zrot_copies ignores cp.z

[RAMilewski]

include<BOSL2/std.scad>
$fn = 32;

zrot_copies(n = 10, cp = [0,0,120], r = 25) color("blue") sphere(2);

[adrianVmariano]

I think this is the correct answer. cp is specifying the location of the axis to rotate around and that axis doesn't depend on cp.z

[pipatron]

This is also exactly what I would expect. What do you think should happen?

[RAMilewski]

OK. But in that case I'm not clear as to why the cp is specified as a [x,y,z] rather than just [x,y]. Consistency with rot_copies, xrot_copies, and yrot_copies maybe. But in the example, it is in fact rotating around [0,0,0], not [0,0,120]. Should the docs clarify that it's not rotating around a point, but about an axis located at [cp.x,cp.y]?

[adrianVmariano]

You don't have to give cp.z, that is, giving just x,y is fine. A zrot() operation is rotating around a line, not a point. There's no meaningful sense in which it is "in fact rotating around [0,0,0]". It's rotating around the line parallel to the Z axis that passes through [0,0,0]. You can the same line if you give a different cp.z value. Or, to turn this around, what would you expect zrot() to do "around the point [0,0,120]"?

[RAMilewski]

For what I was trying to do, it would negate the need to prefix the rotate_sweep() with an up(120). But this late in the game, changing it would just cause backward compatibility problems. Although I would change the docs so it says it's rotating around a z-oriented axis at x,y, instead of saying explicitly (in this case) that it's rotating around a point 120 units above it. If it's ok with you and Revar I can make those changes.

[adrianVmariano]

There is nowhere in the docs where it says you have to give a 3-vector as cp. If you give cp=[3,4] that works fine. The only thing that suggests that cp should be 3d is the listed default of [0,0,0].

Sure, go fix the docs. Presumably you need to change all three rotation distributors to clarify how the axis is determined from cp, and you can note the optionality of cp.z for specifying the axis to zrot_copies. Not optional for the other ones, though.

[RAMilewski]

I've added it to my to-do list right after the Bezier and texturing with images tutorials.