Why doesn't intersect faces work with planar faces?

As you can see in this gif I want to trim an object to fit an identical object. Yes I could use solid tools but I don’t want to.
When I use Intersect with model in the correct context the top and bottom faces should be split where they interact with the other object, but they don’t. The sides do, I guess because they are passing through a face, but why not the planar faces?
Again I know I only need to draw in the line, but that isn’t my point. I don’t understand why it doesn’t work.
Here’s a better example of why I would want and expect it to work. I know I can make the curved plank thicker and it will work, but again why should I need to?
Both these actions work with solid tools, so why not with Intersect Faces?
Is this a Bug or a Limitation!

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I think this is a limitation of how the intersect-algorithm works and is related with the normal of faces. When you reverse faces first, it works :


@MikeWayzovski that just makes me say, why should we have to reverse the faces when solid tools is able to cut the face as is?
And for that matter, Raw Geometry does too.

By the way, I’m not asking this to find workarounds, I’m asking this as a question to someone who actually works with the code that makes up sketchup, the Sketchup Team for example.

I am not showing workarounds, it just illustrates that it could be related to the ‘normal’ of the face, do not shoot me :grinning: (ssenger)

I don’t think that the inner code would be exposed or elaborated by team members on this public forum, we as ‘normal’ users can explicate endlessly about certain behavior being a bug or limitation of used algoritmes, though.
@Wo3Dan has a FR regarding the limitations of solids and their faces:

I think it is related to the original question.

Sorry Mike, didn’t mean it to look like I was shooting the messenger.
It’s just that this is one of those long term issues that never seems to be fixed and I wanted to bring it the teams attention again.
Also this isn’t even confined to solids, two grouped faces on the same plane will fail to intersect when using intersect with model.

I wish you could use @anon77361920 team member to make this pop up on all their computers.


This is a quite strange behavior. When intersecting from the Ruby API you have to intersect twice to include all co-planar faces. When called from the UI all faces are intersect with all other faces but in the API all faces in one set is intersected with all faces in another set. The operation has to be repeated with the order of the sets switched for a correct result.

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Good to know it’s not just me thinking it is strange.

No, it’s not just you!

I’ve occasionally hit this and other peculiar misfires from intersect with. There are other oddities such as the infamous “bowtie” error in which inset faces can get their edge ordering so messed up that SketchUp can’t decide what is going on. And the mysterious “fly off to another galaxy bug” affecting leader texts.

I’ve never seen the developers comment at all.

It would be useful to know whether this kind of issues are from known fundamental limitations of the problem (e.g. there is a basic uncertainty and every choice of how to address it can leave one or another error), from known flaws in the particular algorithm that are too low on the priority list to fix, or remain because nobody has figured out why they even happen.

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Yes, I started a thread on that too (which has never been addressed) which is why I made the comment about not wanting workarounds.

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