Simple Drawing on Canvas - With Timer

by Shawn Lackey

HTML

<button onclick="doDrawing();">Start Drawing</button>
<canvas id="myCanvas" width="400" height="400" style="border:1px solid #d3d3d3;">
Your browser does not support the HTML5 canvas tag.</canvas>

JavaScript

var t = 0;
var c = document.getElementById("myCanvas");
var R = 50;
var ctx = c.getContext("2d");
var r= 45;
var O= 5;
var x = (c.width/2) + (R+r)*Math.cos(t) - (r+O)*Math.cos(((R+r)/r)*t);
var y = (c.width/2) + (R+r)*Math.sin(t) - (r+O)*Math.sin(((R+r)/r)*t);
    
function doDrawing() {
    t = 0;    
     
    // Clear the Canvas
    ctx.clear();
    
    // Create a random color
    var timesRun = 0;
    var color =  '#'+Math.floor(Math.random()*16777215).toString(16);
    
      
    // Start the Drawing
    ctx.beginPath();
    ctx.strokeStyle = color;
    ctx.moveTo(x,y);
      
    //Use the timer to create drawing
    var interval = setInterval(function(){
    timesRun += 1;
    if(timesRun === 200){
        clearInterval(interval); }
        
    drawCircle();}, 5); 
    
}
function drawCircle()
{
  t += 0.8;
    
  x = (c.width/2) + Math.floor((R+r)*Math.cos(t) - (r+O)*Math.cos(((R+r)/r)*t));
  y = (c.width/2) + Math.floor((R+R)*Math.sin(t) - (r+O)*Math.sin(((R+r)/r)*t));
 
    
  ctx.lineTo (x,y)*(c.width/2);
  ctx.stroke();
}

CanvasRenderingContext2D.prototype.clear = 
  CanvasRenderingContext2D.prototype.clear || function (preserveTransform) {
    if (preserveTransform) {
      this.save();
      this.setTransform(1, 0, 0, 1, 0, 0);
    }

    this.clearRect(0, 0, this.canvas.width, this.canvas.height);

    if (preserveTransform) {
      this.restore();
    }           
};