var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var cw=canvas.width;
var ch=canvas.height;
var cBez1=[{x:250,y: 120},{x:290,y:-40},{x:300,y:200},{x:400,y:150}]
drawBez(cBez1);
var cPoints=findCBezPoints(cBez1);
drawPlots(cPoints);
function findCBezPoints(b){
var startPt=b[0];
var controlPt1=b[1];
var controlPt2=b[2];
var endPt=b[3];
var pts=[b[0]];
var lastPt=b[0];
var tests=5000;
for(var t=0;t<=tests;t++){
// calc another point along the curve
var pt=getCubicBezierXYatT(b[0],b[1],b[2],b[3], t/tests);
// add the pt if it's not already in the pts[] array
var dx=pt.x-lastPt.x;
var dy=pt.y-lastPt.y;
var d=Math.sqrt(dx*dx+dy*dy);
var dInt=parseInt(d);
if(dInt>0 || t==tests){
lastPt=pt;
pts.push(pt);
}
}
return(pts);
}
// Given the 4 control points on a Bezier curve
// Get x,y at interval T along the curve (0<=T<=1)
// The curve starts when T==0 and ends when T==1
function getCubicBezierXYatT(startPt, controlPt1, controlPt2, endPt, T) {
var x = CubicN(T, startPt.x, controlPt1.x, controlPt2.x, endPt.x);
var y = CubicN(T, startPt.y, controlPt1.y, controlPt2.y, endPt.y);
return ({
x: x,
y: y
});
}
// cubic helper formula
function CubicN(T, a, b, c, d) {
var t2 = T * T;
var t3 = t2 * T;
return a + (-a * 3 + T * (3 * a - a * T)) * T + (3 * b + T * (-6 * b + b * 3 * T)) * T + (c * 3 - c * 3 * T) * t2 + d * t3;
}
function drawPlots(pts){
ctx.fillStyle='red';
// don't draw the last dot b/ its radius will display past the curve
for(var i=0;i<pts.length-1;i++){
ctx.beginPath();
ctx.arc(pts[i].x,pts[i].y,1,0,Math.PI*2);
ctx.fill();
}
}
function drawBez(b){
ctx.lineWidth=7;
ctx.beginPath();
ctx.moveTo(b[0].x,b[0].y);
ctx.bezierCurveTo(b[1].x,b[1].y, b[2].x,b[2].y, b[3].x,b[3].y);
ctx.stroke();
}
body{ background-color: ivory; }
#canvas{border:1px solid red; margin:0 auto; }
<h4>Black line is context.bezierCurveTo<br>Red "line" is really dot-points plotted along the curve</h4>
<canvas id="canvas" width=500 height=300></canvas>