Why is this behavior "logical"?


#1

Forgive me, if this is a stupid question. I acknowledge that we often learn to live with strange “features” of tools, where some specific implementation has been chosen for various reasons. I have often wondered about this one in Sketchup:

Consider the four geometries embedded in the topmost face (named from left to right)
a) a square containing a face
b) a square NOT containing a face
c) two connected triangles both with faces
d) two connected triangles NOT containing faces
These were all created embedded in the same surrounding face, so I just
deleted three “inner” faces to get this.

Now, delete one of the outermost edges. The surrounding face disappears,
as shown in the middle figure.
Then reestablish the edge to redo the face. You are now left the the bottommost figure.
As can be seen,
a) is as before
b) the outer face “overwrites” the hole (does not recognise the hole?)
c) faces of both triangles are “overwritten” by the outer face
d) the outer face overwrites both holes (does not recognise the holes?).

Naturally, I would have liked that reestablishing the outer edge would
reestablish the face as it was originally.
That not being the case, I struggle to see that this really is a “logical” behavior.
Can anyone shed some insight why this is a “logical” i.e. why has
the implementation been chosen to be as this?


#2

There are no stupid questions :wink:

Your first image shows two bounded rectangles and two sets of two bounded triangles contained in a large bounded rectangle. Some of these bounding edges define a face (as shown by the blue-gray area), while others have no face. Since SketchUp tends to view things as a collection of faces bounded by edges, priority is given to creating a face when closing a complete boundary. Since the first and the third shapes define a face, they are left intact when you delete the boundary edge of the outer rectangle. When you add the edge back in again, it creates a large face defined by the four edges of the large rectangle. This, to me , is perfectly logical from a face-creation point of view. You can recreate the inner faces by “healing” the faces; that is, redrawing any of the edges of the boundary. This is a well-known feature and it can sometimes be important to the overall workflow (having to suddenly heal a hundred-odd details can be very frustrating).

If you accidentally deleted the line originally, an “Undo” or Ctrl+Z (or Alt-Backspace) will restore everything prior to the delete operation.

It’s worth noting that the third and fourth shapes are not properly defined for SketchUp. The diagonal edge defines two separate triangular boundaries that should be a single four-sided shape.


#3

Thanks for replying.
I know that this is a well-known feature, but still I find that it is not really
“logical” - IMHO.
I remove an edge, which causes the enclosed face to disappear… So far, so good.
So I draw the edge again, which restores the face… sort of.
But what I have got now could be very different from what I originally had,
especially from a perspective of “solidity”.
I still find it hard to see that this behavior is really what you would wish for.
Which is why I wanted someone else’s views.
Of course, I guess I just have to live with it…


#4

I’ll have to say here this isn’t a feature , it is a long know annoyance that we have to deal with.
SU does not cope well with edges and faces in 2d.
So often you get a single face ignoring edges, and even if you try to break it into the correct multiple faces it can be impossible. Not to mention the overlapping vertex nonsense where you cant draw a face with one connection to an edge.
Single%20Vertex%20error


#5

I think it has to do with optimization in the face finding algorithm. Creating an algorithm that finds possible planes to draw faces to is a bit complicated. Making a fast algorithm that doesn’t make SketchUp lag every time you draw a line, even in very large models, is probably quite a bit more difficult.