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Carry over from a youtube video comment here

In the video, the presenter manually fills in an example with edges to make faces. A commenter suggested using Alt-F to fill and then Limited Dissolve to reduce the mesh. As this is a video on precision modeling, I was surprised to see vertex positions of the bevels and circles on a simple 10mm plane with 4 holes were changed. Is there a way to dissolve edges without changing vertex positions?

Blend file for reference; 1 degree + 5 degree dissolves

Edit: Thanks to Robin and taking a second look, I realize now that Blender uses single precision floats instead of double... so that explains my scripts output differing. Don't use 5 degree (default?) with Limited Dissolve and you get great output.

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    $\begingroup$ Hi! Well, this is a mystery. I can get no changes of location, only deletions as you would expect, in a diff. output. 2.8, 2.9, your script and mine, all steps. I've added my script to your file, here, just in case it makes a difference to the diff. for you. If the problem persists, ?? $\endgroup$
    – Robin Betts ♦
    Commented Sep 21, 2020 at 19:35
  • $\begingroup$ Hey Robin! Here's my findings: Limited Dissolve at 5 degrees leads to large changes that you can see in the example blend file. 1 degree leads to smaller, but measurable differences. Selecting all but a handful of edges and dissolving them leads to nothing moving (yay!), and of course hand-drawing the lines has the same results. Reloading the file will lead to differences measured only in floating point values (5th decimal or 100,000th of a millimeter) which is expected with floats. $\endgroup$
    – jtmcdole
    Commented Sep 23, 2020 at 2:59
  • $\begingroup$ Your script has better output; I'm happy to stick with 1 degree or less on limited dissolve. $\endgroup$
    – jtmcdole
    Commented Sep 23, 2020 at 3:20
  • $\begingroup$ @RobinBetts - if you leave an answer, I'm happy to mark it as the solution. Thank you for your time! $\endgroup$
    – jtmcdole
    Commented Sep 23, 2020 at 3:38
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    $\begingroup$ Glad there's a satisfactory-(ish) outcome! You found it.. you can answer your own question, and mark it as accepted, if you prefer. It will be useful to others who run into the same anomaly. $\endgroup$
    – Robin Betts ♦
    Commented Sep 23, 2020 at 4:52

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Thanks to Robin, his script, and taking a second look, I realize now that Blender uses single precision floats instead of double. In the first python script, it is printing whatever python thinks (double precision).

  1. Don't use 5 degree (default?) with Limited Dissolve. This lead to original vertices being deleted.
  2. Less than 1 degree limited dissolve doesn't have this problem.
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