HTML5 Video buffered attribute features

How video is buffered is browser implementation-dependent and therefore may vary from browser to browser.

Various browsers can use different factors to determine to keep or to discard a part of the buffer. Old segments, disk space, memory, and performance are typical factors.

The only way to know is to “see” what the browser has or is loading.

For example – in Chrome I played a few seconds then I skipped to about 30 seconds and you can see that it starts to load another part starting from that position.

(The buffer also seem to be bounded to key-frames so it is possible to decode the n-frames in that buffer. This means the buffer can start to load data a little before the actual position).

I supplied a demo video about 1 minute long – however, this is not long enough to do proper testing. Free free to supply video links that contain longer video (or please share if you want me to update the demo with this).

The main function will iterate through the buffered object on the video element. It will render all parts that exist to the canvas right below the video showing in red.

You can click (bit not drag) on this viewer to move the video to different positions.

/// buffer viewer loop (updates about every 2nd frame)
function loop() {

    var b = vid.buffered,  /// get buffer object
        i = b.length,      /// counter for loop
        w = canvas.width,  /// cache canvas width and height
        h = canvas.height,
        vl = vid.duration, /// total video duration in seconds
        x1, x2;            /// buffer segment mark positions

    /// clear canvas with black
    ctx.fillStyle="#000";
    ctx.fillRect(0, 0, w, h);

    /// red color for loaded buffer(s)
    ctx.fillStyle="#d00";

    /// iterate through buffers
    while (i--) {
        x1 = b.start(i) / vl * w;
        x2 = b.end(i) / vl * w;
        ctx.fillRect(x1, 0, x2 - x1, h);
    }

    /// draw info
    ctx.fillStyle="#fff";

    ctx.textBaseline="top";
    ctx.textAlign = 'left';
    ctx.fillText(vid.currentTime.toFixed(1), 4, 4);

    ctx.textAlign = 'right';
    ctx.fillText(vl.toFixed(1), w - 4, 4);

    /// draw cursor for position
    x1 = vid.currentTime / vl * w;

    ctx.beginPath();
    ctx.arc(x1, h * 0.5, 7, 0, 2 * Math.PI);
    ctx.fill();

    setTimeout(loop, 29);
}