[removed] Extract video frames reliably

Question

I\'m working on a client-side project which lets a user supply a video file and apply basic manipulations to it. I\'m trying to extract the frames from the video reliably.

Answer 1

Here's a working function that was tweaked from this question:

async function extractFramesFromVideo(videoUrl, fps=25) {
  return new Promise(async (resolve) => {

    // fully download it first (no buffering):
    let videoBlob = await fetch(videoUrl).then(r => r.blob());
    let videoObjectUrl = URL.createObjectURL(videoBlob);
    let video = document.createElement("video");

    let seekResolve;
    video.addEventListener('seeked', async function() {
      if(seekResolve) seekResolve();
    });

    video.src = videoObjectUrl;

    // workaround chromium metadata bug (https://stackoverflow.com/q/38062864/993683)
    while((video.duration === Infinity || isNaN(video.duration)) && video.readyState < 2) {
      await new Promise(r => setTimeout(r, 1000));
      video.currentTime = 10000000*Math.random();
    }
    let duration = video.duration;

    let canvas = document.createElement('canvas');
    let context = canvas.getContext('2d');
    let [w, h] = [video.videoWidth, video.videoHeight]
    canvas.width =  w;
    canvas.height = h;

    let frames = [];
    let interval = 1 / fps;
    let currentTime = 0;

    while(currentTime < duration) {
      video.currentTime = currentTime;
      await new Promise(r => seekResolve=r);

      context.drawImage(video, 0, 0, w, h);
      let base64ImageData = canvas.toDataURL();
      frames.push(base64ImageData);

      currentTime += interval;
    }
    resolve(frames);
  });
});

}

Usage:

let frames = await extractFramesFromVideo("https://example.com/video.webm");

Note that there's currently no easy way to determine the actual/natural frame rate of a video unless perhaps you use ffmpeg.js, but that's a 10+ megabyte javascript file (since it's an emscripten port of the actual ffmpeg library, which is obviously huge).

Answer 2

Mostly taken from this great answer by GameAlchemist :

Since browsers doesn't respect videos' framerates, but instead "use of some tricks to make a match between the frame-rate of the movie and the refresh-rate of the screen", your assumption that every 30th of a second, a new frame will be painted is quite inaccurate.
However, the timeupdate event should fire when the currentTime has changed, and we can assume that a new frame was painted.

So, I would do it like so :

document.querySelector('input').addEventListener('change', extractFrames, false);

function extractFrames() {
  var video = document.createElement('video');
  var array = [];
  var canvas = document.createElement('canvas');
  var ctx = canvas.getContext('2d');
  var pro = document.querySelector('#progress');

  function initCanvas(e) {
    canvas.width = this.videoWidth;
    canvas.height = this.videoHeight;
  }

  function drawFrame(e) {
    this.pause();
    ctx.drawImage(this, 0, 0);
    /* 
    this will save as a Blob, less memory consumptive than toDataURL
    a polyfill can be found at
    https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/toBlob#Polyfill
    */
    canvas.toBlob(saveFrame, 'image/jpeg');
    pro.innerHTML = ((this.currentTime / this.duration) * 100).toFixed(2) + ' %';
    if (this.currentTime < this.duration) {
      this.play();
    }
  }

  function saveFrame(blob) {
    array.push(blob);
  }

  function revokeURL(e) {
    URL.revokeObjectURL(this.src);
  }
  
  function onend(e) {
    var img;
    // do whatever with the frames
    for (var i = 0; i < array.length; i++) {
      img = new Image();
      img.onload = revokeURL;
      img.src = URL.createObjectURL(array[i]);
      document.body.appendChild(img);
    }
    // we don't need the video's objectURL anymore
    URL.revokeObjectURL(this.src);
  }
  
  video.muted = true;

  video.addEventListener('loadedmetadata', initCanvas, false);
  video.addEventListener('timeupdate', drawFrame, false);
  video.addEventListener('ended', onend, false);

  video.src = URL.createObjectURL(this.files[0]);
  video.play();
}

<input type="file" accept="video/*" />
<p id="progress"></p>