Procedural maps (noise, vraynoisetex, cellular...) generate inverted bump with CPU

V-Ray V-Ray for 3ds Max Issues
1

Hi,

I absolutely don’t know how it didn’t catch my attention up until now but I’ve just realized that a lot of the procedural maps render flipped/inverted when used as a bump input in VRayMtl, meaning that the dark color is convex and the bright color is concave.

Here is the incorrect result (bright is concave, dark is convex, the transform gizmo shows the direction of the sun):

Bump from noise bad result.jpg
Bump from noise bad result.jpg937×533 83.6 KB

And here’s how it should look like (bright is convex, dark is concave):

Bump from noise expected result.jpg
Bump from noise expected result.jpg928×539 83.1 KB

I’ve tested some procedural maps using Vray CPU and the results were:

  • Noise, VrayNoiseTex, Smoke, Splat, Speckle - bump result is always flipped.
  • Cellular - bump result is correct as long as the bright color is in the top and the dark is in the bottom of the three input colors (!).
  • BerconNoise, Gradient Ramp, Tiles, Checker - bump is correct.

Further testing with different rendering engines, different materials gave some mixed and inconsistent results:

  • Vray GPU - bump is correct!
  • Vray CPU with Physical, OpenPBR materials - bump is flipped
  • Arnold - bump with Physical material is correct
  • Art - bump with Physical material is flipped

I also tested some tricks like pushing the map via VRayBump2Normal->VRayNormalMap but the result was the same.
This shows that the resulting bump from procedural maps depends mostly on how the rendering engine interprets it. This is especially confusing if someone uses procedural maps with the Composite or VrayCompTex maps to add some randomness to the material and expects that the color of the map will blend the same as with the regular bitmap when you plug it into the bump input of the material.

The solution I found was to set the Bump Amount in the Output tab of the map to -1 thus reversing the flip. But how does it work with the Composite shaders still confuses me.
The other solution would be to use VRayColor2Bump but the docs state that it’s slower and it’s purpose is to fix “some difference” between CPU and GPU and I wouldn’t consider the opposite bump effect as “some”. :wink:

Since it’s been like that for a long time I’m wondering if it’s a bug or a feature. If it’s on purpose then I’m really curious why. At least it should be mentioned in the docs in every section related to bump which maps are affected and need flipping.

EDIT:
Tested on 3DSMax 2026 with VRay 7.30.02 and 3DSMax 2023 with VRay 6.20.06

Best regards,
Przemek

Bump from noise bad result.jpg
Bump from noise bad result.jpg937×533 83.6 KB

2

It’s a known thing. For V-Ray CPU, we just sample the 3ds Max texture, and afterward, V-Ray calculates the bumped normal. Scanline exhibits a similar behaviour. Arnold probably does some inverting internally. The workaround, as you’ve already found out, is to use VRayColor2Bump after the procedural texture or stick to negative values for the bump amount.

3

Thanks for clearing it Alexandar.
Soo.. is there any reason behind it apart from being compatible with scanline in the early days? :smile:
I’ve been using VRay since version 2.0 and the fact that I never noticed that behavior just blows my mind. I wonder what else did I miss… :shock:

4

It’s more of a 3ds Max thing that we “inherit”. I’ve never noticed it as well…