Fitting the neck opening

We are not going to create some opening that we hope is the right size, we’re going to make sure it is. Here’s how we’ll make sure the neck opening is just right:

function draftBib({
  Path,
  Point,
  paths,
  points,
  measurements,
  options,
  part,
}
) {

  /*
   * Construct the quarter neck opening
   */
  let tweak = 1
  let target = (measurements.head * options.neckRatio) /4
  let delta
  do {
    points.right = new Point(
      tweak * measurements.head / 10,
      0
    )
    points.bottom = new Point(
      0, 
      tweak * measurements.head / 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.quarterNeck = new Path()
      .move(points.right)
      .curve(
        points.rightCp1, 
        points.bottomCp2, 
        points.bottom      )
    delta = paths.quarterNeck.length() - target
    if (delta > 0) tweak = tweak * 0.99
    else tweak = tweak * 1.02
  } while (Math.abs(delta) > 1)
  return part
}

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 within 1 mm of our target value.

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