不好意思，剛剛沒有看清楚你注視了代碼，重新答一次。drawstar這個函數(shù)的功能是根據(jù)指定的上下文在特定的位置繪制特定大小的的星星。根據(jù)老師前面的例子，我們最好只是規(guī)劃出一個基本圖形的單位路徑。具體的位置變換，旋轉(zhuǎn)，縮放都交給外部進(jìn)行。出錯的原因是在外部你進(jìn)行了旋轉(zhuǎn)，在內(nèi)部進(jìn)行了位移，外部的旋轉(zhuǎn)改變了里面的坐標(biāo)系。將以上的代碼改為如下：

var?searchlight?=?{
	x:?400,
	y:?400,
	radius:?150,
	vx:?Math.random()?*?5?+?10,
	vy:?Math.random()?*?5?+?10
};
var?rot?=?0;
window.onload?=?function()?{
	var?canvas?=?document.getElementById('canvas');
	canvas.width?=?800;
	canvas.height?=?600;
	if?(canvas.getContext('2d'))?{
		var?context?=?canvas.getContext('2d');
	}?else?{
		alert('該瀏覽器不支持！');
	}
	setInterval(function()?{
		draw(context);
		update(canvas.width,?canvas.height);
	},?40);
};

function?draw(cxt)?{
	var?canvas?=?cxt.canvas;
	cxt.clearRect(0,?0,?canvas.width,?canvas.height);

	cxt.save();

	cxt.beginPath();
	cxt.fillStyle?=?'black';
	cxt.fillRect(0,?0,?canvas.width,?canvas.height);

	cxt.save();
	cxt.beginPath();
	//?cxt.arc(searchlight.x,searchlight.y,searchlight.radius,0,2*Math.PI);
	cxt.translate(searchlight.x,searchlight.y);
	cxt.rotate(rot/180*Math.PI);
	cxt.scale(searchlight.radius,searchlight.radius);
	drawstar(cxt);
	
	cxt.fillStyle?=?'#fff';
	cxt.fill();
	cxt.restore();

	cxt.clip();

	cxt.font?=?'bold?150px?Arial';
	cxt.textAlign?=?'center';
	cxt.textBaseline?=?'middle';
	cxt.fillStyle?=?'#058';
	cxt.fillText('CANVAS',?canvas.width?/?2,?canvas.height?/?4);
	cxt.fillText('CANVAS',?canvas.width?/?2,?canvas.height?/?2);
	cxt.fillText('CANVAS',?canvas.width?/?2,?canvas.height?/?4?*?3);
	
	cxt.restore();
}
//星星
function?drawstar(cxt)?{
	cxt.beginPath();
	for?(var?i?=?0;?i?<?5;?i++)?{
		cxt.lineTo(Math.cos((18?+?i?*?72)?/?180?*?Math.PI),?-Math.sin((18?+?i?*?72)?/?180?*?Math.PI));
		cxt.lineTo(Math.cos((54?+?i?*?72)?/?180?*?Math.PI)?*?0.5,?-Math.sin((54?+?i?*?72)?/?180?*?Math.PI)?*?0.5);
	}
	cxt.closePath();
}
//碰撞檢測
function?update(width,?height)?{
	rot?+=?1;
	searchlight.x?+=?searchlight.vx;
	searchlight.y?+=?searchlight.vy;
	if?(searchlight.x?-?searchlight.radius?<=?0)?{
		searchlight.vx?=?-searchlight.vx;
		searchlight.x?=?searchlight.radius;
	}
	if?(searchlight.x?+?searchlight.radius?>=?width)?{
		searchlight.vx?=?-searchlight.vx;
		searchlight.x?=?width?-?searchlight.radius;
	}
	if?(searchlight.y?-?searchlight.radius?<=?0)?{
		searchlight.vy?=?-searchlight.vy;
		searchlight.y?=?searchlight.radius;
	}
	if?(searchlight.y?+?searchlight.radius?>=?height)?{
		searchlight.vy?=?-searchlight.vy;
		searchlight.y?=?height?-?searchlight.radius;
	}
}

試了下你的代碼，沒有問題啊?？赡苁歉叨仍O(shè)置的大了點(diǎn)導(dǎo)致屏幕出現(xiàn)了滾動條，你會發(fā)現(xiàn)星星的一部分跑到了黑布的外側(cè)，你可以拖動滾動條看一下其實(shí)星星并沒有到黑布非外面，你可以試試適當(dāng)設(shè)置小一點(diǎn)的高度，比如600就非常完美了。

會不會是瀏覽器兼容問題