Zeger Knaepen wrote:
>    pigment {
>        camera_view{[mask-scene camera settings]}
>        pigment_map {
>            [0 rgbt 1]
>            [1 camera_view{[original camera settings]}
>        }
>    }

This does not create the correct gradient. I'll demonstrate with a simpler 
gradient:

// Incorrect:
color_map {
    [0, red 1]
    [1, rgbt 1]
}

// Correct:
color_map {
    [0, red 1]
    [1, red 1 transmit 1]
}

The former incorrectly gives rgb <1.0,0.5,0.5> transmit 0.5 halfway through. 
That's a semi-transparent pink, not a semi-transparent red.
The latter correctly gives rgb <1,0,0> transmit 0.5 halfway through.

Likewise, if you want to create a patterned blend between some MyPigment and 
a transparent pigment, then you must use for the transparent pigment a 
pigment identical to MyPigment, except that all transmit values are 1. Yes, 
it's cumbersome.

However, in the particular situation we're dealing with, there is a 
workaround. Some image manipulation software, like Photoshop, Paint Shop 
Pro, and probably The Gimp, has a feature to remove a matte. Basically, if 
you open the image created using your proposed method and then use the 
feature "Remove white matte", then the incorrect effect of using a 
transparent pigment that's white will be removed.

Rune
-- 
http://runevision.com

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