Extract Multiple Screenshots Video

extractvideo.js

/*
 * Extract multiple screenshots/frames from a video URL
 * Params
 *   - vidURL : video URL
 *   - frOffsets : array of timestamps
 *   - frameWidth : screenshot width (default: video width)
 * Returns
 *   - frames: object with offset => {imgUrl, blob}
 *
 * An multiple-frame extension of http://cwestblog.com/2017/05/03/javascript-snippet-get-video-frame-as-an-image/
 * In this version, video & canvas elements are reused to extract multiple screenshots. Note, video seeks must
 * occur serially, hence use of async/await.
 */

function extractVideoFrames(vidUrl, frOffsets, frameWidth) {

  function extractFrame(video, canvas, offset) {
    return new Promise((resolve, reject) => {
      video.onseeked = event => {
        var ctx = canvas.getContext('2d');
        ctx.drawImage(video, 0, 0, canvas.width, canvas.height);
        canvas.toBlob(blob => {
          resolve({offset: offset, imgUrl: canvas.toDataURL() , blob: blob});
        }, "image/png");
      };
      video.currentTime = offset;
    });
  };

  async function serialExtractFrames(video, canvas, offsets) {
    var frames = {};
    var lastP = null;

    for (var offset of offsets) {
      if (offset < video.duration) {
        if (lastP) {
          var f = await lastP
          frames[f.offset] = f;
        }
        lastP = extractFrame(video, canvas, offset);
      }
    }
    if (lastP) {
      var f = await lastP;
      frames[f.offset] = f;
      lastP = null;
    }
    return frames;
  };

  return new Promise((resolve, reject) => {
    var vvid = document.createElement("video");
    var vcnv = document.createElement("canvas");
    vvid.onloadedmetadata = event => {
      var aspect_ratio = vvid.videoWidth/vvid.videoHeight;
      vcnv.width = frameWidth !== undefined ? frameWidth : vvid.videoWidth;
      vcnv.height = vcnv.width/aspect_ratio;
      if (vvid.duration) {
        serialExtractFrames(vvid, vcnv, frOffsets).then(resp => {
          resolve(resp);
        })
      }
    }
    vvid.src = vidUrl;
  });
};


export default extractVideoFrames;