On Mon, 24 Apr 2000 13:43:30 -0500, Chris Huff
<chr### [at] yahoocom> wrote:

>>  3) Blur the negative somewhat heavily. 
>
>Is this stage necessary for a filter? It seems that it is only done in 
>"real life" to avoid problems with slight misalignment or with film 
>grain, these wouldn't exist in a computer generated image.

I think there was another reason for blurring the negative, in
addition to worries about film alignment or grain. I think it had to
do with preserving the high frequency content and "local contrast" of
the image.

I'm trying to find some technical reference material on the subject,
but haven't yet. I've had to work strictly from my personal memory so
far, but I'd like to find some technical documentation on this
subject. The internet turns out to be a very poor place to search for
this information, because practically all the search engine hits refer
to the Photoshop unsharp mask effect.

Also, there *are* things in POV-Ray image that could react to this
process in a way similar to film grain. High frequency content such as
crand, fine textures, very small objects, and media "graininess" come
to mind. Without blurring, I think we would be removing, minimalizing,
or distorting much of the high frequency content in the image. This
isn't usually a wanted by-product.


As usual, comments are encouraged.
Thanks,
Glen Berry

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