Thank you, Dade, for kindly giving your thoughts. Yes, my inquiry may be off the beaten path. And I may have been too vague; so I can describe my circumstance better, at the expense of more tedious detail. I hope this will be useful to other LuxCoreRender users, though I cannot be certain that it will be. I feel like a
for posing this. Nevertheless, I'll do the best I can to explain why I have troubled others with the question. You see...
1. In my model case, I have a very irregular mirrored surface to study. The mirror is non-planar in many directions and on many scales. In the real world, it represents a physical surface whose reflection characteristics are important to my work in one branch of physics.
2. Naturally, I cast light on the mirror in a number of ways to study its "performance." If there were some other way to study the mirror performance in Blender, I would give that a try. I'll say more about that later...
3. One illumination case of particular interest involves an area light that casts parallel (0 degree spread angle) light toward the mirror. Another case turns the area light into a thin sheet of light, shown at the mirror. Another case involves an array of "laser"-like parallel emitters that hit the mirror. Another case involves flooding the space near the mirror surface with omni-directional light. For the sake of visualization and for the sake of this discussion, please imagine a rectangular emitter of light shining vertically-down at the mirror under study.
4. So, the goal is to visualize what happens to the light after it leaves the mirror. For all practical purposes, reflection. I am interested in light paths after reflection from the surface... but not before.
5. I need the lamp, only for the purpose of generating light from the mirror surface that obeys the law of reflection. If it were somehow possible to
emit light from the surface of my mirror that would behave as if it were reflected according to that law, I would skip the lamp source altogether. I only want to visualize the light path
after it leaves the mirror surface. I would guess that this stipulation that I have introduced is what makes my question unusual among Blender applications. Most applications do not require that a distinction between incident and reflected light be so draconian.
6. One way of visualizing light paths is to use volumetric scatter. I like this method as well as any that I have found. It lets me see how the light is distributed in space, including any local focusing effects (bright areas in the "fog"), after reflection from the mirror. If there were a way to "see" the light paths without actually scattering light and introducing all of the computational overhead with the scattering process, I would like that even better.
Can we do that? I don't know of any way to get such a ray-tracing diagram...
7. As I have probably stated too many times (sorry), what I want is to see the volumetric scatter of light only
after it leaves the surface of the mirror, without seeing the scatter of light from the lamp source itself. In other words, I
don't want to see the light "coming down" from the source. I only want to see the light "going back up" from the mirror.
8. Finally, the requirements outlined above are the reason that I have hoped to be able to change the color of the light upon reflection - literally to re-define its spectral content after it bounces off the mirror. And why? You see, volumetric scatter gives us control over how light scatters, depending on its color. It gives the option of back-scatter, isotropic scatter and forward scatter... depending on the R,G,B content of the light! That's really convenient. And it underlies my question, however ill-conceived it is. My logic is that, if I have a scattering volume in Blender that only forward-scatters, say, "red" light, then I should never see any of that red light entering the camera in my render, unless the camera is looking directly at the light source. If my camera is off axis (which it always is), then no "red" light will be seen in the rendered volume scatter. So, taking my idea further,
if I could tell the scattering algorithm to scatter only, say, "green" light,
and if the reflected light from the mirror could be
forced to be some other color, like green, no matter what the color of the incident light, then I would have a way to visualize only the reflected light as desired. Reflected light would scatter in my virtual "fog," whereas the source light would remain invisible. And that would be very helpful in my case.
So, Dade, there is my belabored description of what I'm really trying to do and why I am trying to do it. Hopefully, it provides better background and motivation for my question. I simply did not know whether the option exists to force reflected light to take-on a desired color, when I posed my original question. I think I have learned that the answer is "no." Too bad.
But... as I've mentioned, if I back-away from the question as posed and pose a slightly different, more general question, there might still be some way to do what I want and to get the end result that I am targeting for my study in Blender. That is: Is there some
other way to visualize only reflected light from a mirrored surface? Or, is there some other way to eliminate the need for a lamp at all? Could I define my mirror as an emitter with emission that obeys some manually-defined relationship to the surface normal and the global coordinate system? Perhaps the answers to these questions are more important than the "colorshifting" question that I originally posed. I have no idea.
Thank you once again.