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

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>

Answer 2

Here is a different approach taken from this answer and ported to Javascript:
const PI = Math.PI;
const HALF_PI = Math.PI / 2;
const TWO_PI = Math.PI * 2;
const DEG_TO_RAD = Math.PI / 180;
const RAD_TO_DEG = 180 / Math.PI;
const getQuadrant = (_angle) => {
const angle = _angle % (TWO_PI);
if (angle > 0.0 && angle < HALF_PI) return 0;
if (angle >= HALF_PI && angle < PI) return 1;
if (angle >= PI && angle < PI + HALF_PI) return 2;
return 3;
};
// https://stackoverflow.com/a/35977476/461048
const getArcBoundingBox = (ini, end, radius, margin = 0) => {
const iniQuad = getQuadrant(ini);
const endQuad = getQuadrant(end);
const ix = Math.cos(ini) * radius;
const iy = Math.sin(ini) * radius;
const ex = Math.cos(end) * radius;
const ey = Math.sin(end) * radius;
const minX = Math.min(ix, ex);
const minY = Math.min(iy, ey);
const maxX = Math.max(ix, ex);
const maxY = Math.max(iy, ey);
const r = radius;
const xMax = [[maxX, r, r, r], [maxX, maxX, r, r], [maxX, maxX, maxX, r], [maxX, maxX, maxX, maxX]];
const yMax = [[maxY, maxY, maxY, maxY], [r, maxY, r, r], [r, maxY, maxY, r], [r, maxY, maxY, maxY]];
const xMin = [[minX, -r, minX, minX], [minX, minX, minX, minX], [-r, -r, minX, -r], [-r, -r, minX, minX]];
const yMin = [[minY, -r, -r, minY], [minY, minY, -r, minY], [minY, minY, minY, minY], [-r, -r, -r, minY]];
const x1 = xMin[endQuad][iniQuad];
const y1 = yMin[endQuad][iniQuad];
const x2 = xMax[endQuad][iniQuad];
const y2 = yMax[endQuad][iniQuad];
const x = x1 - margin;
const y = y1 - margin;
const w = x2 - x1 + margin * 2;
const h = y2 - y1 + margin * 2;
return { x, y, w, h };
};
jsfiddle: https://jsfiddle.net/brunoimbrizi/y3to5s6n/45/