Extension request - -- front & back faces being equal material?

Discussion on 2024 has prompted me to raise an old idea…
And in 2023/24 this ties in with the recent Overlay mode for extensions.

Can we override SketchUp’s default mode of having front face materials being different to back faces?

IE whenever you apply a material to one, I would like the same material to be applied to the other opposite side of the face.
(And, of course, the face material would be calculated as having one face Area, not two)

I’ve thought long and had about this. Yes, there needs to be a Front/back face for certain extensions to work properly.
But the constant ongoing effort of chasing and correcting (flipping) front and back faces, to me, is not worth the effort. The use of Front and Back faces serve little to no purpose in day-to-day modelling. (except if you are modelling interior spaces and using the “transparent back face” technique to see through your model…that, I get).

For me, the default mode is particularly problematic on projects that utilise Face Material Area as a key piece of quantity information - eg Landscaping plans or urban planning models/maps. In this instance, faces with materials applied to both front and back are calculating incorrectly.


Nevermind. I didn’t catch that this is a request for an extension.

I suppose it could be a feature if enough people think it makes sense.

You’ll know better than anyone the number of times people are recommended to correct their front/back faces.
Aside from applying materials, and the odd extension such as Joint Push Pull, do front/back faces this affect modelling in any other way?

Yes but in practice for me, I almost never have exposed back faces either. I expect for most users improving their modeling technique would go a long way toward eliminating the issue.

Many rendering applications don’t properly render back faces even if they have materials applied to them. And for 2D printing face orientation is absolutely critical because it tells the slicer which side of the face gets print media and which side is toward air.

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There’s ruby script you can run to do this (And I use it after all’s said and done to make all back faces match the front face color.

This is the script I use:

model.start_operation("MATfront2back", true)
### In Model's faces
model.entities.grep(Sketchup::Face).each{|f| f.back_material = f.material }
### In compo/group's faces
  next if d.image?
  d.entities.grep(Sketchup::Face).each{|f| f.back_material = f.material }

But like DaveR said, you probably want to work on getting your front faces be front faces and back faces be back faces, regardless of making the front and back the same color.

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The concept of front and back faces is tied to the way that the edges of a face are sequenced and the normal vector to the face is defined in SketchUp (and other geometry apps). The edges go in a counter-clockwise direction around the perimeter of the face when viewed looking back along the normal vector. It isn’t possible for this to be true for both sides of the face at the same time. So, the idea of having front and back sides is fundamental, not just a requirement of certain extensions.

That said, if you always model paper-thin surfaces and can see both sides, it doesn’t seem wrong to paint them both with the same material. It does seem wrong to use this as a workaround for properly orienting faces, though.

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