How to get width & height of drawn arc(s) in HTML5 Canvas?

Question

Here I have drawn some arcs using Konvajs library, but I cannot get their width and height after the objects have been drawn, How can I do that? for quick read of code:

function drawSurface(idnumber, radius, x, y, startAngleParam, endAngleParam) {
var borderbold = 5;
var surface;
if (typeof startAngleParam !== 'undefined') {
    surface = new Konva.Shape({
        x: x,
        y: y,
        fill: '#ccc',
        stroke: "#ccc",
        strokeWidth: 8,
        id: idnumber,
        opacity: 1,
        drawFunc: function (context) {
            var startAngle = startAngleParam * Math.PI;
            var endAngle = (startAngleParam + 0.5 + endAngleParam) * Math.PI;
            var counterClockwise = false;
            context.beginPath();
            context.arc(0, 0, radius, startAngle, endAngle, counterClockwise);
            context.setAttr("lineWidth", borderbold);
            context.stroke();
            context.fillStrokeShape(this);
        }
    });
}
else {
    surface = new Konva.Circle({
        x: x,
        y: y,
        radius: radius,
        fill: '#ccc',
        strokeWidth: 3,
        id: idnumber,
        opacity: 1
    });
}
return surface;
}

Please support your answer with a code example.

Answer 1

Find the bounding box of your arc and then calculate the width & height from the bounding box.

Geometrically, the only 5 possible bounding box corners are:

• the arc centerpoint,
• the point on the arc (if any) at 0 degrees (0 radians),
• the point on the arc (if any) at 90 degrees (PI/2 radians),
• the point on the arc (if any) at 180 degrees (PI radians),
• the point on the arc (if any) at 270 degrees (PI*3/2 radians),

From these possible bounding box points, find the minimum X, minimum Y, maximum X & maximum Y. The [minX,minY] will be the top left corner of the bounding box. The [maxX,maxY] will be the bottom right corner of the bounding box.

Your arc width will be maxX-minX and height will be maxY-minY.

Here's example code and a Demo:

var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var cw=canvas.width;
var ch=canvas.height;


var PI=Math.PI;
var cx=150;
var cy=150;
var radius=75;
var startAngle=-PI/4;
var endAngle=PI/3;

ctx.beginPath();
ctx.moveTo(cx,cy);
ctx.arc(cx,cy,radius,startAngle,endAngle);
ctx.closePath();
ctx.fillStyle='skyblue';
ctx.fill();
ctx.strokeStyle='lightgray';
ctx.lineWidth=3;
ctx.stroke();

var bb=arcBounds(cx,cy,radius,startAngle,endAngle);
ctx.strokeStyle='red';
ctx.lineWidth=1;
ctx.strokeRect(bb.x,bb.y,bb.width,bb.height);


function arcBounds(cx,cy,radius,startAngle,endAngle){
  var minX=1000000;
  var minY=1000000;
  var maxX=-1000000;
  var maxY=-1000000;

  var possibleBoundingPoints=[]
  // centerpoint
  possibleBoundingPoints.push({x:cx,y:cy});
  // starting angle
  possibleBoundingPoints.push(arcpoint(cx,cy,radius,startAngle));
  // ending angle
  possibleBoundingPoints.push(arcpoint(cx,cy,radius,endAngle));
  // 0 radians
  if(0>=startAngle && 0<=endAngle){
    possibleBoundingPoints.push(arcpoint(cx,cy,radius,0));
  }
  // PI/2 radians
  var angle=PI/2;
  if(angle>=startAngle && angle<=endAngle){
    possibleBoundingPoints.push(arcpoint(cx,cy,radius,angle));
  }
  // PI radians
  var angle=PI;
  if(angle>=startAngle && angle<=endAngle){
    possibleBoundingPoints.push(arcpoint(cx,cy,radius,angle));
  }
  // PI*3/2 radians
  var angle=PI*3/2;
  if(angle>=startAngle && angle<=endAngle){
    possibleBoundingPoints.push(arcpoint(cx,cy,radius,angle));
  }

  for(var i=0;i<possibleBoundingPoints.length;i++){
    var pt=possibleBoundingPoints[i];
    if(pt.x<minX){minX=pt.x;}
    if(pt.y<minY){minY=pt.y;}
    if(pt.x>maxX){maxX=pt.x;}
    if(pt.y>maxY){maxY=pt.y;}
  }

  return({ x:minX, y:minY, width:maxX-minX, height:maxY-minY });

}


function arcpoint(cx,cy,radius,angle){
  var x=cx+radius*Math.cos(angle);
  var y=cy+radius*Math.sin(angle);
  return({x:x,y:y});
}

body{ background-color: ivory; }
#canvas{border:1px solid blue;}

<canvas id="canvas" width=300 height=300></canvas>