Why does the stroke position have to be ‘center’ on an open path in vector programs?

I’ve noticed that in every vector drawing program I’ve used, the options to place strokes ‘inside’ or ‘outside’ are disabled when a path is open (or on text). I assume there is a reason for this, but it makes mimicking CSS hard to do, as borders are never in the ‘center’ of a line segment.

Can someone explain why this is, and if there is a workaround to have a 3 sided shape with the stroke in the inside of the anchor points. Think a “tab shape” that is open at the bottom. It seems to me that if you can still “fill” an open shape, you should also be able to control the border position.

Answer

If a path is not closed, can it be said to have an inside?

Or more specifically, can a naive algorithm optimized for speed reliably determine the inside of an arbitrarily-shaped open path?

It seems to me that for open paths, “left” and “right” (with node1-node2 determining vector) would be useful and the more general case. Sadly, this does not seem to have been incorporated into Illustrator.

Attribution
Source : Link , Question Author : ktomasso , Answer Author : Yorik

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