On Tue, 02 May 2000 20:48:21 -0500, Chris Huff
<chr### [at] yahoocom> wrote:

>Stuff I don't even know the use for, let alone how to implement. Could 
>anyone tell me what these are for?

>posterize

As Ron Parker described, classic posterization is simply reducing the
color depth to a relatively small number, without using any dithering
to hide the banding. I don't believe that one has to perform this in
two passes, however. Simply divide the range between minimum allowable
value (0), and maximum allowable value for a color into a few discrete
ranges. Posterization is basically a color depth adjustment, where you
get to specify any number of possible shades between 0 and 255. For
example, a setting of "5" would allow 5 different shades of red, 5
shades of green, and 5 shades of blue. If you wanted to get a little
fancier, you could specify different depths for the different color
channels. This might be especially handy to use for use with a cartoon
filter. 

Note: A posterization filter would offer much more control than simply
setting the color depth in the povray.ini file.


>solarize

Solarize can be achieved with an "iso-function" type patch. Basically,
if a pixel exceeds a certain value, it is converted to a "negative"
image. The origination of this effect was in a characteristic of older
types of camera film. Early camera film would actually reverse it's
tonal range if subjected to an extremely bright light. That is, what
should appear in the photo as white, would actually start turn dark,
if the exposure to light was severe enough. I have seen examples in
books where the sun appeared black, and the rest of the image looked
normal. Modern films don't exhibit this effect.

It is *very* similar to the Sabattier Effect, which is generated by
flashing light onto film or photo paper while it is still being
processed in the darkroom. Specifically, flashed while in the initial
developer stage of processing.

If you want a sample formula to play with, here is one of many that
will give this sort of effect:

RedOut = 1-(0.5+(cos(RedIn*pi*2)/2))
GreenOut = 1-(0.5+(cos(GreenIn*pi*2)/2))
BlueOut = 1-(0.5+(cos(BlueIn*pi*2)/2))

This generates a fixed response curve that makes a negative image of
anything above the mid-point of the scale. It's a deep and dramatic
solarization effect. A more refined approach would be to offer a user
specified value for the reversal threshold.

>despeckle

Of course, this is a type of blurring/smoothing filter, but
unfortunately I don't have the details for you.

>laplace

This should be some sort of "edges" type filter. I don't know the
details.

*****************

I would *highly* reccomend you visiting this site:

   http://www.tisl.ukans.edu/~jgauch/kuim/

Here you will find info about the Laplacian Edges filter, different
types of blurring filters, different sharpening filters, and much
more. All filters described even come with sample source code. It
should still provide you with a lot of insight.

Later,
Glen Berry

( Remove the "7" from 7no### [at] ezwvcom to email me. )

Post a reply to this message