# How do you use front / back faces?

I’ve been pondering Front and Back faces, which have been a unique part of SketchUp’s design since the beginning.

I spend a lot of time solving problems with incorrect Front/Back face issues. Material quantity calculations, texturing and rendering issues, import/export issues, etc.

The only time I can think of a use for Back faces is the trick which allows a building or roof to roof appear invisible from one side (transparent face material on the back face) while appearing solid from the front. Though, I’ve only used that method a few times in 18 years (xray is my go-to).

I understand some extensions, such as Joint Push Pull, may need to adopt the front face as the ‘normal’ to be pulled (but I’m guessing).

So, I’m wondering…what benefits do you see in Front and back faces being different from one another?

Transparent back faces is a good one as you mentioned (interior inspection).
One maybe not that strong could be with section planes without section fill. Each hollow group can have a different color painted on their back faces to better distinguish between different materials these groups represent (brick, insulation etc.) Just a thought.

A face has a natural notion of “front” based on the direction of the normal vector that defines its orientation in 3D space. But you can orbit around and look from the opposite direction. What are you then looking at? Nothing? What happens if you assemble a box with all the front sides facing inward? If you look at it, does it seem not to exist?

No, because you see through the near sides to the back side of the opposite sides, or inside faces. Of course if you paint both sides of each plane in transparent then yes, it disappears.

I think the alternative is for Front and Back faces (materials) to be identical (ie, materials), not for there to be “void”

If you put a cube within a larger cube with the front faces facing out you effectively have a single solid, a solid within a solid. But if you reverse those faces you have a solid with a void space within. This type of thing is critical for understanding 3d shapes in a 2d framework. Other software, solid modellers, show ‘mass’, sketchup uses back faces to convey the same message.

2 Likes

You may have missed my point. I was asking in the context of “back sides don’t exist”, per the OP’s question. Back sides being transparent is a different situation. You can already do that by applying a transparent material to them. You can’t apply any material to something that isn’t there!

I was just being a bit silly, trying to visualize a version of the impossible inside out cube Yes, I see your point, the idea of a side that does not exist while the opposite side does is a Cartesian mind bender. Perhaps this is how the TARDIS works?

Or maybe one end of a wormhole?

I use them to create 2D floorplans. I reverse the faces of walls wwith Fredo’s reverse orient faces. This allows me to pushpull without painting materials

This is how I was hoping SketchUp would properly treat solids. Including a new “Material” attribute (not to be mistaken for the face painted material but the enclosed space)

No need to paint back faces to mimic material inside walls etc.

I produce a lot of survey drawings and models of semi-derelict historical buildings and often do a preliminary 3D scan of them using Polycam (a mostly free photogrammetry app that uses photos—or LIDAR if you have an iPhone or iPad Pro). The “transparent back material” trick is very useful when blocking out a rough model based upon that 3D scan:

If I reverse the sides of the outer box, and Also make the inner box have wrong direction faces, it’s still a solid with a void, and will cut just fine with the section tool. Maybe a problem in exports to other file formats, but I dont see the big problem inside sketchup, except maybe with materials.

It is a moot question. All 3D applications have to deal with face normal directions. Most of them only show the front sides so looked at from the wrong side the faces are invisible. Rendering applications either do the same, or render them black or some other uniform colour.

That SketchUp has tools to deal with these is a plus. Otherwise you would have to check, for instance, the order you draw edges to form faces. The only apps that get them automatically right is those that only support solid geometry.

Generally, if a model is to be kept within SketchUp, it lets you to keep modelling without caring about face direction. It only comes to play with exports to other applications and rendering, and, internally, in cases like outlined by @Wo3Dan .

It’s more expensive to render front and back faces, also but comes part and parcel with SketchUp always trying to keep things simple - even if it does lead to poor practices and confusion when they try to leave SketchUp land and visit Real-time world or the Rendering kingdom.

There are a few more “benefits” and they also have to do with face normals which has been mentioned before.
1: Rendering programs need to know the face normals. A lot of rendering problems here on the forum are caused by exposed back faces…
2: Slicing programs (3D printing) need to know the face normals to dicide on which side of the face is the print media and which side is air…

SketchUp doesn’t take face orientation into account deciding whether something is solid or not. But it should, as it is one of the requirements of solids!
Solid inspector2 does by the way…

I reread the original post, and I was struck by the phrase “front and back faces”. There is no such thing! There is a single thing, a face, which has front and back sides like a piece of paper. Which side you see depends on what direction you look from.

There are situations in which it is impossible to see the back side and uses such as rendering where back sides are ignored, but they are still there because just like that piece of paper the notion of a face with only one side is meaningless!

And the comparison with a solid modeler is specious because a “face” there is a patch of the outer surface of a solid, not a face as used in SketchUp.

3 Likes

Ah yes, you’re correct.

I’m used to saying “Front face material” or “back face material”
but in fact, it’s “Face’s front material” or “Face’s back material”