Dealing with Flipped Normals and therefore textures painted on the backface of model

I spoke too soon…

The issue with select Same direction faces is that it will select faces that have correct normals and when i hit Fix reversed face material it’ll flip faces which it shouldn’t/which have their normals in the correct direction

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‘Material Tools’ and ‘Remove all back face materials’ can be effective in getting rid of those unwanted materials once all your orientations are correct.

Maybe this step is of use in your workflow.

Does ‘Select same direction faces’ select all the required faces as you require - or just faces that share the same orientation relative to the original selection? E.G Faces pointing in ‘Y’

So It seems to be a bit temperamental. Sometimes it selects just the faces i need and others it doesn’t.
Here it works

And here it correctly selects one side but when I orbit around it has selected the other side faces based on the same axis

I’ll keep experimenting

Try Fredo Tools, Auto Reverse Faces.
Fredo Auto

@Box

Thanks I do use Fredo Tools to flip the normals. However as I said the problem ISN’T flipping the normals.

So If i was to flip the faces then I wouldn’t be 100% sure which faces I needed to fix material wise as the materials are applied to the backface and not the front face.

I hope I’m explaining myself well.

Fredo’s tool flips the faces and puts the the same material on them.

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@Box Fredo tools did an okay job. It flipped about 80% of the faces but then out of those 80%, about 50% didn’t flip the material.

I mostly blame the poor fundamentals of this model which is why the plug-ins are having a lot difficulty with it.

The experimenting continues…

Oh well, that’s why we keep telling people to model cleanly and keep on top of reversed faces.

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If the mesh isn’t manifold it’s impossible to consistently flip faces to ensure they are oriented consistently. Too much ambiguity.

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My ideal solution here would be to be able to marquee select all backfaces and then it would be easy to fix

*Of course I mean the Backfaces visible to the camera

That would be possible. My Vertex Tools extension has an option to not select vertices for faces with the backside towards the camera:

# If this returns true - the backside is facing the camera
view.camera.direction % face.normal > 0

(I hope I didn’t invert the logic here, but you’ll quickly discover that.)

Hi,

So SketchUp is definitely not my strong point. Only really use it to clean up models and export into other software.

I tried out your Vertex tool with ignore backface enabled. Does your script here:

…help just select faces then.

Part of the problem of so-called “flipped” faces is that Sketchup isn’t all that particular about following the “right-hand rule” in normal orientation. Years ago, I played around with 3D Max, a very early version, and was instructed (properly) to always create faces in a counter-clockwise direction so that the normal would face “out” of the surface. Sketchup, however, seems to ignore this, blithely creating inside-out faces NO MATTER IN WHICH DIRECTION the edges of those faces are laid down. To make matters worse, the push-pull tool often turns entire geometries inside-out.
Sure, if these anomalies are caught and rectified early enough, a model that contains faces whose normals always face out can be achieved.
But then, orient faces can muck that all up.
I have the idea that if the construction is a “legal” solid, this isn’t an issue. But it is way too easy to create acceptable geometries that are not legal solids. Acceptable, that is, until applying materials for rendering engines produces problems such as Keith describes.
Then, too, one can apply a material to a poorly-formed group or component, and none of the faces within that group need have any material applied at all. But if they do…applying a material to the group will not affect those faces…and if they’re inside out … Keith, I do not envy you.

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Is it correct to state that a back face simply should never be visible? The fact one can see a back face indicates it is wrongly oriented, correct?

It depends on the nature of the model. If the model contents are solids, yes there should never be a visible back face no matter how your orbit the view. But if the model has any single surfaces, there may be directions from which you can see those back faces. They are not physically realistic (nothing real has zero thickness) but they are legal in a SketchUp model.

Ok, but those will always be troublesome for renderers, typically only rendering the front face?

Yes, they will.