4.0 cycles spotlights produce unwanted refraction and glare #113693
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Reference: blender/blender#113693
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System Information
Operating system: Windows-10-10.0.22621-SP0 64 Bits
Graphics card: Intel(R) Arc(TM) A770 Graphics Intel 4.6.0 - Build 31.0.101.4887
Blender Version
Broken: 4.0
Worked: 3.6 and earlier
the attached screenshot shows the same scene in 4 blender versions: 3.3, 3.5 (top left/top right), 3.6, 4.0 (bottom left/bottom right)
in 4.0, spotlights that were previously not visible are producing glare and refraction even outside of their cone of visibility. (ignore the top row of images.)
on the right bottom side of the case, a spotlight that overlaps with the case is illuminating the case brightly within its radius despite that area not being within the spotlight cone. this is direct illumination (there is no material on the case, and no glossy reflection).
on the right top of side of the case, the light reflects on the glass, also despite not being within the spotlight cone.
last, if you redirect the spotlight to point through the glass, the spotlight will be visible refracting/transmitting through the glass even when the camera is not within the cone. previous versions of blender would only show the light if the camera/viewpoint was directly in the cone.
these make it harder to avoid bright refractions/reflections in glass materials...this scene and another one i authored in 3.6 look markedly different (and worse) in 4.0 with bright reflections and refractions that i had avoided with deliberate light placement and cone size/angles. the refraction issue #3 is the worst one of the lot imho. the other 2 are easier to work around.
The release notes mention some changes to the lamps:
https://wiki.blender.org/wiki/Reference/Release_Notes/4.0/Rendering#Lights
I believe that the changes to Spot lamps are related to this commit:
#109329 Cycles: replace spot light disk sampling with sphere sampling
But in fact this compatibility breakage is strange. Didn't it deserve versioning?
@weizhen can know the details better.
Weizhen is currently away and will be for a few (5 or 6) more months. It's probably better to ask someone else like @brecht to comment on this until they're back.
This is a case of bad visualization. In Cycles the spotlight’s “cone of visibility” has always been wider than the overlay cone. For 4.0 you can imagine a light bulb with a lamp shade, before 4.0 the sampling was quite weird, it’s hard to visualize the shape, but also definitely larger than the cone you see.
The cone is drawn that way partly because overlay is handled by @fclem and EEVEE doesn’t support spotlight radius. We should be able to do better, either use different overlay depending on the render engine used, or change the overlay and support spotlight radius in EEVEE?
Besides, it should be expected that light with larger radius would illuminate a larger radius. If you keep the radius zero the illumination will stay in that cone, and you can change the blend if you just want it to look softer.
The change appears to be intentional and an improvement.
However, there is still a question: can this alteration in the rendering result be addressed through versioning? (Automatically setting a radius zero and maybe slightly increasing the angle of the cones for older files)
It can't be fixed in versioning, the radius also affects the softness of shadow. Which in most setups where the sphere does not overlap other objects is the more important thing to preserve.
There's some work going on to standardize these light shapes for USD, and one interesting behavior I saw in some other renderers is to have this work as a disk light that cuts off the end of the cone, which looks more natural.
In any case, I don't think this is a bug, even we if we can still improve the visualization and/or shape in the future.