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Is there a way to use constraints such that a mesh with reflective material will be at the correct angle to reflect light from a point source to a target object?

Or if not with constraints, is there a way to do this at all?

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Assuming your reflector's normal is along its local Z, the easiest way I can think of is to give it 2 consecutive Damped Track constraints:

enter image description here

One to the light, with influence 1, and then another to the target, with influence 0.5.

Then the normal of the reflector (middle ray, below) will bisect the [light-reflector-target] angle:

enter image description here

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  • $\begingroup$ thanks so much! working great! is there any way to show light paths in the viewport for more accurate tweaking? $\endgroup$
    – Dawn Laing
    May 12 at 20:37
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    $\begingroup$ Tank you! I guess a little GN setup might be good for that... another question, really? $\endgroup$
    – Robin Betts
    May 12 at 21:50
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I think you can accomplish this by using two track to constraints:

Track to constraint

Select the light, then the reflector object, then F3 to do a search and select Track To Constraint, then do the same thing between the reflector object and the subject object.

Result:

enter image description here

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  • $\begingroup$ I could be wrong. But don't you want the reflector to bisect the angle between the light and the target?.. Maybe not, for a diffuse reflector... $\endgroup$
    – Robin Betts
    May 12 at 10:28
  • $\begingroup$ You might be right. I am a high-school dropout who got straight D's in math class after all. :P $\endgroup$
    – Jakemoyo
    May 12 at 10:39
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    $\begingroup$ I just did some tests to check and you're definitely more right than I am. $\endgroup$
    – Jakemoyo
    May 12 at 10:51
  • $\begingroup$ I couldn't be sure, myself, for diffuse reflectors, without looking back at how the diffuse shader works. Thanks for the tests! $\endgroup$
    – Robin Betts
    May 12 at 11:11
  • $\begingroup$ I think your intuition has more to do with the fundamental way light bounces work and less to do with the surface reflectivity. So it should apply to any Principled Shader. $\endgroup$
    – Jakemoyo
    May 12 at 12:47

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