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Bidirectional path tracing and pinhole camera

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I'm implementing a bidirectional path tracer as described in chapter 10 of the Veach thesis: http://graphics.stanford.edu/papers/veach_thesis/ Now my problem is how to calculate the probability density function (PDF) with respect to projected solid angle for the primary ray (z0->z1) for a pinhole camera. This is important to calculate the relative probabilities with which the other sampling techniques would have created the same path and needed to calculate the weight for the contribution of the path. According to equations (10.9) from the thesis the relative probabilities can be calculated. As an example consider the path generated by a (1,2) sampling technique. Then the ratio p2/p1 = ( P(x0->x1) * G(x0<->x1) ) / ( P(x2->x1) * G(x2<->x1) ), where P(x->x') is the PDF with respect to projected solid angle for extending the path at x into the direction to x'. Now the term P(x2->x1) is the same as P(z0->z1), where z0 is the location of the camera (and origin of all primary rays) and z1 is the vertex hit by the primary ray. So how to compute the value of P(z0->z1) ? [Edited by - nmi on February 27, 2010 2:05:06 AM]

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