// from https://codepen.io/tculda/pen/pogwpOw
// (https://stackoverflow.com/questions/22356012/multiple-points-colors-gradient-on-html5-canvas)

static double getProjectionDistance2(DoublePt a, DoublePt b, DoublePt c) {
  ret getProjectionDistance2(a.x, a.y, b.x, b.y, c.x, c.y);
}

static double getProjectionDistance2(double ax, double ay, double bx, double by, double cx, double cy) {
  var k2 = bx*bx - bx*ax + by*by -by*ay;
  var k1 = ax*ax - bx*ax + ay*ay -by*ay;
  var ab2 = (ax - bx)*(ax - bx) + (ay - by) * (ay - by);
  var kcom = (cx*(ax - bx) + cy*(ay-by));
  var d2 = (k2 + kcom) / ab2;
  ret d2;
}

Began life as a copy of #1034858