Alain <ele### [at] netscapenet> wrote:
> If you go to deep and max_trace kicks in, the color returned is black.
>
> If the contribution goes under the adc_bailout value, you don't return black but
> the calculated color so far. Bailout never return black, unless it have only
> sampled black.
>
> Thus, you ONLY get black if, and only if, max_trace_level kicks in before
> adc_bailout.

I know what you mean, but you're mistaken here:

*Both* max_trace_level and adc_bailout cut-offs do *not* return pure black. They
both do, however, both pretend as if the not-followed portion of the ray was
black. So effectively they both return the calculated color so far - *mixed*
with a more or less big deal of black.

That mix-in will be most notably in shots with high reflectance or refraction,
where max trace level kicks in and places an abrupt end to the ray, with the
contribution of the black mix-in being high enough to be percieved as as a
distinct object of pitch-black color, while the actual pixel values may differ
due to overlaid highlights, diffuse components etc. It is typically far less
obvious with adc_bailout because that mechanism by definition only kicks in
when the not-followed portion of the ray contributes very little to the overall
pixel color, making the outline of the mixed-in black patch far less obvious.

However, even with the adc_bailout, the mix-in of black color may still cause a
slight (and non-uniform) overall darkening of the image.


The math behind this is simple: If a pixel's color is, say, a mix of pure red
weighted at 60%, pure green weighted at 30%, and an untraced portion weighted
at 10%, POV-Ray will make that 0.6*<1,0,0> + 0.3*<0,1,1> + 0.1*<0,0,0>, i.e.
<0.6,0.3,0>. If, however, we assume that the unaccounted-for 10% are the same
color as the average of the accounted-for 90%, we would have to extrapolate
that value to 100% weight, giving <0.6,0.3,0>/0.9 = <0.667,0.333,0.000>, which
is a slightly brighter color.

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