Canvas学习笔记——弹动

如果有一根橡皮筋拴住一个小球，将小球拉开一定距离后释放，小球将会弹动。距离越远，橡皮筋对小球施加的外力越大，小球的加速度就越大。也就是说，小球的加速度与距离是成正比例关系的。这和缓动很相似，缓动是速度与距离成正比例。假设弹力系数为spring，则有公式：

ax = (targetX - currentX) * spring;
ay = (targetY - currentY) * spring;

vx += ax;
vy += ay;

举个例子：

 

// > 16 & 0xff,
g = color >> 8 & 0xff,
b = color >> 0xff,
a = (alpha 1) ? 1 : alpha);

if(a === 1) {
return 'rgb('+r+','+g+','+b+')';
} else {
return 'rgba('+r+','+g+','+b+','+a+')';
}
};

window.utils.parseColor = function (color, toNumber) {
if (toNumber === true) {
if (typeof color === 'number') {
return (color | 0); //chop off decimal
}
if (typeof color === 'string' && color[0] === '#') {
color = color.slice(1);
}
return window.parseInt(color, 16);
} else {
if (typeof color === 'number') {
color = '#' + ('00000' + (color | 0).toString(16)).substr(-6); //pad
}
return color;
}
};

window.utils.containsPoint = function (rect, x, y) {
return !(x rect.x + rect.width ||
y rect.y + rect.height);
};

window.utils.intersects = function (rectA, rectB) {
return !(rectA.x + rectA.width 0) {
ctx.stroke();
}
ctx.restore();
};

Ball.prototype.getBounds = function () {
return {
x: this.x - this.radius,
y: this.y - this.radius,
width: this.radius * 2,
height: this.radius * 2
};
};

function Ship () {
this.x = 0;
this.y = 0;
this.width = 25;
this.height = 20;
this.rotation = 0;
this.showFlame = false;
}

Ship.prototype.draw = function (ctx) {
ctx.save();
ctx.translate(this.x, this.y);
ctx.rotate(this.rotation);

ctx.lineWidth = 1;
ctx.strokeStyle = "#ffffff";
ctx.beginPath();
ctx.moveTo(10, 0);
ctx.lineTo(-10, 10);
ctx.lineTo(-5, 0);
ctx.lineTo(-10, -10);
//ctx.lineTo(10, 0);
ctx.closePath();
ctx.stroke();

if (this.showFlame) {
ctx.beginPath();
ctx.moveTo(-7.5, -5);
ctx.lineTo(-15, 0);
ctx.lineTo(-7.5, 5);
ctx.stroke();
}
ctx.restore();
};

(function() {
var canvas = document.createElement('canvas'),
a = document.getElementById('a');
canvas.id = 'c1';
canvas.width = 500;
canvas.height = 500;

a.appendChild(canvas);

var canvas = document.getElementById('c1'),
ctx = canvas.getContext('2d');

var ball = new Ball(20,Math.random() * 0xffffff),
mouse = utils.captureMouse(canvas),
targetX = canvas.width / 2,
targetY = canvas.height / 2,
vx = 0,
vy = 0,
ax = 0,
ay = 0,
requestId,
friction = 0.95,
gravity = 1,
spring = 0.01;

(function() {
requestId = window.requestAnimFrame(arguments.callee,canvas);
ctx.clearRect(0,0,canvas.width,canvas.height);

ctx.beginPath();
ctx.moveTo(mouse.x,mouse.y);
ctx.lineTo(ball.x,ball.y);
ctx.stroke();

ax = (mouse.x - ball.x) * spring;
ay = (mouse.y - ball.y) * spring;

vx += ax;
vy += ay;
vy += gravity;

vx *= friction;
vy *= friction;

ball.x += vx;
ball.y += vy;

ball.draw(ctx);
})();
})();
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