A: it is possible to bake the normal map of the asset plus decals as Object/World space?
DECALmachine does not bake the normals of an asset, it only bakes a flat decal normal map.
You are suppossed to combine the flat decal normals with the asset normal map on your own. This can easily be done in a substance product are even in Photoshop.
Baking them in object space could be done but I see no benefit in doing sot.
B: There is any issue if you use decals on assets which use material with a normal map already included?
Not that I’m aware of.
One nice trick for a normal map without skew and waviness is baking as Object/World, then convert it as a Tangent, sadly the Blender baker can’t do that conversion but Xnormal and Substance Designer can do it. So I wonder if there are some room to use that trick (baking as Object/World through DecalMachine, OR baking the decals normal map inside another normal map you’r using as a material) along with DecalMachine without encountering crappy bake result. https://www.artstation.com/artwork/ba3Qav 2
Baking in object space does not magically get rid of skewing. Baking in object space is an old technique used in cases the baker’s tangent base does not match the target engine’s.
These days, everyone uses MikkT I think.
To get rid of skewing in your high to lowpoly bakes, you can paint it out in tools like marmoset or substance designer(painter?).
In the DECAmachine workflow, the decals would sit on the lowpoly object, not the highpoly. Maybe, after you’ve baked your main normals from high to low. Since the decals then follow the lowpoly object perfectly, there won’t be any skewing, when you bake the decal normals.
You’d then have two normal maps, that you can combine.