confusing behaviour of the unit parameter of Param

[Real-John-Cheung]

When I use the unit parameter in vsketch.Param the value seems to be multiplied by something (I assume that convert the value to CSS pixel?) However vsk.scale() will completely ignore that which makes it very confusing.

For example, for the following code

import vsketch


class BugSketch(vsketch.SketchClass):
    # Sketch parameters:
    a = vsketch.Param(1.0, unit = "cm")
    b = vsketch.Param(1, unit = "cm")
    def draw(self, vsk: vsketch.Vsketch) -> None:
        vsk.size("a4", landscape=False)
        vsk.scale("cm")
        print(self.a, self.b)
        vsk.rect(0, 0, self.a, self.b)

    def finalize(self, vsk: vsketch.Vsketch) -> None:
        vsk.vpype("linemerge linesimplify reloop linesort")


if __name__ == "__main__":
    BugSketch.display()

self.a and self.b will be something around 37, so I get a rectangle that's larger than the paper instead of a square of 1cm by 1cm. I am not sure if that's the desired behaviour it should have but for me I will say that's quite counter intuitive

[abey79]

Yeah, unit and vsk.scale() dont play well together as they have a "multiplicative" interaction.

My preferred workflow is avoid vsk.scale() and use vsketch.Param(..., unit=...). That way, everything is in CSS pixel (the default vsketch unit), but the UI for parameters does the $UNIT <-> CSS pixel conversion. In that case, I would use vsk.scale() only for relative scaling, if at all.

The alternative workflow is also possible when starting with vsk.scale("cm"). This means that "all units are now cm instead of CSS pixels". In that case, unit=... should really be avoided, because you dont want the UI to apply further scaling.