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动画原理——摩擦力
书籍名称：HTML5-Animation-with-JavaScript
书籍源码：https://github.com/lamberta/html5-animation1
1.有摩擦力的情况
摩擦力在物体发生相对运动时产生，是运动方向的反向加速度。
代码表示意思如下
if (speed > friction) { speed -= friction; } else { speed = 0; }
06-friction-1.html
<!doctype html><html> <head> <meta charset="utf-8"> <title>Friction 1</title> <link rel="stylesheet" href="http://www.mamicode.com/include/style.css"> </head> <body> <header> Example from <a href="http://amzn.com/1430236655?tag=html5anim-20"><em>Foundation HTML5 Animation with JavaScript</em></a> </header> <canvas id="canvas" width="400" height="400"></canvas> <script src="http://www.mamicode.com/include/utils.js"></script> <script src="http://www.mamicode.com/classes/ball.js"></script> <script> window.onload = function () { var canvas = document.getElementById(‘canvas‘), context = canvas.getContext(‘2d‘), ball = new Ball(), vx = Math.random() * 10 - 5, vy = Math.random() * 10 - 5, friction = 0.1; ball.x = canvas.width / 2; ball.y = canvas.height / 2; (function drawFrame () { window.requestAnimationFrame(drawFrame, canvas); context.clearRect(0, 0, canvas.width, canvas.height); var speed = Math.sqrt(vx * vx + vy * vy), angle = Math.atan2(vy, vx); if (speed > friction) { speed -= friction; } else { speed = 0; } vx = Math.cos(angle) * speed; vy = Math.sin(angle) * speed; ball.x += vx; ball.y += vy; ball.draw(context); }()); }; </script> </body></html>
2.简单的方法
摩擦力实际就是速度逐渐变相，在动画程序中，我们可以简单的使每一帧的速度都变小。类似。
vx *= 0.9;vy *= 0.9;06-friction-2.html
<!doctype html><html> <head> <meta charset="utf-8"> <title>Friction 2</title> <link rel="stylesheet" href="http://www.mamicode.com/include/style.css"> </head> <body> <header> Example from <a href="http://amzn.com/1430236655?tag=html5anim-20"><em>Foundation HTML5 Animation with JavaScript</em></a> </header> <canvas id="canvas" width="400" height="400"></canvas> <script src="http://www.mamicode.com/include/utils.js"></script> <script src="http://www.mamicode.com/classes/ball.js"></script> <script> window.onload = function () { var canvas = document.getElementById(‘canvas‘), context = canvas.getContext(‘2d‘), ball = new Ball(), vx = Math.random() * 10 - 5, vy = Math.random() * 10 - 5, friction = 0.95; ball.x = canvas.width / 2; ball.y = canvas.height / 2; (function drawFrame () { window.requestAnimationFrame(drawFrame, canvas); context.clearRect(0, 0, canvas.width, canvas.height); vx *= friction; vy *= friction; ball.x += vx; ball.y += vy; ball.draw(context); }()); }; </script> </body></html>
动画原理——摩擦力
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