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Smart UV mapping provides significantly different results since C rewrite (Island Margin non zero)
Confirmed, NormalPublic

Description

System Information
Operating system: Linux (NixOS)
Graphics card: Intel

Blender Version
Broken: 2.91.2
Worked: 2.83.5

Short description of error
Smart UV mapping seems to work significantly worse (at least for my test case) since the C rewrite, which landed in 2.91.2 (I think).

Before:

After:

Test mesh:

Exact steps for others to reproduce the error

Perform Smart UV mapping on the mesh, as detailed in this video: https://www.youtube.com/watch?v=illIxYKb-1k

Event Timeline

Philipp Oeser (lichtwerk) changed the task status from Needs Triage to Needs Information from User.Feb 25 2021, 1:57 PM

Please try a fresh build from https://builder.blender.org/download/

There have been a couple of fixes, now I am getting matching results (first is py version, second is C in current master):


Do you see the same results now?

Please try a fresh build from https://builder.blender.org/download/

There have been a couple of fixes, now I am getting matching results (first is py version, second is C in current master):


Do you see the same results now?

I just tried with 2.93, and I get the same results as the older version.

Philipp Oeser (lichtwerk) changed the task status from Needs Information from User to Confirmed.Feb 25 2021, 5:35 PM
Philipp Oeser (lichtwerk) added projects: Modeling, Restricted Project.

Watched the video now.
That is because of the Island Margin of 0.03, if I set it to 0.00 all is fine in the C version.

But you are right, in this particular file, the python version could still keep up, even with a Island Margin of 0.03.
Will confirm for now, needs some further investigation.

Philipp Oeser (lichtwerk) renamed this task from Smart UV mapping provides significantly different results since C rewrite to Smart UV mapping provides significantly different results since C rewrite (Island Margin non zero).Feb 25 2021, 5:35 PM