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w=0 problem in shadow volume

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in dx sample:Shadow volume, in the shader file: ShadowVolume.fx, on the function which deal with extrde the vertex away, you can get the
    float4 PosView = mul( vPos, g_mWorldView );

    // Light-to-vertex vector in view space
    float3 LightVecView = PosView - g_vLightView;

    // Perform reverse vertex extrusion
    // Extrude the vertex away from light if it's facing away from the light.
    if( dot( N, -LightVecView ) < 0.0f )
        if( PosView.z > g_vLightView.z )
            PosView.xyz += LightVecView * ( g_fFarClip - PosView.z ) / LightVecView.z;
            PosView = float4(LightVecView,0);

        // Transform the position from view space to homogeneous projection space
        oPos = mul( PosView, g_mProj );
    } else
        oPos = mul( vPos, g_mWorldViewProjection );

why when it set w=0 when PosView.z <= g_vLightView.z, it works and otherwise it doesn't; what happened in the clipping stage? when I set w=0 on all situations, there is one more strange shadow; sorry I don't know how to paste picture here; you can see it by modify the code; the homogeneous coordinates, the clipping algorithm, puzzle me;

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I can't give a real answer, but setting the w component means that you have a vector instead of a point. The gpu knows how to handle this. If you have a triangle where one of the points has w=0, you'll actually get a three sided rectangle where one side can never be seen because it's off at infinity. You can see a cool example from nvidia visualizing this: http://download.developer.nvidia.com/developer/SDK/Individual_Samples/3dgraphics_samples.html

Search for "External Triangles".

As for this specific case, it has to do with fixing problems with general stencil shadows. Using a special shadow mesh, there are cases that can be fixed by sending off some of the faces to crazy town with this method. I'm not an expert on this, but I know how to find one: http://developer.nvidia.com/object/robust_shadow_volumes.html

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