Questions on Volume emission

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Jul 31, 2023, 6:28:20 AM7/31/23

I am reading free chapters from pbrt-v4 and also the "null-scattering" paper referenced here.

For the general null scattering equation. It requires multiplying the emission value by absorption probability (or coefficient).

Here comes the confusing part for me. If I am rendering volume with pure white particles, could it emit light or not?
To my understanding, pure white particles don't absorb any energy, so the absorption(emitting) probability should be zero. As conclusion, it will not emit any all.

Then, how to select or calculate the absorption probability? May I simply use the (1.0 - particles_colour) as absorption probability.

Assume the equations below I borrowed from pbrt-v3 book and pbrt-v4 free chapters. (Maybe I lost something important during reading)
sigma_t = sigma_s + sigma_a 
majorant = sigma_t + _null_coefficient

If using predefined sigma_a and sigma_s as pbrt-v3, it will not reflect the feature of colourful volume.
If using (1.0 - particles_colour) as absorption probability, it seems the value will lost the relation to sigma_t.

Best Regards
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