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Re: [mediacapture-record] Feature request: Concat filter (#166)

From: guest271314 via GitHub <sysbot+gh@w3.org>
Date: Tue, 26 Mar 2019 16:30:29 +0000
To: public-webrtc-logs@w3.org
Message-ID: <issue_comment.created-476732810-1553617826-sysbot+gh@w3.org>
@Pehrsons Do you mean re-record the recorded media fragments? There are several approaches that have considered and tried.

The concept is briefly outlined at https://github.com/w3c/mediacapture-main/issues/575.

https://github.com/guest271314/ts-ebml provides assistance "remux"ing recorded webm files. https://github.com/guest271314/whammy and https://github.com/guest271314/webm-writer-js provide a means to write images to a webm file created "on the fly", though without support for adding/including audio.

If there were a means to write the Matroska or Webm file directly using human-readable format https://github.com/Matroska-Org/matroska-specification/issues/129, https://github.com/Matroska-Org/ebml-specification/issues/156 the image and audio chunks could be written "on the fly" to a single file. 

What this feature request basically asks for is some form of a flag, or property at options passed to `MediaRecorder` to NOT change `state` to inactive when a single `<video>` or `<audio>` element being captured changes `src`. Beyond the basic feature request, for the ability of `MediaRecorder` to record multiple tracks in sequence even if a track that is e.g., at index 1 of an array passed to `MediaStream` is ended before the track at index 0, then write the data to the resulting webm file. Instead of using several different API's (`AudioContext`, `canvas`) to try to achieve the expected result of concatenating several audio and/or video fragments (or full videos) while simultaneously trying to synchronize audio and video.

The functionality is essentially possible using `canvas` and `AudioContext`, though could be far simpler if `MediaRecorder` was extended to handle multiple video and audio tracks.

In the absence of changing or extending `MediaRecorder` specification then the feature request is for a webm writer - which provides a means to concatenate _video_ data, similar to what is possible using `.decodeAudioData()` and `OfflineAudioContext()` and/or concatenating `AudioBuffer`s (see https://github.com/w3c/mediacapture-main/issues/575) - using human-readable structure, e.g., `XML` that is then written to `EBML` (https://github.com/vi/mkvparse), where the API is `Promise`-based (the media at index N might be read before media at index 0) though the mechanism "awaits" completion of all input media before writing the output: single webm file.

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Received on Tuesday, 26 March 2019 16:30:31 UTC

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