#3StrandBraids
As I mentioned in
https://pixelfed.social/p/Splines/797555432624206626, the geometry of braid strands is not at all obvious despite how familiar they look.
The geometry is defined in a series of steps starting with two coil-shaped
#helix curves that we project on two perpendicular flat surfaces to create two flat
#sinusoidal curves. Then, we transform the two flat sinusoids back into a composite 3D curve that acts as the rail for each strand. Then, we arrange three strands to form a braid. If you have never seen the geometry of a braid strand, you might be surprised at how it looks.
A braid strand is
#periodic just like the
#sinusoid curves it is based on. That means the pattern repeats at a fixed period and the motif can go on forever. For our purposes, a braid that is µ = 144 units long is sufficient, but to get that, we have to start with a helix that is longer than µ.
In the front view, start at the origin and draw a helix of length µ*4/3 = 192 units. This is shown in blue in the top-left portion of the figure. Directly underneath that is the view as seen from the top, which looks very similar to the one in the front view, but is distinct. Since a helix is like a round coil, it looks like a circle if viewed from a side as shown in bottom left. Only in the perspective view seen on the right is the coil shape readily apparent.
The blue helix has a fixed distance equal to 1/2 part (4 units) from the axis and it makes 8 full turns along the entire length of the axis, meaning it repeats with a period (or wavelength) of 192/8 = 24 units or 3 parts. This is crucial: For a 3 strand braid, the period must be a multiple of 3.
Duplicate the blue helix and double it in size while centered on the origin. Then, shift the larger helix by 1/4 of period (6 units) to the left. This larger helix is shown in magenta.
These helices are still in 3D. Make sure that the front, top, and right views look like what's shown here. Otherwise, the next step won't work.