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GL projection shadows w/ stencil buffer

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Hello everyone, I've been working implementing projection shadows mainly for projecting multiple faces and I indeed did implement it and so far it looks like its a success however; there is a problem in the projection matrix calculation that I fixed but the problem is that I don't understand why the fix works. I am using the well-known matrix with parameters of a plane in the form of a*x + b*y + c*z + d = 0, a point P on the plane and light position L. Plane normal N is normalized( a, b, c ) and d = -dot( N, P ). Vec4D for planeNormal is ( a, b, c, d ) and lightPos is ( x, y, z, 1 ). Below is the matrix calculation. Renderer::_calculateShadowMatrix( Vec4D lightPos, Vec4D planeNormal ) { float dotPL = dot( planeNormal, lightPos ); shadowMatrix_.x11 = dotPL - lightPos.x * planeNormal.x; shadowMatrix_.x12 = -lightPos.x * planeNormal.y; shadowMatrix_.x13 = -lightPos.x * planeNormal.z; shadowMatrix_.x14 = -lightPos.x * planeNormal.w; shadowMatrix_.x21 = -lightPos.y * planeNormal.x; shadowMatrix_.x22 = dotPL - lightPos.y * planeNormal.y; shadowMatrix_.x23 = -lightPos.y * planeNormal.z; shadowMatrix_.x24 = -lightPos.y * planeNormal.w; shadowMatrix_.x31 = -lightPos.z * planeNormal.x; shadowMatrix_.x32 = -lightPos.z * planeNormal.y; shadowMatrix_.x33 = dotPL - lightPos.z * planeNormal.z; shadowMatrix_.x34 = -lightPos.z * planeNormal.w; shadowMatrix_.x41 = -lightPos.w * planeNormal.x; shadowMatrix_.x42 = -lightPos.w * planeNormal.y; shadowMatrix_.x43 = -lightPos.w * planeNormal.z; shadowMatrix_.x44 = dotPL - lightPos.w * planeNormal.w; } If I directly use it as it is, some shadows slide across the surface and they even move with the camera. Image Hosted by ImageShack.us I fixed it by scaling the d of plane with a trial-error value as below which happened to be 0.75: polyNormal.w = -dot( polyNormal, point ) * 0.75f; Image Hosted by ImageShack.us And with that it works properly so far. I tested it on several situations and it does work fine. Though I can't really explain why, I suspect that after transforming the normals of a face using the mesh's transformation, a 0 may turn into a very small number close to 0 such as -1.15e16 and I wonder if that may build up to an error. Can anyone shed a light on it? Thanks.

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