Agfdhyk

Having a closed filled polygon defined as a set of points, question is how to get a list of internal angles between each 2 connecting segment lines that make up this polygon.

Paper.js allows to get angle between 2 vectors by calling getDirectedAngle (See class Point). So we can get angles between all segments, just we still need a way to decide which angle value is internal.

After spending some time i solved the task using this method.

First, i calculate angle between 2 connecting segments A and B. Next, i evaluate median vector M for A and B starting from the connecting point C where segments A and B meet. Finally i test whether point that lies on median between A and B is inside of polygon or outside of it and based on that i decide if this angle is internal or external.

Solution is below.

function checkPolygonAngle(path) {

  if (path.curves.length<=1) {

    return;

  } 



  const angleText = function(point, angle) {

    const text = new paper.PointText(point);

    text.justification = 'center';

    text.fillColor = 'black';

    text.content = ""+(Math.round(angle*100)/100);

    return text;

  }

  for (let i=1;i<=path.curves.length;i++) {

    console.log(i);

    const curve1 = path.curves[i-1];

    const curve2 = i==path.curves.length ? path.curves[0] : path.curves[i];

    const v1 = curve1.segment1.point.subtract(curve1.segment2.point);

    const v2 = curve2.segment2.point.subtract(curve2.segment1.point);

    const point = curve1.segment2.point;

    (new paper.Path.Circle(point,3)).fillColor='red';



    const angle = v1.getDirectedAngle(v2);



    const medianAngle = angle/2;

    const medianPoint = curve1.segment1.point.rotate(medianAngle, point);

    const medianVector = medianPoint.subtract(point);

    medianVector.length=30;



    const pointOnMedian = point.add(medianVector);

    (new paper.Path.Line(point,pointOnMedian)).strokeColor='green';

    let insideAngle = null;

    if (path.contains(pointOnMedian)) {

      insideAngle = Math.abs(angle);

      (new paper.Path.Circle(pointOnMedian,3)).fillColor='magenta';

      angleText(pointOnMedian, insideAngle);

    }



    const oppositeMedianVector = medianVector.multiply(-1);

    const pointOnOppositeMedian = point.add(oppositeMedianVector);

    (new paper.Path.Line(point,pointOnOppositeMedian)).strokeColor='blue';

    if (path.contains(pointOnOppositeMedian)) {

      insideAngle = (360 - Math.abs(angle));

      (new paper.Path.Circle(pointOnOppositeMedian,3)).fillColor='magenta';

      angleText(pointOnOppositeMedian, insideAngle);

    }



    console.log(i+":", v1, v2, 'angle: ', insideAngle);

  }

}