webRTC convert webm to mp4 with ffmpeg.js - ffmpeg

Question

I am trying to convert webM files to mp4 with ffmpeg.js.
I am recording a video from canvas(overlayer with some information) and recording the audio data from the video.
stream = new MediaStream();
var videoElem = document.getElementById('video');
var videoStream = videoElem.captureStream();
stream.addTrack(videoStream.getAudioTracks()[0]);
stream.addTrack(canvas.captureStream().getVideoTracks()[0]);
var options = {mimeType: 'video/webm'};
recordedBlobs = [];
mediaRecorder = new MediaRecorder(stream, options);
mediaRecorder.onstop = handleStop;
mediaRecorder.ondataavailable = handleDataAvailable;
mediaRecorder.start(100); // collect 100ms of data
function handleDataAvailable(event) {
if (event.data && event.data.size > 0) {
recordedBlobs.push(event.data);
}
}
mediaRecorder.stop();
This code works as expected and returns a webm video
var blob = new Blob(recordedBlobs, {type: 'video/webm'});
Now I want a mp4 file and checked the ffmpeg.js from muaz-khan.
The examples just show how to convert to mp4 when you have 2 single streams (audio and video). But I have one stream with an additional audio track. Can I convert such a stream to mp4? How can that be done?

Answer 1

As per the provided code sample, your recorder stream is having only one audio & one video tracks.
If your input file is having both Audio & Video, then you need to specify output codec for both tracks here as following.
worker.postMessage({
type: 'command',
arguments: [
'-i', 'audiovideo.webm',
'-c:v', 'mpeg4',
'-c:a', 'aac', // or vorbis
'-b:v', '6400k', // video bitrate
'-b:a', '4800k', // audio bitrate
'-strict', 'experimental', 'audiovideo.mp4'
],
files: [
{
data: new Uint8Array(fileReaderData),
name: 'audiovideo.webm'
}
]
});
Trans-coding the video inside browser is not recommend, as it will consume more CPU Time & Memory. And ffmpeg_asm.js is heavy. May be ok for POC :)
What is your use case? webm(vp8/vp9) is widely using these days.
Chrome will support following mime types:
"video/webm"
"video/webm;codecs=vp8"
"video/webm;codecs=vp9"
"video/webm;codecs=h264"
"video/x-matroska;codecs=avc1"
So you can get mp4 recording directly from chrome MediaRecorder with following hack
var options = {mimeType: 'video/webm;codecs=h264'};
mediaRecorder = new MediaRecorder(stream, options);
.....
//Before merging blobs change output mime
var blob = new Blob(recordedBlobs, {type: 'video/mp4'});
// And name your file as video.mp4