And I've finally nailed the last problem that had been bugging me for some time
about mixing rendered and photographic material in the same scene - colour
correction and diffuse lighting (to give realistic shadows).

If rendered material doesn't affect the photographic material then you can just
declare it ambient 1.0, and it will come out in the render the same as it was
shot.

If the rendered material does interact however, for example by casting shadows
onto the photograph, then you need to use diffuse lighting. But if your lights
aren't white (or don't sum to white), then your photograph will get a colour
cast from the lights. In effect the photograph is getting lit twice - once in
real life, and then again in the render, and this can be quite noticeable.

I fixed it by adding a illusion_colour_correction variable to Rune's
illusion.inc file that adjusts the colour of the photograph by exactly the
opposite of the light, so that when the light is a applied, it shifts the
colour back to the original.

I constructed a test scene, and it looks like it's doing the job pretty well to
me :-)

Cheers,
Edouard.

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