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I am trying to render a dielectric sphere inside the Cornell Box. Some circles are generated on the sphere. Here is the photo generated by path tracer:

enter image description here

And below is my main function:

function trace(surfaces::Array{Surfaces, 1}, ray::Ray, depth::Int64, maxDepth::Int64)
if depth >= maxDepth
    return Vec3(0,0,0)
end
t, material = nearest(surfaces, ray, Inf)
if typeof(material) == Empty
    return Vec3(0,0,0)
end
if material.isLight == true
    return material.emittance
end
ρ = material.reflectance
n = material.normal
if material.isDielectric == true
    cosθ = -dot(n, ray.d)
    ref = material.ni/material.nt
    if cosθ < 0
        cosθ = -cosθ
        n = -1.0*n
        ref = 1/ref
    end
    disc = 1 - ref*ref*(1-cosθ*cosθ)
    if disc < 0
        R = ray.d - 2*dot(ray.d, n)*n
        In = trace(surfaces, Ray(ray.s + t*ray.d, R), depth+1, maxDepth)
        return In
    elseif disc > 0
        R0 = (ref-1)/(ref+1)
        R0 = R0*R0
        Rθ = R0 + (1-R0)*(1-cosθ)*(1-cosθ)*(1-cosθ)*(1-cosθ)*(1-cosθ)
        u = rand()
        if u < Rθ
            R = ray.d - 2*dot(ray.d, n)*n
            In = trace(surfaces, Ray(ray.s + t*ray.d, R), depth+1, maxDepth)
            return In
        else
            R = ref*ray.d + (ref*cosθ - sqrt(disc))*n
            In = trace(surfaces, Ray(ray.s + t*ray.d, R), depth+1, maxDepth)
            return In
        end
    end
end      
BRDF = ρ/Π
R = hemiRand(n)
In = trace(surfaces, Ray(ray.s + t*ray.d, R), depth+1, maxDepth)
return Π*BRDF*In
end

And I think the fresenl effect of the area light on the sphere looks strange and wrong! Am I correct?

EDIT: I searched a bit and I guess it's the moire effect. But I don't know how to get rid of it.

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    $\begingroup$ The usual way to get rid of moire banding is to add a small amount of noise. $\endgroup$ – user1118321 Oct 14 '19 at 18:18
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    $\begingroup$ Those circles are usually a symptom of numerical problems, these tend to come from the intersection distance. Sometimes the starting point for a reflected ray starts slightly below the surface because of rounding errors. Try subtracting a small epsilon from the intersection distance to see if it improves things. $\endgroup$ – PaulHK Oct 15 '19 at 2:07
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I solved my problem by adding a small value to t value when the ray is getting refracted. But I still doubt the result is true. Below is the new generated photo:

enter image description here

Why is the area light reflected twice on top of the sphere? Is there still a bug in my path tracer? The glass sphere is closer to the camera than the area light.

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  • $\begingroup$ The double reflection might be from internal reflections. Maybe you're getting one reflection from the light above and one reflection of the bright spot from the bottom of the sphere? $\endgroup$ – user1118321 Oct 15 '19 at 18:37
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    $\begingroup$ This photo of a real-world Cornell box model does show multiple images of the light source in the glass ball, FWIW: commons.wikimedia.org/wiki/… $\endgroup$ – Nathan Reed Oct 15 '19 at 21:34
  • $\begingroup$ A good test would be to compare your result against an image generated by a well tested path tracer fed with the exactly same scene and materials. I usually use Mitsuba as my ground truth (or even smallpt in some cases). $\endgroup$ – Christian Pagot Oct 16 '19 at 19:39
  • $\begingroup$ One useful debugging technique is to display contribution of path of certain length(s): length equal to 1 is just directly visible emission of materials, 2 is just direct illumination, >2 is whole indirect illumination, 2 is single-bounce indirect illumination, >3 multi-bounce indirect illumination, etc. This way you can separate certain groups of paths to better understand what's going on under the hood. $\endgroup$ – ivokabel Oct 17 '19 at 15:33

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