Ellipse mode radius changes behavior of radius labelled argument

[R4chel]

I would expect that these two lines of code would produce the same output
vsk.circle(0,0,radius=5) vsk.circle(0,0,radius=5, mode="radius")
but was suprised to find that
vsk.circle(0,0,radius=5, mode="radius")
draws a circle with radius 10!

I created a repo to write a test and explore this outside the context of the complex code where I had come across it: https://github.com/R4chel/vsketch-circle-mode-debugging
And here is an output image from that showing the problem. The lines are drawn from the center of the circle to a point distance radius away.

[R4chel]

Another issue with vsk.circle if the radius / diameter is a negative number it draws a triangle!
vsk.circle(0,0,-1)

[R4chel]

vsk.rect() with negative values also does strange things

[R4chel]

I found the line responsible for drawing triangles when the radius is negative.

https://github.com/abey79/vpype/blob/15ae8110eac724226d1050ea88f4f9301a39cbec/vpype/primitives.py#L131
n = max(3, math.ceil((stop - start) / 180 * math.pi * max(rw, rh) / quantization))

if radius (rw and rh) are negative the whole right side will be negative and n will be 3.

[R4chel]

I think it's not immediately obvious what vsk.cricle(0,0,-1) should draw. Here is a start of potential answers

• Reject, invalid input
• In polar coordinates, $(-r, \theta)=(r,\theta+\pi)$
• Im not sure about the implementation details, in p5.js negative values for width and height of rect are drawn inverted (going up instead in down) and circles appear to be drawn with the absolute value but I haven't explored this too deeply. Here is an p5.js discussing this error when ellipse has negative width and height processing/p5.js#413

[abey79]

I think the clean way would be to draw with the absolute value and invert the circle rotation direction. This latter point would be completely pointless in the current situation IMO, so I'd drop it. I'm actually thinking of the future of vpype/vsketch data model, with first class support for arcs, bezier, etc. When/if that happens, it'll be easy (and more meaningful) to do the "right thing".