Here’s how we’ll make sure the neck opening is just right:

let tweak = 1;
let target = (measurements.headCircumference * options.neckRatio) /4;
let delta;
do {
	points.right = new Point(tweak * measurements.headCircumference / 10, 0);
	points.bottom = new Point(0, tweak * measurements.headCircumference / 12);

	points.rightCp1 = points.right.shift(90, points.bottom.dy(points.right)/2);
	points.bottomCp2 = points.bottom.shift(0, points.bottom.dx(points.right)/2);

	paths.neck = new Path()
	  .curve(points.rightCp1, points.bottomCp2, points.bottom);

	delta = paths.neck.length() - target;
  if (delta > 0) tweak = tweak * 0.99;
  else tweak = tweak * 1.02;
} while (Math.abs(delta) > 1);

We’ve added a few new variables:

  • tweak: A tweak factor that we’ll use to increase or decrease the neck opening by making it more or less than 1
  • target: How long our (quarter) neck opening should be
  • delta: How far we’re off. Positive numbers mean it’s too long, negative means too short

Now that we know what target is, we construct our path as we did before. But this time around, we multiply our point coordinates with our tweak variable (1 at the start).

Then, we compare our target to the result of paths.neck.length() which — you guessed it — returns the length of our neck path.

If the delta is positive, our path is too long and we reduce the tweak factor.
If the delta is negative, our path is too short and we increase the tweak factor.

We keep on doing this until Math.abs(delta) is less than 1. Meaning that we are withing 1mm or our target value.

It might look the same as before, but now it's just right

Now that we’re happy with the length of our quarter neck opening, let’s construct the entire neck opening.