Severi Salminen wrote:
> Invisible wrote:
> 
>> It's used for CSG. (E.g., if you "difference" the sphere against
>> something else, the "back" intersection might actually be where the
>> solid surface begins.)
> 
> 
> Hmm. I still don't get it. Why should I ever need to trace the ray in
> the "wrong direction"?
> 
> The ray starts at camera and goes forward. If anything interesting is
> found, it is in front of the ray, right? And the only exception is if
> the camera is inside an object. But even then: shouldn't it be enough to
> trace forward. Please enlighten me :)

You will never need an intersection from *behind* the camera.

However, a ray typically intersects a sphere in two places. POV-Ray 
generates both points of intersection because you might need them both. 
(E.g., if the object is transparent, you'll want to trace a ray out of 
it. If the object is involved in CSG with other objects, the "back" 
surface might actually be the first visible surface. etc.)

-- 
http://blog.orphi.me.uk/
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