let video = document.getElementById('my_video_id');
let capture = new cv.VideoCapture(video);
let frame = new cv.Mat(video.videoHeight, video.videoWidth, cv.CV_8UC4);
capture.read(frame);

执行之后报错
Error: Bad size of input mat: the size should be same as the video.

在控制台打印一下video.videoHeight，video.videoWidth
分别是448和960，也确实跟视频一样大

解决办法：
加上如下两行代码：
video.height = video.videoHeight;
video.width = video.videoWidth;

let video = document.getElementById('my_video_id');
video.height = video.videoHeight;
video.width = video.videoWidth;
let capture = new cv.VideoCapture(video);
let frame = new cv.Mat(video.videoHeight, video.videoWidth, cv.CV_8UC4);
capture.read(frame);

可能是opencv源码要用到video.height 和 video.width变量吧
然而这两个变量默认都等于0，所以跟448和960对不上

如果直接把video.height和video.width 代入cv.Mat函数的话，那就会报另外一个错，而且是capture.read(frame)导致的报错
DOMException: Index or size is negative or greater than the allowed amount
视频的长和宽都是0，这显然是不科学的。