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Path.CCW() confusion #335

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konskoehler opened this issue Feb 3, 2025 · 2 comments
Closed

Path.CCW() confusion #335

konskoehler opened this issue Feb 3, 2025 · 2 comments

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@konskoehler
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Hey,

I have been experimenting with the Path functions and encountered a confusing result.

When I run:

canvas.MustParseSVGPath("M0 0H10V10H0z").CCW()

It returns true, even though the given SVG path is clearly drawn in a clockwise direction (considering SVG's coordinate system, where x increases to the right and y increases downwards).

I expected CCW() to return false for this case. Could you help clarify if this is intended behavior or a bug?

Thanks for your time!
@tdewolff
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tdewolff commented Feb 3, 2025

Yes, canvas is in the first Cartesian quadrant coordinate system. That is, the bottom-left is the origin and X and Y increase along the right and towards the top. This is common in math and in general, but drawing coordinate systems are usually in the fourth Cartesian quadrant. This has been a decision from early on since it is confusing to think in the fourth quadrant (for me at least) and well all papers and algorithms are usually focused on the first quadrant.

That said, you can use https://github.com/tdewolff/canvas/blob/master/canvas.go#L303 to draw in the fourth quadrant, but internally all paths remain in the first quadrant.

@konskoehler
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thanks for the explanation!

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@tdewolff @konskoehler