canvas 像素级碰撞

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demo地址http://06wjin.sinaapp.com/html5/hitTest/ 用鼠标拖动

1首先先判断两个图片的矩形区域有无碰撞

判断两个矩形的中心距离即可

2如果矩形区域有碰撞则检测上图红色矩形区域的像素，这里有两种方法
a 直接判断，循环检测两位图在红色矩形内是否有一点像素的alpha点都不为0，如果有则说明碰撞

context.drawImage(img1, x1, y1);//画第一张位图
var data1 = context.getImageData(minx, miny, maxx - minx, maxy - miny).data;//获取第一张位图红色矩形内像素
context.clearRect(x1, y1,img1.width,img1.height);//清除第一张位图
context.drawImage(img2, x2, y2);//画第二张位图
var data2 = context.getImageData(minx, miny, maxx - minx, maxy - miny).data;//获取第二章位图红色矩形内像素
for(var i = 3; i < data1.length; i += 4)
{
if(data1 > 0 && data2 > 0) return true;//循环判断alpha值，这里可以设置透明阙值，比如把0改为0.2，意味着透明度为0.2时就认为不碰撞了
}
return false;

b将绘图模式改为xor(xor是指相交部分透明，具体见站长上篇教程)，也可以判断
context.drawImage(img1, x1,y1);//画第一张位图
      context.globalCompositeOperation = 'xor';//改绘图模式为xor
      context.drawImage(img2, x2,y2);//画第二张位图
      var data = context.getImageData(minx, miny, maxx - minx, maxy - miny).data;//获取位图红色矩形内像素
      context.globalCompositeOperation = 'source-over';//把绘图模式改回去

      for(var i = 3; i < data.length; i += 4)
      {

            if(data == 0 ) return true;//若有透明像素，则碰撞
      }
      return false;

下面是一个2d精灵类，碰撞直接用sprite1.hitTest(sprite2)就好

function Sprite(x, y, img, width, height)
{
this.x = x;
this.y = y;
this.img = document.getElementById(img);
this.width = width;
this.height = height;
this.halfWidth = this.width/2;
this.halfHeight = this.height/2;
this.angle = 0;角度
this.scaleX = 1;//水平缩放
this.scaleY = 1;//竖直缩放
this.alpha = 1;//透明度
this.isDrug = false;//是否拖到
}

Sprite.prototype.draw = function()
{
context.save();
context.translate(this.x + this.halfWidth, this.y + this.halfHeight);
context.globalAlpha = this.alpha;//修改透明度
context.rotate(this.angle);//旋转角度
context.scale(this.scaleX, this.scaleY);//缩放
context.drawImage(this.img, -this.halfWidth, -this.halfHeight);
context.restore();
}

Sprite.prototype.hitTest = function(sprite)
{
      var minx = this.x > sprite.x ? this.x :sprite.x;
      var maxx = this.x + this.width < sprite.x + sprite.width ? this.x + this.width : sprite.x + sprite.width ;
      var miny = this.y > sprite.y ? this.y : sprite.y;
      var maxy = this.y + this.width < sprite.y + sprite.width ? this.y + this.width : sprite.y + sprite.width;

      if (minx >= maxx || miny >= maxy) {return false;}

      var canvas = document.createElement('canvas');
      canvas.setAttribute('width', 550);
      canvas.setAttribute('height', 400);
      var context = canvas.getContext('2d');

      /*第一种方法*/
      context.drawImage(this.img, this.x, this.y);
      var data1 = context.getImageData(minx, miny, maxx - minx, maxy - miny).data;
      context.clearRect(0, 0, 550, 400);
      context.drawImage(sprite.img, sprite.x, sprite.y);
      var data2 = context.getImageData(minx, miny, maxx - minx, maxy - miny).data;

      for(var i = 3; i < data1.length; i += 4)
      {
            if(data1 > 0 && data2 > 0) return true;
      }
      return false;

      /*第二种方法
      context.drawImage(this.img, this.x, this.y);
      context.globalCompositeOperation = 'xor';
      context.drawImage(sprite.img, sprite.x, sprite.y);
      var data = context.getImageData(minx, miny, maxx - minx, maxy - miny).data;
      context.globalCompositeOperation = 'source-over';

      for(var i = 3; i < data.length; i += 4)
      {

            if(data == 0 ) return true;
      }
      return false;*/
}

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