Solution
After some significant Googling I managed to find the missing piece to the puzzle: MediaSource
Effectively the process goes like this:
- Create a
MediaSource
- Create an object URL from the
MediaSource
- Set the video's
src to the object URL
- On the
sourceopen event, create a SourceBuffer
- Use
SourceBuffer.appendBuffer() to add all of your chunks to the video
This way you can keep adding new bits of video without changing the object URL.
Caveats
- The
SourceBuffer object is very picky about codecs. These have to be declared, and must be exact, or it won't work
- You can only append one blob of video data to the
SourceBuffer at a time, and you can't append a second blob until the first one has finished (asynchronously) processing
- If you append too much data to the
SourceBuffer without calling .remove() then you'll eventually run out of RAM and the video will stop playing. I hit this limit around 1 hour on my laptop
Example Code
Depending on your setup, some of this may be unnecessary (particularly the part where we build a queue of video data before we have a SourceBuffer then slowly append our queue using updateend). If you are able to wait until the SourceBuffer has been created to start grabbing video data, your code will look much nicer.
<html>
<head>
</head>
<body>
<video id="video"></video>
<script>
// As before, I'm regularly grabbing blobs of video data
// The implementation of "nextChunk" could be various things:
// - reading from a MediaRecorder
// - reading from an XMLHttpRequest
// - reading from a local webcam
// - generating the files on the fly in JavaScript
// - etc
var arrayOfBlobs = [];
setInterval(function() {
arrayOfBlobs.append(nextChunk());
// NEW: Try to flush our queue of video data to the video element
appendToSourceBuffer();
}, 1000);
// 1. Create a `MediaSource`
var mediaSource = new MediaSource();
// 2. Create an object URL from the `MediaSource`
var url = URL.createObjectURL(mediaSource);
// 3. Set the video's `src` to the object URL
var video = document.getElementById("video");
video.src = url;
// 4. On the `sourceopen` event, create a `SourceBuffer`
var sourceBuffer = null;
mediaSource.addEventListener("sourceopen", function()
{
// NOTE: Browsers are VERY picky about the codec being EXACTLY
// right here. Make sure you know which codecs you're using!
sourceBuffer = mediaSource.addSourceBuffer("video/webm; codecs=\"opus,vp8\"");
// If we requested any video data prior to setting up the SourceBuffer,
// we want to make sure we only append one blob at a time
sourceBuffer.addEventListener("updateend", appendToSourceBuffer);
});
// 5. Use `SourceBuffer.appendBuffer()` to add all of your chunks to the video
function appendToSourceBuffer()
{
if (
mediaSource.readyState === "open" &&
sourceBuffer &&
sourceBuffer.updating === false
)
{
sourceBuffer.appendBuffer(arrayOfBlobs.shift());
}
// Limit the total buffer size to 20 minutes
// This way we don't run out of RAM
if (
video.buffered.length &&
video.buffered.end(0) - video.buffered.start(0) > 1200
)
{
sourceBuffer.remove(0, video.buffered.end(0) - 1200)
}
}
</script>
</body>
</html>
As an added bonus this automatically gives you DVR functionality for live streams, because you're retaining 20 minutes of video data in your buffer (you can seek by simply using video.currentTime = ...)
