The mystery of the suddenly reversed faces

Usually I’m pretty careful about face orientation. I minutely inspect a new drawing to ensure the faces aren’t reversed. It’s always possible some tiny, hard-to-see face may slip through the cracks, but usually I get them all. So it was a surprise to say the least when I inadvertently deleted all my materials (don’t even ask…) only to see my model has a bunch of reversed faces. They’re a small percentage of the whole, but still way more than there ought to be, considering the care I took. They are distributed among multiple groups and almost random-looking.

All I can think of is that I’ve used Purge/Fix and cleanup3 a few times. Could either or both of these cause the random faces, or is it likely to be something else? (This is all modeled entirely in SUP 2022.)

You might try editing your default style to change the back face color something that is easier to see. Some people use a pink or magenta color. I use a green that I wouldn’t use as a material and that I find easy to see even when the faces are very tiny.

I expect it’s something else in the work flow. You might have induced them with some step in your modeling process.

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Good tip on the colors! I’ve always stuck with the default white/grey and never even thought of altering those.

As for the work flow, I honestly cannot think of anything that would mess with the faces. Like I said, I complete a drawing, inspect it all over and inside out for reversed faces, then add the materials (always plain solid colors, so no overhead), make it a group and then add the new drawing to the model.

Years ago I had a work-issued laptop with a lousy display. The shaded white front faces would appear the same as the default blue back face color so I changed it to make them easier to ID.

Maybe you’re doing something that results in two faces sharing the same space and CleanUp is removing one of them. I don’t know how it happens but I’ve run across it in models I’ve gotten from others.

FWIW, changing the Face Style to Monochrome would make your back faces visible without removing the materials.

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also solid tools. I’ve had the case in a few complex models where using the solid tools would flip a face for no reason. I guess a pair of faces was close / on the same spot maybe.

yep. I model in the regular style, but as soon as I apply the first colour / material, I’ll revert to monochrome to further the 3d work. it really helps thinking about your volumes and not the materials.

As of default colours, I based my default template on the Blue style years ago, white faces on grey background would burn my retinas at night. Dull blue recto, darker dull blue verso, like Dave, a pair of colours I would never use in any project, but still contrasted.

Maybe the native Solid Tools. I don’t use those. Haven’t ever seen reversed faces with Enroth Solid Tools, Bool Tools 2, or Trim and Keep.

You don’t need to delete your materials to see if you have back faces exposed, just select the monochrome style and you’ll be able to see that.
I don’t know why but sometimes when I intersect geometry some faces get flipped, I either reverse them or select a well oriented face and on the contextual menu select orient faces.

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Thank you for repeating what I wrote an hour ago.


A solid tool extension will reveal them, there is one from thomthom, Solid Inspector2.

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You’re welcome

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Dave, I’m either stupid or blind (or both) but I can’t find a monochrome style. Is it included with the default install or is it an add-on?

EDIT: Just discovered I get the same effect by selecting “Display shaded using all same” option under the Edit tab of Styles.

There are several ways to get quickly to Monochrome
One is to go View/Face Style/Monochrome
Another would be to activate the Styles Toolbar, so that you can easily click Monochrome whenever you want, or xray or hidden line or shaded with textures etc

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@Saul, … and while you are at it, create shortcuts to get to them at any time.
For instance:
[Alt] [T] = 'Shaded with Texture"
[Alt] [M] = ‘Monochrome’
[Alt] [X] = ‘X-ray’


So, Fredo Tools AutoReverseFaces does a pretty great job of fixing the problem; only I notice some parts of the drawing that previously were not affected now have some odd face reversal issues, especially (semi-)cylindrical elements. Whereas before they were correct, there is now a kind of “striping” going on, where the only fix is to draw lines to isolate the reversed area, reverse it, then hide/soften/smooth the lines.


Is this common or normal?

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If you turn on “hidden geometry” then select a white front face, right click and select “orient faces” does it fix the problem? I will admit once in a great while the “orient face” command seems to go crazy on me and mixes the faces up all different directions. It doesn’t happen very often though when it does running Solid Inspector 2 will clear things up. Also to clarify things I run older versions of SU.

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Not sure about the Windows interface, but on Mac I customized the tool bar like this. I click that button to check faces a fair amount. It also helps to have a saved style and select that for checking.

Screen Shot 2023-08-12 at 10.34.25 PM

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That striping on a cylinder would make me look inside to check for extra faces.

you don’t need to draw lines, in fact drawing lines could be causing the problem as it may be creating internal faces, turn on hidden geometry to work with the individual faces.
Hidden geometry on and off should be set as a shortcut key so you can quickly use it as needed.

In certain situations drawing an edge can create an internal face, faces like that will confuse face orientation.
GIF 13-08-2023 12-49-54 PM

But hidden geometry lines don’t form borders of regions that can be filled or reversed. So far as I know that can only be achieved by drawing solid lines vertically between the face segments (in the case of my example). Then I can reverse or fill those newly created areas, before hiding/softening/smoothing the solid lines.

This is hidden geometry.
GIF 13-08-2023 1-02-56 PM


Sure, the reversed areas are bounded by hidden geometry lines; but to remedy this, solid lines must first be drawn over the hidden lines.