Volume Tau() with zero absorption

[Lord Crc]

Hi,

how should we handle ClearVolume/HomogenousVolume Tau() if one of the components are zero?

The RGB -> spectrum conversion may lead to some portion of the spectrum being negative, so I added a clamp to the absorption (in SigmaA) as the negative values caused the Exp in Scatter() to blow up.

The problem is that I forgot that of course zero times infinity = NaN, and ray.maxt is often +infinity...

So, should we instead clamp the absorption to some very small value, say 1e-30 or something? Perhaps we could add a check to see if all components are black, in which case we return 0, otherwise clamp?

If not, what would be the right thing to do? If sigma_a is absolutely zero then it should of course be zero, but one can imagine the input going to zero, in which case the value is indeterminate.

[jeanphi]

Hi,

I think that the fix should go in the places where Tau is used because if the value is 0 then we can save an exp computation since the value should be kept unmodified. Having a small value won't fix the issue as inf*epsilon=inf and this will lead to complete absorption.
Alternately, we can enforce a non inf value for maxt.

Jeanphi

[Lord Crc]

I think it's bad to have an interface where it's expected that Tau() may return NaN for some components. I think can come back and bite us down the line.

Thinking a bit more about it I think using a huge, but non-inf, maxt value would be best as it would be the least surprising for users.

[jeanphi]

Hi,

Maybe just check the value of sigma and return 0 if sigma is <= 0, that should be safe enough, I hadn't rechecked the code and had forgotten about the exact return of Tau.

Jeanphi

[Lord Crc]

Yeah, I guess that's just as good. We'll have to document for users in any case that they may get surprising results if they have sigmas which have parts of the spectrum zero and parts non-zero.

For example I'm going to modify the luxblend exporter to use the "colordepth" texture as it should be less prone to these special cases.