Manually Placed Seams¶
Flatiron gives you the option to preserve already existing seams during the unwrapping process.
When you apply Flatiron without checking the ‘Keep seams’ checkbox it will discard all seams for the map channel and create its own set of seams to unwrap the selected meshes according to the given specification. If you check the box to keep already existing seams Flatiron will use the already existing seams fo the given source map channel to unwrap the meshes.
Please consider that manually placed seams and the specified stretch limit might be incompatible with each other. If the desired stretch limit cannot be maintained with the existing seams, Flatiron will introduce additional seams in order to comply with the limitation. In order to resolve this issue you can either introduce additional seams yourself or relax the stretch limitation. With a stretch setting of 1.0 Flatiron will try to use your manual seams exclusively, no matter how extremely it will stretch the charts. New cuts are only used when it’s absolutely impossible for the algorithm to unwrap the meshes at all with the existing seams. With a very low stretch setting Flatiron will usually have to introduce a significant amount of additional seams to comply with the stretch restriction.
Depending on the subsequent processing of the UV map you will have to find a compromise between the stretch limitation and your seam placement.
Flatiron will remove a manually placed seam if the algorithm considers it to be redundant. This usually happens with seams that don’t separate charts completely but just cut a bit into the surface in areas where it’s not required to spread the surface to maintain stretch requirements (i.e you make a partial cut into a plane). When the two adjacent sides of a seam end up lying directly next to each other on the UV map they will be attached to remove the redundant gutter padding between them.
If you use the polygonal unwrapping mode but have manually placed seams that split polygons along their inner triangle edges, those seams will be kept even though they violate the polygonal unwrapping constraint. If Flatiron introduces additional seams they will conform to the constraint and not separate polygons further.