Zach kindly points out an error in this slide:
d^2 = cos^2(theta) / ||p' - p ||^2
||p' - p ||^2 = d^2 / cos^2(theta)
or more simply:
||p' - p || = d / cos(theta)
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What is d, particularly for the current assignment? In the paper they indicate that it is the distance to the exit pupil, but I couldn't figure out what that was supposed to be either. (I'm trying to use this to figure out the appropriate weight for GenerateRay to return.)
EDIT: Now I see that d is arbitrary since we are integrating over area, which scales with d^2, so the effect of scaling d will be canceled out by the dA' differential in the integral.