Hi all and happy New Year.

Hope this is the right place for this question. Got my brain stuck in an infinite loop and need someone to whack me on the head please.

It relates to the Union Path operation and how it behaves with compound and single paths. Can’t seem to find the answer anywhere.

Purely in terms of outline shapes - ignoring formatting etc.

1. Union on two non-touching paths - let's say a perfectly upright 5 point star and a square - creates a single compound path with two sub-paths.

2. and performing break apart on that path returns the two independent sub-paths shaped as before.

3. OK - Now think of two sets of paths - set 1 is the two shapes previously mentioned and set 2 is two squares.

4. Set 1 - Perfectly align the two shapes - star vertically below  square so that a point of the star perfectly meets the outline of the bottom edge of the square with no overlap (I used a bounding box 600 tall, made the two shapes 300 tall and aligned them to the bounding box top and bottom). Union and break apart works as in points 1 & 2 - BUT if, after union, one uses the unified shape to apply division to the lower bounding box it cuts out the two unified shapes individually and cuts the bounding box where the two shapes meet. This behaviour suggests that the two shapes are being treated a separate entities in one respect  (the two shapes are cut out individually) but a single entity in another respect (the bounding box is divided at the point where the two shapes meet).

5. Performing the same set of operations with the squares in set 2 results in completely different behaviour – Union creates a non-compound path which therefore cannot be broken apart and division with the bounding box cuts out the unified shape as a single entity etc.

6. The only difference in the two situations seems to me to be the length of the interface where the shapes meet. In set 1 it is a point and in set 2 it is the length of the side of the square.

7. So apparently the difference in behaviour is something to do with the meeting point of the two shapes yes?. My question therefore is how is this defined? How, or at what point or in what circumstances does the union operation (for want of better technical expression) decide what becomes a compound path and what becomes a non-compound path?   

Sorry if this is obvious and thanks for any help.

Best.

Dave

Please add an example SVG. Thanks.

Hi Polygon.

Attached.

Thanks

Path direction is important which you can not see with parametric objects - you´ll need to convert into paths. And "Fill rule" can render the same object differently.I'm not sure if this is just a philosophical discourse or if it has a practical use. 😅

Hi Polygon and thanks.

Can you expound on how path direction and fill rule might result in the behaviour as described?

I don't regard this as purely philosophical - path operations can be confusing/counter intuitive for the lesser experienced, shall we say. Full understanding leads to correct application and vice-versa and so tying down why something happens the way it does is inherently practical IMO. 

Thanks again.