Intent to Implement: Media Capture from HTMLMediaElement

[Miguel Casas-Sanchez]

Contact emails

mca...@chromium.org, nik...@chromium.org

Spec

Media Capture from DOM Elements: HTML Media Element Media Capture Extensions

Summary

“Media Capture from DOM Elements” document by W3C defines captureStream() method that allows the capture of a <video> and/or <audio> elements in the form of a MediaStream. We want to implement the necessary Blink and Chromium sections that would create this stream by accessing the canvas output according to the given frame rate constraints. 

The implementation is rather straightforward once the security constraints are cleared, so not much need for a DD per se. It adds a captureStream() and a captureStreamUntilEnded() static methods. EME-labeled content cannot be captureStream()ed, obviously.

Motivation

This feature lets users stream <video> contents through RTC easily. Also, it already has an implementation by Mozilla Firefox.

Compatibility Risk

This feature is well defined by the W3C spec and already supported by Mozilla Firefox.

Ongoing technical constraints

Currently, there is no transparency support in MediaStream as well as encoders, which would mean that alpha channel output would be lost.

Will this feature be supported on all six Blink platforms (Windows, Mac, Linux, Chrome OS, Android, and Android WebView)?

Yes

OWP launch tracking bug

https://crbug.com/569976

Link to entry on the feature dashboard

https://www.chromestatus.com/features/5522768674160640

Requesting approval to ship?

No. We are planning to implement it behind a runtime flag.

[Harald Alvestrand]

Non-owner LGTM.

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[Joel Weinberger]

Hi there! Taking a brief look at the spec, I'm a bit confused about the implications of using this API on HTTP. Should it be disallowed over insecure origins, similar to what we've done with getUserMedia(), or is there a reason there isn't a similar security concern which I'm missing? Thanks!

--Joel 

[Harald Alvestrand]

The reason why getUserMedia() is deemed a "powerful feature" is that it gives access to camera and microphone - something that the page doesn't have access to before this functionality is available.

The content of a <video> tag is available to the page already, so we're not adding access to anything it doesn't already have (security-wise).

[Joel Weinberger]

Ah, I was thrown off by the "capture" portion of the API name :-) Thanks!

--Joel

[Philip Jägenstedt]

An Intent to Implement needs no LGTM, but a few comments anyway. I've filed these spec bugs:

• Expand on captureStream() for CORS-cross-origin media data
  
• When and how does a MediaStreamTrack end?
  
• Tie MediaStreamTrack lifetime to the underlying AudioTrack or VideoTrack?
  
• Strange description of captureStreamUntilEnded
  

Overall, mapping an HTMLMediaElement to a MediaStream is complicated by the fact that a HTMLMediaElement can represent multiple media resources over its lifetime, and each media resource can have tracks added and removed.

Would it solve the same uses cases to instead map a AudioTrack or VideoTrack to a MediaStreamTrack, leaving the rest of the decisions up to the application?

[outdre...@gmail.com]

This does not seem to work with <video> captureStream on chrome 54.

[PhistucK]

It is still behind a flag. Did you enable the experimental web platform features flag?

If so, you can use crbug.com to report it.

☆PhistucK

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[Bubble Goos]

When will it be in the release? I know the workaround, using audiostream + canvas.StreamCaputre combo. but it's messy.

[Miguel Casas-Sanchez]

The Intent-To-Ship discussion is in https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/SKLhE1LhjOg ; it has been stalled for a while now.