"grammophone" <eml### [at] ingr> wrote:
> during internal ray shooting towards the "other side", i might indeed hit the
> "other side" or a "bone". Light integration is then performed in both cases. In
> the former case, it works like yours. In the latter, the bone is completely
> hidden from lights, thus it would contribute the desired "blackness" to the
> effect with its ambient color setting.
>
> Yes, this is dirty! No computation takes place for the light scattered on the
> bone. But i pose the approximation hypothesis that the light reflected from a
> bone is usually quite less than the one penetrating the surface.

The problem is not so much the light coming directly from the bone.

The main problem is that in a highly scattering medium, the light does not
spread along a straight line, so testing for interfering objects via a "naive"
ray tracing approach is prone to error.

To take an extreme example, imagine a bright laser shining at a finger: With
your approach, the finger bone would cast a precise, sharp shadow on the
opposite surface, which of course would be far from realistic. Instead, a great
deal of light should be transported around the bone by diffusing through the
skin and flesh, illuminating the whole surface with a very low gradient.

And the problem not only manifests in the shadow cast on the opposite, but also
in the brightness of the non-shadowed areas: With your approach, the finger
bone would have no effect on these (because the only light source - the laser
dot - would be in direct line of sight). This is unrealistic as well; instead,
as the bone absorbs light, in reality there would be less light scattered back
to the surface.

The effect will probably not be as prominent with normal lighting, as it will
illuminate not single points but rather patches of skin, so the bone shadow
will be quite diffuse; however, it shows that the effect cannot be neglected,
and results must be expected to differ significantly from reality.

Of course just ignoring the bone isn't right either. So the best solution is yet
to be found.

> [photons]

That would solve the issue if it was only about light scattering back from the
bone, but as demonstrated that's not even the main problem.


> Thank you for your great work!

You're welcome. There's still an awful lot to do though.

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