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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset=utf-8 /> 
<title>Time based animation using the parameter of the requestAnimationFrame callback</title>
  <script>
    var canvas, ctx;
    var width, height;
    var x, y, incX; // incX is the distance from the previous drawn rectangle to the new one
    var speedX; // speedX is the target speed of the rectangle in pixels/s
    
    // for time based animation
     var now, delta=0;
    // High resolution timer
     var oldTime = performance.now();
    
    // Called after the DOM is ready (page loaded)
    function init() {
      // init the different variables
      canvas = document.querySelector("#mycanvas");
      ctx = canvas.getContext('2d');
      width = canvas.width;
      height = canvas.height;
      
      x=10; y = 10;
      // Target speed in pixels/second, try with high values, 1000, 2000...
      speedX = 200;
      
      // Start animation 
      requestAnimationFrame(animationLoop);
    }
     
    function animationLoop(currentTime) {
      // How long between the current frame and the previous one ?
      // the first time oldtime is not defined.
      delta = currentTime - oldTime;        
       
        
      // Compute the displacement in x (in pixels) in function of the time elapsed and
      // in function of the wanted speed
      incX = calcDistanceToMove(delta, speedX);
        
      // an animation is : 1) clear canvas and 2) draw shapes, 
      // 3) move shapes, 4) recall the loop with requestAnimationFrame
      
      // clear canvas
      ctx.clearRect(0, 0, width, height);
      
      ctx.strokeRect(x, y, 10, 10);
      
      // move rectangle
      x += incX;
      
      // check collision on left or right
      if(((x+10) > width) || (x < 0)) {
        // inverse speed
        x -= incX;
        speedX = -speedX;
      }
      
      // Store time
      oldTime = currentTime;
      
      // asks for next frame  
      requestAnimationFrame(animationLoop);
    }  
    
  var calcDistanceToMove = function(delta, speed) {
    //console.log("#delta = " + delta + " speed = " + speed);
    return (speed * delta) / 1000; 
  }
    
  </script>
</head>
  
<body onload="init();">
  <canvas id="mycanvas" width="200" height="50" style="border: 2px solid black"></canvas>
</body>
</html>
 
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