Volume blue halo with "White" RGB vs constant SPECTRUM. about pbrt-v4 HOT 6 CLOSED

The issue here is that the RGB values for sigma_a/sigma_s end up being represented using RGBSpectrum, which multiplies the value it returns by color space's illuminant. What we want here is RGBReflectanceSpectrum, which doesn't do that.

But, RGBReflectanceSpectrum only handles values in [0,1] (i.e., reflectances). Plus, it's clearly misnamed then.

So the proper fix will probably be to rename RGBReflectanceSpectrum to RGBSpectrum and fix it to handle values outside of [0,1], rename the old RGBSpectrum to RGBIlluminantSpectrum, and to patch everything up accordingly. This should be straightforward; I'll try to get to it soon.

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I don't think this has to do with the conversion, for example rendering a cloudy torus with rgb values gives an interesting faint blue grid structure.

This looks similar to what surrounds the bunny, you can see some of the structure in the ears especially.

cloud.zip
Rendered with https://github.com/mmp/pbrt-v4/tree/ca0db69 on the cpu.

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(Sorry I haven't had a chance to look into this yet, but yes, it doesn't look right..)

Possibly related, via @anderslanglands: the attached scene is a big white diffuse quad, lit by a white light. If the reflectance is 0.999, the average RGB channel values are all just under one, as would be expected

B: min      0.98291 max      1.01758 avg     0.999213 (0 infinite, 0 not-a-number)
G: min     0.994629 max      1.00098 avg     0.999048 (0 infinite, 0 not-a-number)
R: min     0.992676 max      1.00586 avg     0.998826 (0 infinite, 0 not-a-number)

If the diffuse reflectance is 1, then things go blue-ish:

B: min      0.98584 max      1.02051 avg        1.002 (0 infinite, 0 not-a-number)
G: min     0.975098 max     0.981445 avg     0.979393 (0 infinite, 0 not-a-number)
R: min     0.958984 max     0.971191 avg     0.964616 (0 infinite, 0 not-a-number)

x.pbrt.txt

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This is looking good when rendered with https://github.com/mmp/pbrt-v4/tree/0e4b5ae

Edit for clarity
This is a render of the above torus scene, but with a sigma_a, sigma_s of (0.999, 0.999, 0.999) to test the majorant grid artifacts fixes.

Thanks Matt!

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(Are you sure that one was rendered with RGB? I was still seeing a slight blue tinge with that cloudy doughnut scene that is now fixed(*) with e6b4d90. Here is ToT now:

(*) RGB of exactly 1,1,1 still comes out blue-ish; I need to chase that down but think I will file that as a separate bug. The attachment is rendered with 0.99 0.99 0.99. I've also confirmed that bunny-cloud comes out correctly with those RGB values.

Thanks for reporting this!

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Opps sorry Matt, that was my bad I should have been more clear in what I was testing. My render in #28 (comment) was with a sigma_[as] of 0.999 as I was just testing the fix for majorant grid artifacts.

Thanks for making the Issue #54

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