Light clamping from Normal or Bump maps near shadow terminator in Cycles #107579
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Reference: blender/blender#107579
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System Information
Operating system : Windows 11
CPU : AMD Ryzen 9 3900X
Graphics card : RTX 3090
RAM : 32GB
Has also been tested on a Ryzen 7 Laptop with Ubuntu.
Blender Version
Has been tested on every major release post 2.9, and on 3.6 build
d5fc1b9ba4, identical behavior on all of them.All provided screenshots of the test file (and the test file itself) were made with 3.5.1 Stable (build
e1ccd9d4a1)Short description of error
All normal information from a Height map passed through a Bump Node or from a Normal Map passed through a Normal Map node introduces some sort of light-clamping around the shadow terminator. The effect starts being visible at an angle of around 66° from the light source.
This is especially visible when the target object is Cylindrical and/or Spherical, and lit by a single Light source (Sun Lamp).
It looks like this behavior doesn't affect the Displacement node, since using it in Bump Only mode instead of the Bump node gives different results (they should both give identical results, as I understand it).
From looking at other renderers (and aesthetically speaking) it looks like the way the Displacement node does it is supposed to be the correct one.
Exact steps for others to reproduce the error
A test Blend file is provided.
Simplest way to reproduce it from scratch is to put a simple repeating height pattern on a smooth cylinder, using a bump node and a simple diffuse node, and lighting that cylinder from the side with a single Sun lamp.
The behavior of the Displacement node can be tested (and compared with the Bump/Normal) by plugging the same Height pattern into a Displacement node with the Shader Option set to "Bump Only", and plugging that displacement node into the Material Output.
I was initially thinking it to be related to (or the same issue as) this one :
#95729
But it has apparently been fixed (#105776), while the clamping is still clearly visible even in very recent builds.
Can't quite make sense of why is that, but I'd assume there's will be some differences between normal or bump map from a height map. Also, from a quick level adjustment on that screenshot, you can see the difference actually starts at the very beginning (as opposed to some 60 degrees position), it's just not that visible.
Between a Normal map and a Height map, I'd agree (in theory, it should be identical, but some small degree of differences would be expected), but as I understand it, the same height map used in a Bump node and in a Displacement node in "Bump Only" mode should give identical results, as the operation done in the node should be the same, barring some normal-space shenanigans necessary for displacement maps. And for the most part, at angles perpendicular (or close) to the light-vector, it is the case.
By the looks of it though I don't think the issue stems from the way the Normal/Height information is interpreted/converted, but rather from the way the shader somehow deals with it. Using a Bump node and a Normal map Node passes the information through the shader node, which the displacement (if I understand it correctly) does not, which could be where the issue comes from.
A rather dramatic example of the issue (and the way I found out about it) :
It's clearly visible here that, when using the Displacement node, a set angle (after the Bump has been accounted for) from the light always gives the same "lightness" level, while it looks like the current "lightness" level is calculated only using the geometry, and not the Normal/Bump info, resulting in something that looks like clamping...
That is correct, using extreme Gamma it starts being noticeable around 33° from the light, but it is likely present at all angles...
Please note that, while #110271 's issue may be fixed in the current version, i'm still seeing the same erroneous behavior on the Bump node in the latest 3.6.1 stable release, so #105776 likely wasn't the culprit.
