[英]Capture group of HTML5 video screenshots
我正在尝试使用以下代码使用canvas在浏览器中从HTML5视频中生成一组缩略图:
var fps = video_model.getFps(); //frames per second, comes from another script
var start = shot.getStart(); //start time of capture, comes from another script
var end = shot.getEnd(); //end time of capture, comes from another script
for(var i = start; i <= end; i += 50){ //capture every 50 frames
video.get(0).currentTime = i / fps;
var capture = $(document.createElement("canvas"))
.attr({
id: video.get(0).currentTime + "sec",
width: video.get(0).videoWidth,
height: video.get(0).videoHeight
})
var ctx = capture.get(0).getContext("2d");
ctx.drawImage(video.get(0), 0, 0, video.get(0).videoWidth, video.get(0).videoHeight);
$("body").append(capture, " ");
}
捕获的数量是正确的,但是问题是在Chrome中所有画布都显示为黑色,而在Firefox中它们始终显示相同的图像。
也许问题在于循环太快而无法绘制画布,但我读到.drawImage()是异步的,因此,从理论上讲,它应该在跳转到下一行之前先绘制画布。
关于如何解决此问题的任何想法? 谢谢。
经过数小时的努力,我终于提出了基于“寻找”事件的解决方案。 为此,视频必须完全加载:
代码如下:
var fps = video_model.getFps(); //screenshot data, comes from another script
var start = shot.getStart();
var end = shot.getEnd();
video.get(0).currentTime = start/fps; //make the video jump to the start
video.on("seeked", function(){ //when the time is seeked, capture screenshot
setTimeout( //the trick is in giving the canvas a little time to be created and painted, 500ms should be enough
function(){
if( video.get(0).currentTime <= end/fps ){
var capture = $(document.createElement("canvas")) //create canvas element on the fly
.attr({
id: video.get(0).currentTime + "sec",
width: video.get(0).videoWidth,
height: video.get(0).videoHeight
})
.appendTo("body");
var ctx = capture.get(0).getContext("2d"); //paint canvas
ctx.drawImage(video.get(0), 0, 0, video.get(0).videoWidth, video.get(0).videoHeight);
if(video.get(0).currentTime + 50/fps > end/fps){
video.off("seeked"); //if last screenshot was captured, unbind
}else{
video.get(0).currentTime += 50/fps; //capture every 50 frames
}
}
}
, 500); //timeout of 500ms
});
这在Chrome和Firefox中对我有用,我已经了解到,在某些版本的特定浏览器中,搜寻到的事件可能有错误。
希望这对任何人都有用。 如果有人想出一个更清洁,更好的解决方案,那么很高兴看到它。
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.