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Why are my ray traced reflections wrong?

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#1 BloodOrange1981   Members   -  Reputation: 383


Posted 02 April 2014 - 08:41 PM

Hi! After some fooling around with basic ray tracing with spheres and planes I added reflections, but using the formula seen on many websites


ray direction - 2 * DotProduct(rayDirection , surfaceNormal) * surfaceNormal 


the reflections seem to be orientated the wrong way.




but if I use


ray direction - 2 * DotProduct( -rayDirection , surfaceNormal) * surfaceNormal 


The reflections on the spheres seem ok but isn`t the reflection on the plane a little strange?




The code that determines the color for a pixel goes as follows

//intersection point at time t
point = ray.GetOrigin() + t * ray.GetDirection();

surfaceNormal = sceneObj->GetSurfaceNormal(point);

//determine the reflection direction vector
reflectDirection = ray.GetDirection() - (2.0f * DotProduct(-ray.GetDirection(),surfaceNormal))*surfaceNormal;

//generate the color for this ray and generate a secondary ray, add the generated color
return GenerateColor(point,sceneObj,surfaceNormal) + TraceRay(Ray(reflectDirection,point+reflectDirection * 0.001f),depth-1);

Thanks in advance


#2 Bacterius   Crossbones+   -  Reputation: 11933


Posted 02 April 2014 - 10:24 PM

Since your plane doesn't appear to be reflecting light in the same way as your spheres regardless of the reflection formula you use, I would check that your surface normal is pointing the right way for both objects. It's almost certain your sphere surface normals are pointing inwards of the sphere whereas your plane surface normal is pointing upwards (or vice versa).


Remember: the surface normal is typically defined as always pointing "outwards", that is, if V is the direction of the ray as it hits the geometry, then dot(V, N) < 0 (so that dot(R, N) > 0 where R is the reflected ray). You can define it the other way, with it pointing inwards if you want, but you have to be consistent and make sure this is true for all your geometry, since the reflection formula kind of "needs to know" which way to reflect your incident vector, and that depends on the orientation of the normal vector.


It's fairly easy to enforce if you're only doing reflection, since you can do the dot product check and flip your normal as needed, but it gets messy when you start doing refraction/transparency where non-watertight meshes are physically meaningless (they have no boundary) and you can't flip normals since you need to keep track of which object(s) your ray is currently inside of, therefore you have to be a lot more careful in these situations to make sure your geometry is self-consistent. For opaque reflection you can just hack the surface normal to make it point in whichever direction you need and you're good to go (again, because with reflection only your ray is always "outside" and you know it).


EDIT: if you could show your code for the GetSurfaceNormal() method, that should help track down the bug.

“If I understand the standard right it is legal and safe to do this but the resulting value could be anything.”

#3 BloodOrange1981   Members   -  Reputation: 383


Posted 02 April 2014 - 11:20 PM

Ahh, the Normals were in fact fine, but thanks for the suggestion.


The problem I has having and why the formula didn`t work was 1) not normalizing the generated ray Direction and 2) in my sphere intersection code I was simply checking for the lowest value of t, and not checking if they were positive values as well, hence shapes behind the ray`s origin influencing the reflected colour. Now it looks an awful lot better!


Thank you.



Edited by BloodOrange1981, 02 April 2014 - 11:24 PM.

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