Inverting a bump texture that has UV tiling disabled produces borders around the texture

[Alex_M]

I have a texture that I want to use as a bump map. I need it to be a decal so I disabled the U & V Tiling in the texture's Coordinates tab. Since the B&W colors of that texture are the other way around, I also needed to swap them with an Output map that has the Invert option enabled. And here comes the issue. If I invert the texture's colors and have the U & V tiling in the texture disabled at the same time, a border appears around the texture in the rendered image. This doesn't happen with the CPU render engine. Please see screenshots below.

[hermit.crab]

There must be another factor for reproducing the issue since it works fine with the two mentioned (tested both pre and post hotfix). Could you attach the scene here or send it via the contact form so we can take a look?

[Alex_M]

Hi Aleksandar,
No problem. Please find attached the simplified Max file.

[hermit.crab]

Thank you. There seems to be a difference in how CPU and GPU handle the background of non-tiling bitmap. In GPU, the diffuse color is always used, whereas CPU seems to adapt and set it to white (when inverting). I am not entirely certain which behavior is correct; I'll ask the devs and write back. However, I did figure out a workaround - inverting the texture through a VRayCompTex (set to Difference) by combining the texture with a white VRayColor. This makes the background white as well, eliminating the issue.

[Alex_M]

Thanks for the thorough explanation! I will be using this clever workaround until then.