Fusion
No, it cannot be turned off. You need to change your lighting.
If yoo can share your model and describe what you are trying to achieve we can provide more specific advice.
You may want to demonstrate the effect in a screenshot or screencast so I can get a better understanding of what you are seeing.
Few things of note:
A flat prismatic object without any filleted edges is going to look incredibly boring and unrealistic, in the most sophisticated render engine.
The orientation of your part is odd.
The normal orientation is Z-UP and the HDRI light sources are oriented in reference to the Z-Up direction. So if you had a HDRI light source with a single top down light, it would shine down from the Z direction as I show in the screenshot:
The screenshot you are showing does not show a proper raytraced render but only the viewport preview render.
As such I find it difficult to determine what you are trying achieve.
@johnAMKDR wrote:
I am not interested in a rendering; I just want an uncluttered image of the part where the glare doesn't obscure other things I might be adding, like an offset plane.
Oh ... OOPS 😉
My bad. there is indeed a light source of sorts to make things in the viewport visible. That cannot really be turned off.
You can adjust the screen settings to "uncluttered" the viewport to a degree.
You could also enable component color cycling:
Not sure any of that helps, but its worth a try.
@johnAMKDR wrote:...then clicked FIT and the glare appears.
Ah, that was the key - I have never used FIT. (I use a 3D Connexion device and my screen is always in motion).
Follow these instructions: Right click...
