Here are a few test images of my color_matrix post_process patch, see 
povray.unofficial.patches for details.
I have done two things with post_process: I have allowed them to be 
specified in the camera statement(which is where I think they belong, 
they can still be specified in global_settings with my patch though), 
and I added a filter which transforms the colors of an image with a 
matrix. The syntax is:
camera or global_settings {
    post_process {
        color_matrix {
            < N, N, N,
              N, N, N,
              N, N, N >
        }
    }
}

Here are 4 of the test images: the first is the unprocessed image, the 
second is a simple black-and-white version, the third is a sepia tone 
version, and the last is an attempt at a pastel look.
I will upload the source soon, maybe later today.

-- 
Christopher James Huff - Personal e-mail: chr### [at] yahoocom
TAG(Technical Assistance Group) e-mail: chr### [at] tagpovrayorg
Personal Web page: http://chrishuff.dhs.org/
TAG Web page: http://tag.povray.org/

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Attachments:
Download 'color_matrix Plain.jpg' (26 KB) Download 'color_matrix BW.jpg' (14 KB) Download 'color_matrix Sepia.jpg' (15 KB) Download 'color_matrix Pastel.jpg' (19 KB)

Preview of image 'color_matrix Plain.jpg'

Preview of image 'color_matrix BW.jpg'

Preview of image 'color_matrix Sepia.jpg'

Preview of image 'color_matrix Pastel.jpg'