Use boolean operations with lines does not work

[Jan Kowalewski]

Hi, 

I am trying different boolean operations with lines (curves) and it always fails. Is there no way to e.g. use the "Add" operation to combine two lines or a line and another shape? The line(s) always just disappear when I try it. Is it supposed to be like this?

Cheers

[GarryP]

Welcome to the forums @Jan Kowalewski

Sometimes, the Boolean operations only work with closed shapes. See posts below for more information.
They work by calculating the areas of shapes and then subtracting/adding/etc. those areas.
Since open curves/lines often have no areas there’s nothing for them to work with, so the lines/curves sometimes disappear.

If you want to combine open lines/curves you can use the “Layer → Geometry → Merge Curves” functionality instead.

Edited June 11, 2022 by GarryP
Added more details.

[NotMyFault]

In Case you are using Designer, you could use „expand stroke“ to convert open curves to closed curves. It depends on your intentions/ further edits which way to go.

[R C-R]

9 hours ago, GarryP said:

The Boolean operations only work with closed shapes.
They work by calculating the areas of shapes and then subtracting/adding/etc. those areas.
Since open curves/lines have no areas there’s nothing for them to work with, so the lines/curves disappear.

That isn't quite right. The booleans work with open curves if they enclose some area. So for example, try it with this 2 open curves.afphoto example.

[GarryP]

Interesting.
I’ve never seen that before as I’ve never tried to do that as I assumed it wouldn’t work.
I’ve put a little video together showing some examples of subtracting curves, some of which ‘work’, some of which don’t.
Is there a ‘rule-of-thumb’ to tell us which curves can successfully have Boolean operations applied to them?

Note: I've modified my earlier answer to reflect this new information.

[NotMyFault]

27 minutes ago, GarryP said:

Is there a ‘rule-of-thumb’ to tell us which curves can successfully have Boolean operations applied to them?

Just add a fill. Straight lines wont enclose an area. Curved or multi  node lines who show an effect of fill will work.

If the nodes of an curve cover a 2D area, it works. If the nodes are on one single line (1D), it wont. If all nodes are at one position (0D), it won't. Unless you use node tool to make a curve (not straight) by only two nodes. But this would be cheating 😂

[GarryP]

I’ve done a bit more experimenting and I think I can now understand that, for two-node curves:

• if all of the curves are straight lines (even if some cross each other) then the operation will have no effect;
• if at least one of the curves is not a straight line then the operation will do <something>, but all the straight lines will be ignored by the operation and then removed.

However, for more-than-two-node curves it works differently.
I think I need to experiment more with this (or just leave it alone until I really need to use it).

[R C-R]

5 hours ago, GarryP said:

However, for more-than-two-node curves it works differently.

As @NotMyFault said, it just depends on if the curves cover a 2D area or not. For example in this 2 five node curves.afdesign all the nodes of each curve are in a straight line so they do not cover any area, & the booleans do not have any effect on them.