   question to volume rendering, ray casting
  Hi therei found 2 example shader codes for volume rendering usingray casting. I have a question for getting the ray start position.example A:// find the right place to lookup in the backside bufferfloat2 texc = ((IN.Pos.xy / IN.Pos.w) + 1) / 2; // the start position of the ray is float4 start = IN.TexCoord;example B:vec3 rayStart = texture2D(RayStart, gl_TexCoord[0].st).xyz;vec3 rayEnd   = texture2D(RayEnd, gl_TexCoord[0].st).xyz;My question is, why do they calculate following((IN.Pos.xy / IN.Pos.w) + 1) / 2in example A but not in example B ?I mean, basic idea is to render front and backface of a colored unit cube with front face culling / backface culling. Both are rendered to textures.Then the RGB value stored in these textures give the start point and endpointfor the ray.In example a, only backface is rendered to texture and frontface is drawn withglMultiTexCoord3fARB.c-mos   