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I apologize if my methods seem way off because this is my first time trying to build a path tracer and I'm struggling quite a bit. Currently, I am trying to mimic the "mix shader" node in Blender 3D that combines two different shaders. I would like to add this functionality to my path tracer because it would allow me to make a plastic-like material. Reading through a few explanations, it seems it takes a probabilistic approach to picking one or the other when a ray hits the surface.

Here is a rendering of an object with a 50/50 mix of ideal reflection and ideal diffuse at 80 samples per pixel: enter image description here

Something is just off about it. Especially those weird underside highlights on the leg extending farthest to the left. I have a strong feeling I'm implementing it wrong.

Here is a pseudo implementation of the path tracing part. It uses explicit light sampling. Based off of smallpt path tracer.

rayColor(ray r, depth, int E=1)
{
    if(r doesn't hit triangle)
        return 0
    if(r hit is a light)
    {
       if(E)
         return light_emission
       else
         return 0
    }

    vector x = r.origin + r.direction*t // x is point where r hit tri
    vector n = normal where ray hit triangle
    n.normalize()
    vector nl = n.dot(r.d) < 0 ? n : n*(-1) // properly orient normal

    if(++depth > 5) return 0 // max bounces

    float triangle_area   = area of emitting triangle
    vector x_light_random = random point on emitting triangle
    vector light_normal   = normal of emitting triangle

    vector d = x_light_random = x_converted;


    if(light_normal.dot(d) > 0) light_normal *= -1; // make it emit
                                                    // both sides

    object_normal.normalize();
    light_normal.normalize();

    BRDF = 1/PI // perfect diffuse
    light_emitted = 1 // emission of 1
    vector light_out = 0
    if(ray starting at x towards d hits light (i.e. not in shadow))
    {
         light_out = BRDF*light_emitted*(object_normal.dot(d))*
                            (-1*light_normal.dot(d)*triangle_area)/
                            (d.length*d.length*d.length*d.length)
    }
    vector direct_light = color_of_object_triangle*light_out;



  //----SPECULAR-----
  vector d2 = r.dir-n*2*n.dot(r.dir); // ideal reflection
  vector light_color = 1 // white since dialectics don't change spec
  vector specular = light_color*rayColor(createRay(x, d2),depth)

  float P = 0.5; // 50/50 chance of mirror/diffuse
  if(erand(Xi) < P)
      return direct_light/P
  else
      return spec/(1-P)

Thank you to anyone who can offer me some direction! Again, sorry if this is really convoluted. I've made it so far and I'm about to give up, but I feel like I'm reaaaally close.

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    $\begingroup$ Do you have a reference picture of the image you expect to get? It could be easier to first replicate an exiting image and compare your result. $\endgroup$ – Julien Guertault Jan 6 '17 at 5:47
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    $\begingroup$ @JulienGuertault thank you so much for the suggestion! That was brilliant and I wish I had thought of it earlier. That would've helped with a lot of hair pulling. See my edited post. T $\endgroup$ – Henry Bergin Jan 6 '17 at 6:32
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    $\begingroup$ I've edited out the "one more thing" section now that you've posted it as a separate question. Notice that the separate question now appears in the "Linked" section on the right hand side of this page, because that question contains a link to this question. $\endgroup$ – trichoplax Jan 7 '17 at 12:20
  • $\begingroup$ In case it's helpful, here's a good short read about path tracing basics with some simple c++ source code accompany it: blog.demofox.org/2016/09/21/… $\endgroup$ – Alan Wolfe Jan 7 '17 at 18:30
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As suggested in the comments, I created an exact scene in Blender using the same .obj. After adding a mix shader node and setting one side to diffuse and the other to a perfect mirror, you can see that the results are almost identical. This would indicate that my implementation is correct.

Here is a side by side comparison. Mine is on the left, Blender's is on the right:

enter image description here

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