Shading issue with opacity maps and close distance between surfaces

[greymiura]

I often use duplicated parts of the original surface geometry to create decals, with a distance of 0.1 or even 0.01 millimeters between both of them. There may be other methods but most of the time I'm happy with this one.

Recently I prepared a device roughly the size of a coffee machine with a logo and some writing applied to it, both with opacity maps. The product renders looked fine, but when I integrated the device into a rather large scale setting with a camera distance of about 15 meters, some shading issues started to occur in the transparent parts of the decals. And at some point these transparent parts become totally black.

I isolated the issue in this little scene, just move the camera back and forth and you'll see what I mean.

https://we.tl/t-jeytsFkP9G

You can fix it by increasing the distance between the decal and the underlying surface, but nonetheless it would be interesting to know what this is. Is there some sort of ray bias parameter to fix it? And why are only the transparent parts of the material affected?

[viktor_angelov]

Hello greymiura,

This is happening because the transparency and the secondary rays in general affect the scenes differently. There are various factors that could influence the outcome in this case. We highly recommend you to not use this workflow!

We would encourage you to use V-Ray Decal.

Also, increasing the Secondary Ray Bias could help the GI to calculate scenes that contain overlapping geometry. Be aware, that you might need to assign a 2-sided material to the overlapping objects to make it work.
Please, let it be known that even after doing so success is still not guaranteed. This workflow creates unexpected results, more often wrong than correct, and is best to be avoided.

[Buck]

you could try increasing the secondary ray bias, yes.

[greymiura]

Buck, thanks for your reply! Unfortunately that makes things even worse.

[viktor_angelov]

Hello greymiura,

This seems like it is an overlapping issue. Both objects are simply too close to one another.

Could you let us know why do you need a duplicated part to use the V-Ray Decal on? Could you not simple apply the Decal over the original part?

[greymiura]

Hey viktor_angelov,

thanks for your reply! To be clear, this is not a V-Ray Decal, just geometry with an ordinary alpha mapped material applied to it.

Why I do it this way? I admit that the main reason is laziness. I don't like creating complex uv layouts of curved cad surfaces with several imprints applied to it. And I don't use V-Ray Decals in this case because a flat projection is sometimes not what you need and if you're switching engines for whatever reason you have to start all over again. Don't get me wrong, V-Ray Decals are great, but for this purpose I prefer these little geometry patches.

I agree that this is some sort of overlapping issue, but why doesn't it affect the opaque parts of the material as well like the shading issues that occur with perfectly congruent surfaces? In the opaque areas V-Ray is still able to determine which surface sits on top, regardless of how small the distance gets.

[viktor_angelov]

Hello greymiura,

This is happening because the transparency and the secondary rays in general affect the scenes differently. There are various factors that could influence the outcome in this case. We highly recommend you to not use this workflow!

We would encourage you to use V-Ray Decal.

Also, increasing the Secondary Ray Bias could help the GI to calculate scenes that contain overlapping geometry. Be aware, that you might need to assign a 2-sided material to the overlapping objects to make it work.
Please, let it be known that even after doing so success is still not guaranteed. This workflow creates unexpected results, more often wrong than correct, and is best to be avoided.

[greymiura]

Dear viktor_angelov,

thanks for your detailed reply! Good to know, I'll definitely keep that in mind for my next stickers!

Best regards & have a nice day!