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In section 16.1.1 of Physically Based Rendering the authors describe how we can check, if a given ray $r$ corresponds to one starting from the film area.

They implement this check such that it works for cameras with finite and pinhole apertures. They assume that $r$ has origin $p$ and direction $\omega$. Maybe I didn't get it, but are they assuming that $p$ is a point on the lens? That seems to be the case from Figure 16.1.

Now they want to find the point on the film that $r$ corresponds to. Let $w$ denote the camera viewing direction $(0,0,1)$ in world coordinates. Then they compute the cosine of the angle $\theta$ between $\omega$ and $w$, $$\cos(\theta)=\langle\omega,w\rangle.$$ Assuming we're dealing with a pinhole aperture, why do we find the point on the film by intersecting the ray with a plane arbitrary set at $z=1$? And why is this intersection at $p+t\omega$, where $t=1/\cos(\theta)$? Shouldn't the intersection be at $t=\frac{\langle w-p,w\rangle}{\langle\omega,w\rangle}$?

And anyway, doesn't $p$ need to be equal to the eye point of the camera?

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