yes.
Anyway, it's like trying to fit an electric motor to a 1980's car. You need knowledge of both techs.
Unless somehow this works for you: http://www.nvidia.com/dev_content/nvopenglspecs/GL_NV_copy_depth_to_color.txt

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There is no way I could write something into jk2 source since There isn't such thing accessible. I can only make changes using q3 virtual machine which won't let me do things like that. How do I use these open gl extension? It's written that it doesn't add any new procedures or functions does it mean that only thing I have to do is to load it? From Inside of program or outside of it?

edit: Ok I see now that I have to call glCopyPixels with DEPTH_STENCIL_TO_RGBA_NV as type. However as I wrote I can't add anything to game engine. Maybe I repeat myself but aren't we in a place where hooking an opengl function and adding this line instead of adding it to the game code will make it work?

[Edited by - fau on December 20, 2008 3:15:12 PM]

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There is a tool there called GLIntercept. It is basically a opengl32.dll that you drop where you exe is and it intercepts calls.
You can modify the source code to call glCopyPixels at the right moment. I guess that would be when SwapBuffers is called.

PS : GL_NV_copy_depth_to_color is a nvidia extension. I think it is not available in ATI.

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It's exactly thing I've been looking for. Thank you very much, both of you are in the end credits :)
I'm trying to make it work right now by writing a plugin but it seems that putting it before or after wglSwapbuffers doesn't work. I've also tried before and after rendering call. The extension is supported by my card however I'm not sure if I have to load it somehow? When I try "IsGLExtensionSupported" I get "
GLDriver::IsExtensionSupported - Unable to determine if a extension is supported". Do you have any suggestions where exactly put glCopyPixels?

edit: Now I see that I was calling IsGLExtensionSupported in wrong place. It returns true. I got "some" effect one moment ago but it isn't exactly what I want. I'll have to look into it. Should I put glCopyPixels before or after wglSwapBuffers?

edit 2:I've found out that it works when I turn in-game menu on (It may stop rendering frames in the background - I'm not sure) and stops working when I turn it of.
Here is part of GLIntercept logfile with gl calls made by the game with menu turned off and here with turned on (working one).

[Edited by - fau on December 21, 2008 6:55:15 PM]

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