关于HTML5在网页上调用摄像头

代码

var video = document.getElementById("video");
navigator.getUserMedia = navigator.getUserMedia || navigator.webkitGetUserMedia;
if (navigator.getUserMedia) {
if (navigator.webkitURL) {
navigator.getUserMedia("video", function (stream) {
video.src = window.webkitURL.createObjectURL(stream);
}, function (error) { alert(error); });
}
else {
navigator.getUserMedia("video", function (stream) {
video.src = window.webkitURL.createObjectURL(stream);
}, function (error) { alert(error); });
}
}
else {
alert("navigator.getUserMedia");
}
为何打开网页直接运行到了alert ，有没有大神知道，求指导

相关了解……