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Does anyone know how to select all Z-Up only faces in this monstrocity please? I've selected a few as an example:

enter image description here

I've tried:

  • Select Similar > Normal: that selects Z-Down as well. I've also tried calculating normals from faces and repeating the step with the same result.

  • Select Similar > Linked flat faces: that seemed to select pretty randomly (I'm not surprised - this is entirely made of Ngons.

  • Selecting all side faces with Select Similar, seperating them out and then from a top down view box trying to select them, hoping they would obscure the selection of the bottom faces in ortho view and I could add them layer by layer to a selection group then hide them but for some reason this often selects Z-Down too.

  • Splitting the model at the edges and trying the above steps.

I'm guessing this would be simple in code if anyone has a snippet.

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  • $\begingroup$ If Select Similar > Normal doesn't work you might need to add your file here. As @RobinBetts answered and I commented there, if your geometry is correct without any issues, the Z down faces should not be selected. $\endgroup$ Mar 6, 2023 at 13:51

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After Alt N 'Recalculate Outside', maybe M merge by distance? and checking for all-blue with Overlays > Face Orientation, Shift G Select Similar > Normal should work fine.

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  • $\begingroup$ I agree, if all faces are correctly oriented, none flipped, none maybe duplicated or anything, then Select Similar > Normal should not select faces that are Z down. Just tried it myself to be sure. $\endgroup$ Mar 6, 2023 at 13:48
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    $\begingroup$ Hi @GordonBrinkmann! However simple a problem seems, I never post an answer without trying it myself, immediately before posting, I've been burned too often :D $\endgroup$
    – Robin Betts
    Mar 6, 2023 at 14:16
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    $\begingroup$ Yeah, this was more directed to the OP ;) $\endgroup$ Mar 6, 2023 at 14:23
  • $\begingroup$ @RobinBetts Yup, Overlays > Face Orientation identified the problem. Looks like I've got some manual normal flipping to do, but still quicker than manually selecting all those faces (there are 3 other similar models to go!). On that note, is there any way to select all non-correct normals? They still exist after a recalc. Thanks. $\endgroup$
    – Absinthe
    Mar 6, 2023 at 14:26
  • $\begingroup$ @Absinthe .. IMO, depends on the failure of recalc, and whether you want to keep the characteristics that make it fail. Maybe share a representative file on blend-exchange.com. $\endgroup$
    – Robin Betts
    Mar 6, 2023 at 14:38

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