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OpenGL zFail & infinite view frustum PROBLEM!

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metalcrusader    122
Hello! I have problems implementing Stencil Shadow Volumes with zFail. To prevent that the volume is clipped by the far plane, I have to use an „infinite view frustum“. But how can I make that in OpenGL ( not the theory and need the OpenGL-Call!). And why do I need that infinite view frustum when I limit the volume to for example 10 and the far clipping plane is at 100000? I ask because in my implementation I get a second „ghost shadow“ and i don´t know how to remove it. I know that the volume must be closed so I use the reversed backfaces as the front cap and the extruded backfaces as back faces but the „ghost shadow“ remains. Please help me!!!

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godmodder    828
Closing the volumes only isn't enough. The farplane should always be removed. This is accomplished by setting it to infinity like this:

gluPerspective(fov, aspect, 1.0f, -1.0f);

As you can see, I've set the farplane to -1.0f. This should solve your problem. Remember, for nvidia cards you can recover the 1% depth-precision loss by using the GL_NV_depth_clamp extension, but you won't notice any significant improvement with a 24-bit zbuffer.

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metalcrusader    122
Thank you very much for answering! It seems that only a very few people have experiences with zFail!
I tried my program with the infinite far plane, but I still get the same second "ghost shadow". Is there anything else I have to consider for using this infinite far plane?

Another question: I found an extension called "EXT_depth_bounds_test". Can this extension solve by problem and how can I use it?

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godmodder    828
There are a few things you need for zfail to work:

- an infinite farplane
- a nearplane of 1.0f (depth precision)
- be sure to set 8 bits of stencil
- be sure to use glClearStencil(0) at init
- draw front faces with:
glStencilFunc(GL_ALWAYS, 0, ~0);
- draw back faces with:
glStencilFunc(GL_ALWAYS, 0, ~0);
- be sure to use the wrapping extension in the above operations!
- your volume geometry must face outwards (with the normals)
- you have to draw front and back caps
- draw your volume geometry (last 2 verts of the quads) AND your backward caps at infinity (use w=0.0f or draw at a VERY LARGE offset (100000 or so))
- be sure to use some polygon offset like: glPolygonOffset(1.0f, 4.0f)

When you get these 11 requirements straight, I think it'll work. It works for me!

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metalcrusader    122
Thank you again!
I did everything you advised me to do - but the "ghost shadow" didn't want to disapear. Then I compared my code with an old zPass implementation and I noticed that I have set the mask parameter of glStencilFunc to 0 instead of 1. That was all! The ghost shadow disappers and is fine!

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