In article <3c5b0ee1@news.povray.org> , "Tom Melly" <tom### [at] tomandlucouk> 
wrote:

> What are the difficulties of implementing, or the disadvantages of having
> some sort of automatic precedence?

The coincident surface problem is exactly this.  Ray-tracing works by what you
call "automatic precedence", that is sorting the intersections found.
However, when a surface is at "exactly" the same place the intersections don't
really have to be and mostly are not because the precision available is
limited and causes some intersections to appear slightly closer than others.
As this effect changes from ray to ray, you see the coincident surface
problem.

    Thorsten

____________________________________________________
Thorsten Froehlich, Duisburg, Germany
e-mail: tho### [at] trfde

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