What I want to achieve is to use the circles, for example to apply them in opacity and thus have a perforated steel mesh. As you can see the same material applied to a box works fine, but it doesn't work applied to the Decal on the green box.
Hi.
It seems to be either wrong material setup, either incorrect mapping.
Corona Decal does not support real world scale currently. Decals use actual 1x1 UV space for the texture, even if the the texture is set to use real world scale.
What is set in the real world scale size is divided from 1. If 1cm is the system unit - 1cm in real world scale will be 1/1 = 1 tiling, 2cm will be 1/2 = 0.5 tiling in UV space.
See the images attached.
Try using decals without the real world scale mapping, for now.