Still after years I don't understand why these 2nd layer when connecting two lines on a plane

This happened to me many times and I just go and delete the newly created surface
I never bothered to research the why and how to avoid it as I can’t explain it well
So here I am trying to create a walking path along, across some circles.
The line in blue is the tangent that I just created and bang it gives me that
2nd layer that I have no need for.
What is it that creates situations like that and how to avoid it
I know I am doing something wrong.
Sometimes I also have a VERY VERY hard time to cut out a shape of a face
it will not single out the space within my lines unless I cut into the face with a circle.
Like a created these spiral staircases within a glass cylinder
and when they were done the top created a face like a jar.
To open it up partially as to enter the spiral I only need half the opening
became a game of like chipping away at it starting with a circle and then cutting away edges of it with small curves.
I don’t know what to call the problem/issue so it’s difficult to look for the solution.
So today here I said, finally I will ask for it in this fantastic forum that always shows me the way.
Thank you

Context. it’s always a matter of context.

The reason why sketchup is making faces whenever it can is because it NEEDS faces to switch from 2d to 3d. so whenever you make a line, arc, or sneeze to loud next to sketchup, it’ll spit out some faces.

now, why did some faces appear in your drawing ?
well because there were none off course !

the textured face you’re using as a ground is (I’m guessing judging from the Z-fighting happening) inside a group or component. That makes it in a different context.
But you’re not drawing inside this group, you’re drawing on top of it, in a different context. and in that context, there is no face there, only a group.
Therefore, as soon as you draw a few lines, sketchup will make face. and you’ll wonder why it’s making a second set of faces where there are already some (inside the group)

well if you don’t enter the group your face is in BEFORE you start drawing the part you want to remove, it’ll be hard indeed.

here again it’s a matter of context. Outside the group is a context. inside the group is a different context. both don’t interact by default.

Me too, I’ve been using SU since 2004 and just assume the unnecessary face creation is just part of the ‘experience’.
When I’m patching in terrain I’ll regularly get double faces that I have to hunt down and eliminate, often taking the original material with them requiring me to reapply.

If there’s a way to prevent/fix this ‘feature’ I too would love to know.

Oh WOW thank you so much. Now I totally get the layer way
I am terrible I never use layers
I know I should but how to go back on a project that took two years?
I have groups and components

Yes in this context I am using a base and I want to design pathways that I will import
and pose on the base as a group. And I don’t want them to become one with the floor.
Now actually I will let it make some faces as I can use them to make different materials
in between the pathways (its supposed to become sort of a gentle maze)

If I follow my understanding of your explanation
When I cut into a face with irregular lines and then want to delete what’s in the middle
and it won’t let me (it also had to do with groups)

But usually I click on the face first to make sure I am in or on it
but still like in this example I just made
I create a rectangle, line out a space
and yet can’t select it
then when I make a circle or pentagon it instantly shows as a separate space
why is that?
Thank you so much .


something and it won’t open up it’s because

Layers/tags are not the problem. Your wording sounds like you are making the frequent misunderstanding of thinking SketchUp’s layers isolate contents as they do in a 2D drawing app. SketchUp’s layers don’t do that (which is one of the reasons they have been renamed to “tags”).

You are drawing in the model context atop geometry that is already captured in a group. One of the main functions of a group is to isolate its contents from interaction with things in other contexts. So, when you draw atop the group a new face forms and because it is in the same location in 3D space as a face in the group, “z-fighting” results.

SketchUp doesn’t make unnecessary surfaces and the logic of when surfaces are created is not random but always consistent and predictable: any coplaner (contained on a single plane) edges that form a closed loop in the same context will always form a plane or surface inside them. So any three edges as a closed loop will form a surface no matter where you draw them in space. Any number of edges drawn as a loop in a single plane will form a surface.

You can only get double faces when drawing on an existing surface but outside of the existing surfaces context. By definition 2 faces can only exist in the same space if one of them is isolated inside a group or component, otherwise if they were in the same context they would instantly merge and become the same surface. You can draw a rectangle, then trace another rectangle over it as many times as you want, you won’t get “double” surfaces, the surfaces just merge into a single surface. However, if you make a rectangle, isolate that rectangle inside a group or component, then draw another rectangle over it by tracing the edges with the pencil tool a second rectangle will form in exactly the same space that does not merge with the isolated first but coexists in the same space, and thus causes Z-fighting as the graphics engine can’t decide which surface to show on top as they are in exactly the same plane.

As has been said, it’s all about context.

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Thank you.
I see now I did not close properly the “freeform”

Sketchup has forever had an issue with forming extra faces or linked faces, sometimes referred to as the bowtie bug. It can often be fixed with a simple intersect, or tracing an edge, or even adding a new edge to divide a face, but sometimes it can be very persistent and difficult to fix. In this case it isn’t a context issue as it occurs with raw geometry.
GIF 2-06-2024 11-38-48 AM

Yea, for me it’s when I’ve got a complex bit of landscape geometry that I’ve hand triangulated such as as curved sloping driveway. At least one triangle will have 1 or more identical faces co-existent that need to be deleted.

This is a WIP project, the ground drops about 12m with 5 houses and multiple retaining walls. All the ground has been essentially hand triangulated to get each surface to flow correctly. There have been multiple duplicate faces that I’ve had to fix.

Screencap of the model with hidden faces visible using my iPad (I haven’t used the iPad to model it, I’m no sadomasochist)

Finished image from a different project but similar sloped terain to show where the above is heading:

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