Wasn't it Renderdog who wrote:
>Hi All,
>
>Using an area light with color values greater than one to
>'blow out' the lighting on parts of an image, I noticed
>some odd effects from the bright area light, so I produced
>a scene file to isolate and examine the effects.
>
>The scene file is posted at povray.text.scene-files and
>includes some comments; wondering if my conclusions are
>correct that this is normal and simply a result of the
>color scaling necessary for colors greater than '1'?
>
>This image shows the soft shadow from a very bright
>light source, on a surface that has white and dark gray
>colors. The top part of the image was rendered using
>a bright white light (<1,1,1>*4).
>
>The bottom part was rendered using a bright yellow light
>(<1.00, 0.65, 0.35>*4).
>
>The soft shadows on the dark grey parts appear normal
>because no scaling was performed on that part, while the
>white parts show a truncated soft shadow (and a color
>tinge when lit by a yellow light).
>
>Thanks,
>Mark Slone

I can't see anything other than colour clipping going on here.

With the white light
====================

In the grey areas the fully lit regions are 45.7% grey (I looked at the
output file with an image editor that tells me the colours of pixels
when I point at them), the fully shaded parts are completely black and
the intermediate parts are shades in between.

So, in the white areas the fully lit regions should be five times
brighter, i.e. RGB <2.25,2.25,2.25> but this gets clipped to <1,1,1>.
The shaded parts are still completely black. The intermediate regions,
before clipping, range from <0,0,0> to <2.25,2.25,2.25> but all those
brighter than <1,1,1> get clipped to <1,1,1> and that happens at 1/2.25
of the distance from the fully shaded zone leaving the rest of the
"shadow" zone to be clipped to 100% white.

With the coloured light
=======================

In the grey areas the fully lit regions are <0.457,0.297,0.160>, the
fully shaded parts are completely black and the intermediate parts are
shades in between.

So, in the white areas the fully lit regions should be five times
brighter, i.e. RGB <2.25,1.48,0.80> but this gets clipped to <1,1,0.8>,
a light creamy colour. The shaded parts are still completely black. The
intermediate regions, before clipping, range from <0,0,0> to
<2.25,1.48,0.80> but each colour channel gets independently clipped to
1. Because the channels start to get clipped at different distances, the
hue of the light changes.

For example, if we consider a point that is 10% illuminated, the colour
will be <0.225, 0.148, 0.080>. A point that is 20% illuminated will have
a colour which is double that - a brighter colour of the same hue.
However a point that is 50% illuminated gets the red channel clipped,
giving <1, 0.742, 0.400> instead of <1.143, 0.742, 0.400> so that's a
different hue.

-- 
Mike Williams
Gentleman of Leisure

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