Compound Path issue: can anyone explain how I got here . . .?

Question

I have – what is probably the reverse of a normal problem – I have the answer, but I appear to have just stumbled upon it.

Could anyone explain the background thinking, so that I can get a better grasp of the fundamentals. This would help me in designing future projects.

Firstly in picture A – what is white is actually transparent.

So I need to create a reverse of this in two sections – on the back and the front – so that transparent becomes blue and the blue becomes transparent

In picture B I have created two rectangles and, by selecting the background design: Object > Compound Path > Make

I had to apply the “Use Even-Odd fill rule” from Attributes.

The result, which is exactly what I wanted, is in Picture C.

My problem is that I cannot understand how colour is created in an area that was previously transparent and that it gets filled with the main background colour.

Can someone explain the background principles to this process (keep it simple please!)

Thank you!

Doug A Roberts · Accepted Answer

guess so. anyway compound paths work by making the overlapping area of shapes do one of two things:

1. have a fill

2. not have a fill

based on two rules: the non-zero winding and even-odd rules.

the non-zero winding rule determines the fill by comparing the direction of the paths that make up the compound paths. paths with the same direction will produce a filled overlap; paths with opposite directions do the opposite. creating a compound path by default applies this rule and reverses the direction of the shape at the bottom.

this means that if you have many objects in a compound path, they might end up with mostly filled overlaps depending on the stack order.

the even-odd fill rule is, I think, more intuitive. it fills (or doesn't fill) overlaps based on the number of overlapping shapes. if an odd number of shapes overlap in an area, there will be a fill (kind of obvious, otherwise lone shapes would be unfilled). even numbers get no fill. thus you'll always get your outcome no matter where shapes are in the stack order, since your box (a fill) overlaps the other shapes (which makes 2 fills, which makes no fills).

Doug A Roberts · Answer

what's your blue background pattern made of? just closed paths?

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