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I am writing a basic raytracer and have implemented normal maps. However, when using normal maps, sometimes the rays generated are opposite to the surface's geometry normal, so that the rays are reaching light sources behind the object. If I kill the ray, the shading results looked faceted and the surface normals are apparent. If I don't kill the ray, the ray reaches light sources on the opposite side of the geometry.
How is this handled normally?
That is, to my knowledge, a problem without a proper solution. You're seeing the discrepancy between shading normal and geometry normal and it becomes obvious, that the shading normal is just a trick. PBRT has a paragraph on this, their solution is to look at the geometric normal to determine whether to call the BRDF (reflection) or the BTDF (transmission), then to pass the shading normal to the BxDF. Still, this doesn't work robustly in all situations.
This problem is also known and described for production proven render engines: http://blog.irayrender.com/post/29042276644/shadow-acne-and-the-shadow-terminator The solution suggested for iRay is to use displacement mapping instead of normal maps. This way, shading and geometry normal are in agreement again.
One thing that I see being done, is that when the ray is generated, the location is shifted on the normal. Basically the normal multiplied by a small amount is added to it. If you are shading the backside of the face, then the normal needs to be inverted.
The reason why, is, for example, when a light is behind a face, and you cast the light ray. Without the offset the origin is exactly on the face, so t is 0. Algorithms see this as not a hit, so the offset along the normal, makes it so that t isn't 0 anymore.
I hope this answers it.
