I'm just brooding over the issue of max trace level and ADC bailout.

If I'm not mistaken, at the moment this is handled such that a ray that is
either "too deep" or is unlikely to make much of a difference is truncated to
black. Which is not ideal, especially when dealing with low max trace level and
high bailout values (or low weights), as it happens during radiosity sample
gathering.

I have a few ideas that could help improve the results in such cases:

(A)

Have the user specify a "cutoff color", to be used instead of the current
default black when a ray exceeds the max trace level or ADC bailout limit; this
way, the color can be adapted to the scene, e.g. for a scene dominated by a
bright blue sky the user could choose a rather bright, blueish color.

(B)

Stick to the black, but automatically "boost" each pixel that has suffered
"cutoff" according to the "lost weight"; e.g. if during tracing for a pixel one
ray is "cut off" that would have contributed an estimated 10% of color to the
pixel, and another that would have contributed 5%, multiply the resulting pixel
color by 1/0.85. (Alternatively, such a correction could be applied at each
"fork" where a ray is cut off; however, it would then be unable to catch cases
where multiple "forked" rays are cut off.)

This may still be wrong, but hopefully not as wrong as the current "blackout".

(C)

Borrow some approach from path tracing: If at a "fork" both rays would drop
below the bailout threshold, but together they would still be above it, roll a
dice and trace one of them.

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