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sylpheed

OpenGL How do you do post-process in OpenGL + CG?

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First of all, sorry for my poor English, and sorry again if this thread is not in the correct section, I am new in the forum :) I have an application that uses C++, OpenGL and CG. I have no problem rendering my objects with CG shaders. In fact, I'm using FX Composer to create Shaders, so I have to use Techniques and Passes. The problem is that I do not know how to apply a post-process shader to the whole scene. I have downloaded a shader from Nvidia library, specifically this: http://developer.download.nvidia.com/shaderlibrary/packages/post_deepMask.cgfx.zip It works in FX Composer, but I can't use it in my app because I'm not sure how to implement in my code. For standard shaders, I do this:
Quote:
CGpass pass = cgGetFirstPass(technique); while (pass) { cgSetPassState(pass); glCallList(...); // Render the mesh cgResetPassState(pass); pass = cgGetNextPass(pass); }
Please, could you help me? I've searched for two days in Google but I don't find anything :(

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Well I haven't downloaded and looked at the shader you link to, but all (?) post process effects shaders are applied by first rendering your scene to a texture*, and then rendering this texture as a fullscreen quad using the post processing shader (either two another texture if you intend to apply more post process effects**, or to the backbuffer).


* Framebuffer Object [FBO] on newer hardware, Pixelbuffer Object [PBO] on older hardware, or render to backbuffer and copy to texture on even older hardware.
** Usually for multiple post process effects which cannot be combined into a single shader (this is much preferable, as changing target, regardless of type, is one of the most costly operations, if not the most costly) the most common technique is "ping-pong", where two targets are used:
1) first the scene is rendered to A,
2) then B is rendered to using the contents of A,
3) then A is rendered to using the contents of B
4) if not done goto 2)

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I'm almost there... need help though. I've downloaded another shader from Nvidia Library, its algorithm is very similar and provides various techniques, one of them is very "simple":

- Pass1 -> Calc normals.
- Pass2 -> "Do something" (particularly detect edges from a object) using normals from the pass before.

The technique is like this:
Quote:
technique NormsOnly <
string Script =
"Pass=Norms;"
"Pass=ImageProc;";
> {

/*
* PASS 1
*/
pass Norms <
string Script = "RenderColorTarget0=gNormTexture;"
"RenderDepthStencilTarget=gDepthBuffer;"
"ClearSetColor=gClearColor;"
"ClearSetDepth=gClearDepth;"
"Clear=Color;"
"Clear=Depth;"
"Draw=Geometry;";
> {
VertexProgram = compile vp40 simpleVS(gWorldITXf,gWorldXf,
gViewIXf,gWvpXf,gWorldViewXf);
DepthTestEnable = true;
DepthMask = true;
CullFaceEnable = false;
BlendEnable = false;
DepthFunc = LEqual;
FragmentProgram = compile fp40 normPS();
}

/*
* PASS 2
*/
pass ImageProc <
string Script = "RenderColorTarget0=;" // re-use
"RenderDepthStencilTarget=;"
"Draw=Buffer;";
> {
VertexProgram = compile vp40 edgeVS(QuadScreenSize,gNPixels);
DepthTestEnable = false;
DepthMask = false;
BlendEnable = false;
CullFaceEnable = false;
DepthFunc = LEqual;
FragmentProgram = compile fp40 normEdgePS(gNormSampler,gThreshhold);
}
}


Now, thanks to bluntman, I have rendered the normals to a texture (using frame buffer objects) and now the Shader has access to this texture. This is executed in pass number one and it works, I have tested it:

http://www.subirimagenes.com/otros-norms-1655474.html

It's a teapot on a triangled plane :).


Problems come in Pass2 due to FX Composer Scripts. I don't know how to perform the script "Draw=Buffer;" using Open GL. I've readed SAS from CGFX:

Quote:

Passes can also specify what to draw in each pass – either the geometry from
the scene, or a screen-aligned quadrilateral that will exactly fit the render
window. We use the pass Script “Draw” command to chose: either
“Draw=Geometry;” for (you guessed it) geometric models, or “Draw=Buffer;” for
a full-screen quad. If neither is specified, a “Draw=Geometry;” will be implied at
the end of your pass Script.


That sounds pretty cool using FX Composer, but I have no idea how to implement using Open GL and CGFX API. How could I do this?

This is a nightmare, ten hours non stop, I only see code around me :(

Please, anyone could help me? If anybody has an example of post-processing using OpenGL + CGFX, please share it, I would appreciate it so much.

Best regards.

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Well I haven't used FxComposer, but from what I understand from what you say, the draw=buffer is used to cause a full screen quad to be drawn to the screen using the specified shader. So to do this in OpenGL with Cg you need to:

1) set the shader you want to apply to the fullscreen quad (the edge detection shader I guess).
2) attach the texture (your FBO containing the render of the normals) to the shader parameter.
3) draw a quad that fills the entire screen.

That should be it!
I have code but it is quite modularised and split up, so wouldn't be easy to immediately understand, but it should be simple (ish) to do what you are trying to do.

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Quote:
Original post by bluntman
Well I haven't used FxComposer, but from what I understand from what you say, the draw=buffer is used to cause a full screen quad to be drawn to the screen using the specified shader. So to do this in OpenGL with Cg you need to:

1) set the shader you want to apply to the fullscreen quad (the edge detection shader I guess).
2) attach the texture (your FBO containing the render of the normals) to the shader parameter.
3) draw a quad that fills the entire screen.

That should be it!
I have code but it is quite modularised and split up, so wouldn't be easy to immediately understand, but it should be simple (ish) to do what you are trying to do.


Again, thanks a lot for your help.

For now, we have moved the post-process for two weeks, we're in a hurry with other things of the application.

I will tell you when we take up again.

Thanks for your time.

Best regards.

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Finally... I DID IT!!

Tears in my eyes, is a beginner code, but it has been soooo long... and now is a happy end :)

Thanks Bluntman for your help, you give me the trick! The steps are:

1) Render all previous passes in FBO's (the last pass uses all FBO's textures for calc the results).
2) In the last pass, render a full screen Quad (it is equivalent to "target=buffer").

For those who have the same problem, dont't forget binding FBO's textures to CGFX texture samplers!! ;)

Best regards.

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