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Additional details:

If you wish to interpolate rays from a triple of images you can use barycentric coordinates. Barycentric 
coordinates are a way of representing a 2D point inside a triangle in terms of the surrounding three 
vertices (barycentric coordinates are actually more general and apply to N-dimensional spaces, but for 
this assignment, let’s just worry about N=2). Given the three vertices of the triangle (p1,p2,p3) and 
a point q, here’s a function to compute the barycentric coordinates (u,v,w) of the point q:

/*************************************************************************

  Given three points and a point q, compute the barycentric coordinates
  of the point q.

**************************************************************************/
static void
ComputeBarycentricCoords(float q[2], float p1[2], float p2[2], float p3[2],
                         float *u, float *v, float *w)
{
   // 3x3 determinant of the denominator
   float d = p1[0]*p2[1] - p2[0]*p1[1] - p1[0]*p3[1] + 
             p3[0]*p1[1] + p2[0]*p3[1] - p3[0]*p2[1];

   // each numerator divided by common denominator
   *u = ( q[0]*p2[1] - p2[0]* q[1] -  q[0]*p3[1] + 
         p3[0]* q[1] + p2[0]*p3[1] - p3[0]*p2[1]) / d;

   *v = (p1[0]* q[1] -  q[0]*p1[1] - p1[0]*p3[1] + 
         p3[0]*p1[1] +  q[0]*p3[1] - p3[0]* q[1]) / d;

   *w = (p1[0]*p2[1] - p2[0]*p1[1] - p1[0]* q[1] + 
          q[0]*p1[1] + p2[0]* q[1] -  q[0]*p2[1]) / d;
}