We have a 3D model in ZBrush and we extract the UV map from it. Exporting it in SubstancePainter and trying to set the textures however , leaves white spots in some places. I have seen that depending on the size of the UV Map the white spots appear in different areas. What could the reason for this behaviour be? Computer Graphic Perspective , what is the reason for this problem and what would be a good idea to overtake , in order to fix it ?
First of all, you might use something like ShareX to easily screenshots and screencasts: it's better than taking photos of your screen;
Next, to my knowledge Zbrush UV Master doesn't provide production-quality UV maps, it's more like a starting point — and mostly for organic models. With hardsurface models generally you want for straight edges of a model to be straight on the UV. This is the first issue with this generated UV — all straight UV islands are rotated which will create weird looking artefacts on lower resolution textures.
Second issue comes from the first one: generally you want UVs to be packed as tight as possible to not waste image space and because your long straight islands are rotated, there's a lot of wasted space which means that you have to have an image of higher resolution to achieve a decent result.
And the third issue comes from the way ZBrush packed the islands. UV islands should always have a bit of space between them (called edge padding) because when the texture is downsampled, pixels around the UV island will start to bleed in these islands. Here's a good explanation of what's happening, I'm going to paste just one image from this Polygon artice:
A 512x512 texture with no edge padding. White pixels are the used areas (shells), black pixels are the unused parts (gutters), and the UV shapes are shown in pink. As the texture is downsampled, pixels from the gutters bleed into the shells.
To sum up. You have them like this:
But you need them like this: