# Concept of Reversed Faces and Purpose in SU

I have no knowledge of what the concept of reversed faces is nor do I understand the purpose of same.

Could someone point me to a wiki or tutorial that will explain same so that I can better understand their role in model drawings?

TIA

In brief, every face has two sides. Imagine a cube, for example. It has six faces enclosing a volume. In order to know which is the inside of the box and which is the outside, each face needs an orientation. By convention, the blue shaded faces are on the āinsideā and the white faces are on the āoutside.ā Mathematically speaking, outside means that the normal vector to the plane points āoutwards.ā When constructing geometry, sometimes the faces get reversed. Like having five white faces and one blue face on the cube. By reversing the blue face, the orientation flips and the cube can be treated as a solid and its volume can be computed based on SketchUp knowing which is the inside the cube and which is the outside. In addition, 3d printing software makes use of the orientation to treat the joined faces as a solid. Similarly, rendering software typically only deals with the āfrontā faces and ignores the ābackā faces.

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To extend you cube example,
if you put a smaller cube inside the original,
White faces out and it would be a solid within a solid
Blue faces out and it would be a void within a solid.

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Brilliantly clear and really useful descriptions

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OK, I understand the point of 2 faces and even guessed that they functioned differently.

Is there any reason that sometimes the āinnerā face appears in drawing a fresh āobjectā and on other occasions the āouterā face appears? Is there some inferencing going on that Iām not picking up on?

There is some attempt to figure out what you meant, but it sometimes goes astray. The best course is to draw in monochrome style so reversed faces are evident, fix them at an early chance, and just move on. Whatever logic is involved is beyond me.

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I noticed something else since my last post. I drew a figure that appeared with face reversed. I āpulledā it up to a thickness and the apparent face changed from āinnerā to āouterā which I didnāt have to reverse before applying a color/texture and I had to change the reverse to an outer face. It was as if pulling the face to a thickness caused SU to infer that the pulled faces would be outer and inferred the face left would remain and inner face. It seems to me if it inferred that the pulled face was outer it might have inferred that the remaining face was also outer. Iāve noticed this before which seems to add to the number of surfaces that have to be reversed. During the same drawing activity, I noticed a different object portion had reverted to the āinnerā face without my action. When I reversed it, a color/texture previously applied to that portion appeared.

Itās these anomalies that led to me questions about faces.

UPDATE: I misspoke. I erased and redrew the object that I extruded. When I did so the face did change from reversed to outer AS DID THE āBOTTOMā (which is what I misspoke about above. I was able to select the entire object and apply color/texture that appeared on all āsurfacesā.

One common reason for reversed faces is when a larger surface is divided into smaller regions and then one of them is extruded. Itās a common thing when users are drawing floor plans like this. (Note the green is my templateās default back face color.)

In a case like this there wonāt be any faces at the bottom of the walls, either.

The thing that is going on here is exactly the same as if you were pushing a recess into a face but the direction youāre viewing it from is opposite. The keey thing is to consider what is going on to result in the reversed faces and either back up and fix the geometry before extruding it or reverse the faces if that is appropriate. Donāt leave those reversed faces and donāt apply materials to them to hide the incorrect orientation.

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Thanks, Dave!

I think Iām beginning to get my head around it. I have a tendency to ignore faces and jump right to coloring or texturing. Iām going to have to train myself to leave no reversed faces unless Iām certain that they are to be inner surfaces.

Just think of me looking over your shoulder and rapping you on the knuckles when you leave sloppy modeling in place. Clean up as you go.

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There is a native tool that can help, Orient Faces.
You select one correctly orientated face and right click to activate it. If the shape is fairly simple Sketchup will orient all the faces correctly. It is not infallible and does strange things on very complex geometry, but generally works well on good clean modelling.

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Dave, my knuckles are sore already!

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Another tip that @DaveR mentioned in passing, but is worth having explained is that many seasoned users of SketchUp set their default style to apply a bright garish color on back faces, something seldom used and more obvious than the stock grey/blue. I use neon green, I have seen others use a fuchsia or electric purple. That way any reversed faces scream out to be corrected as you model.

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Admitting that Iām as dumb as a box of hammers, I was unable to find how to change the default colors mentioned. I went through all the options in both Model Info and Preferences and failed to find any that would change my default blank model color defaults.

???

The current Style sets up the default-colorsā¦
So note how these can vary across scenes, which use different Stylesā¦

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After you make any edits to the style be sure to click on the large thumbnail to update it.

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OK, got it done!

Now for another dumb question: Why arenāt Style options with or under Preferences or Model Info? I almost never use the tray for anything but Materials and Components. Iām just puzzled by the choices for navigation.

I canāt answer that question. It really shouldnāt be a Preferences thing because itās specific to the model. Remember that a template is simple a SketchUp file that normally has no model in it until you add one. That is unless you leave the scale figure in the template. I guess it doesnāt belong in Model Info, either or if it was in Model Info it would bloat that section because there are so many options. Styles are closer to materials and components. There are collections of styles and you can make more of them.

You really should be keeping Entity Info open all the time because that gives you a lot of useful information.

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Styles are not a global preference that applies to every SketchUp model you make or even to one single model. You can have many different styles present in a single model, all assigned to different scenes. In addition it is very common to be switching styles around and employing different styles for different reasons within a given model. Itās a complicated powerful tool pallet that adjusts a lot of different visual parameters for how your geometry is displayed. If you play around with some of the different styles that are included in the the stock setup, you will see why all that info deserves its own pallet to control it.

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Still a lot of hammers in the box (not Box )

Placement of options is always a trade off and works best when they enhance learning and ease-of-use.

Iāve now found a style (Pencil on Tracing Paper1) whose background color I like. I would like to create and save this background color as a template. Iāve been able to add the style to an empty drawing file but it comes over with the pencil line flourishes which I donāt want.