Smart UV project sometimes doesn't align the islands properly #116355
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Reference: blender/blender#116355
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System Information
Operating system: Win11
Graphics card: 4090 RTX
Blender Version
Worked: (3.5)
Related issue
#110724
Short description of error
I've reported problems with the "Smart UV Project" operator's auto-align of the UV islands and they were fixed for the model that I supplied last time but now the issue seems to still be there for another mesh that I share here. Bug_Smart-UV-Project_Still-A-Problem.blend
Exact steps for others to reproduce the error
Open .blend, in the 3D view go to UV > Smart UV Project > OK
Then you can see that some of the islands are aligned horizontally and some are vertically. I am not sure why but they should all be aligned vertically and they used to be in Blender versions before 3.5.
Blender 4.0
Blender 3.3.12
cc @Chris_Blackbourn @brecht
That last screenshot looks odd, but the model for that is not attached so I can't investigate that. Can this be attached?
For the other case, it's not obvious that all islands should be oriented along the longest axis. For this model it makes sense. But it's not necessarily the most efficient way of packing, or the direction that a user would consider natural.
I don't think either the new or old packing method was designed to do this, it was mostly a side effect where depending on the model it may or may not work out like that.
The Pack Islands operator in the UV editor is the place where there is more control over the packing strategy. Using a different Rotation Method there seems like it orients the islands along the longest axis, but I think this is also somewhat accidental. Adding a rotation method that does this explicitly would be a good feature to add.
It's not obvious to me that Smart UV Project should be changed to work like that, whatever strategy we use there will be suboptimal for some model.
@ideasman42, do you have an opinion on the packing strategy for Smart UV Project?
Smart UV Project (the original Python version) intentionally aligned islands vertically, for wood-grain for e.g. tended to be aligned more usefully - in general, I'll check on adding support back.