On 8/25/25 17:17, William F Pokorny wrote:
>> But for some reason, the unrefracted light beam isn't showing.
> 
> Interesting! I didn't even try 'media_interaction off' because I didn't 
> think any actual scattering contribution could darken the media photons 
> like that. I don't understand what is happening with the calculated 
> scattering value where we have collected media photons...
> 
> This scene unlike the bug report one doesn't have the photon path cross 
> back over the scattered media path...
> 
> (The initial beam probably isn't showing up because it depends 
> completely on the scattering result - but I don't 'see' the geometries 
> well enough to be sure.)

OK. The origin of the light source is inside the box{} media container - 
  which is invalid for scattering media calculations; Here in a way 
tangled with the photon + scattered media calculations(1).

In the light_source{} block:

     <0, -HALFH - 0.99, -3>, rgb 1

should read:

     <0, -HALFH - 1.01, -3>, rgb 1

With that change and adding:

     photons { target collect off reflection on refraction on }

to the media-box{} block - we get the, additive, results in the top row 
of the attached image as one might expect.

If we then add:

     media_interaction off

to the light_source{} block (as you already have), we get the results in 
the bottom row of the attached image.

Bill P.

(1) No. I've not run down 'exactly' why the scattering + photons 
calculation goes wrong when the light source is inside the media box. 
Given we know that's an invalid set up, I'm not motivated at the moment 
to run it down.

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