Why does SketchUp have Back Faces?

Why does SketchUp have Back Faces?

Postby pbacot » Fri Apr 26, 2019 7:28 pm

Why not have all faces be just faces?
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Re: Why does SketchUp have Back Faces?

Postby TIG » Fri Apr 26, 2019 7:47 pm

In Sketchup a face needs to have a 'front' and a 'back'.
A face has a 'normal' - that is, a vector that's perpendicular to its plane.
So a face's front always has a 'normal' that projects directly in front of it.
When you PushPull a face it's always along its 'normal'.
Otherwise, how might SketchUp know which way to extrude the face ?

All 'faces' are by definition [obviously] 'faces' - but the direction of each face's 'normal' dictates which way that face is facing ! a.k.a its 'orientation'.
Two otherwise identical faces can differ by one thing - their 'normal'.

Try drawing two identical rectangular faces, and 'flip' one so it faces the other way [use View > Monochrome mode with a Style using a distinctive back-face color], the do some manipulations - like PushPull or FollowMe - and the resultant geometry will vary...

Materials can be assigned to a face's front or back [rarely needed for 'backs'] that material affects extruded faces etc...
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Re: Why does SketchUp have Back Faces?

Postby pbacot » Fri Apr 26, 2019 8:11 pm

So it helps make the tools like push-pull work. Though it seems pushpull knows what to do by the way you move the mouse. Doube-click push-pull wouldn't work but that sometimes gets confused as it is. I wondered if it was underlying CG principles. Some rendering programs know you have the wrong faces showing.
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