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Blender 2.79 & 2.8 - Limit Dissolve does not have the right 'Max Angle' when the Delimit mode is Material mode
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Description

System Information
Operating system: Ubuntu 17.10
Graphics card: GTX 1080

Blender Version
Broken: 2.79, 2.80 @ rB23284e4dde5, and 2.83 @ rBec116e3d492511298

Short description of error
Limit Dissolve does not have the right 'Max Angle' when the Delimit mode is both Normal and Material

Exact steps for others to reproduce the error

  1. Open attached blend file
  2. Press x -> l to limit dissolve
  3. In the tool options for limit dissolve, change it to Material Mode
  4. Notice how the circle's form is lost
  5. change the max angle to 0.015d
  6. Notice how the form is somewhat tehre now, but the angle is definetly not 0.015d

This is the same with the modifier (decimate / planar decimation), as well as blender 2.79 & 2.8

Event Timeline

{F5804179}whops forgot to attach the file! Sorry!

Sebastian Parborg (zeddb) lowered the priority of this task from 90 to 50.

I can reproduce this issue:

Before decimate:

After decimate:

I agree that the angles around the circle shape are much bigger than 0.015° after decimating. With an angle of 0.025° the circle is gone:

To illustrate, this is actually a single-arrow empty rotated 0.025° away from the Z-axis:

However, this could just be a misinterpretation of the "Max Angle". When you lift the green area up in the Z direction, everything starts working well. If the angle indicates curvature of the mesh (rather than angle between consecutive edges) the entire feature works just fine, and this is more a clarity/documentation issue than a bug in Blender.

I'll defer to @Campbell Barton (campbellbarton) here, it looks like he implemented the feature.