W3C home > Mailing lists > Public > public-media-capture@w3.org > March 2012

Re: Constraints and Capabilities API for getUserMedia: more detailed proposal

From: Rich Tibbett <richt@opera.com>
Date: Tue, 13 Mar 2012 19:28:28 +0100
Message-ID: <4F5F91CC.5030702@opera.com>
To: public-media-capture@w3.org
Dan Burnett wrote:
> Group,
>
> Here is the proposal for Constraints and Capabilities for the getUserMedia draft.

> Example:
>
> {camera001:{
>    video-min-width:  800,
>    video-max-width:  1024,
>    video-min-height:  600,
>    video-max-height:  768,
>    video-min-aspectratio:  1.333333333333,
>    video-max-aspectratio:  1.333333333333,
>    video-min-framerate:  24,
>    video-max-framerate:  60,
>    video-min-pixelrate:  15,
>    video-max-pixelrate:  47,
>    video-min-timebetweenkeyframes;  20,
>    video-max-timebetweenkeyframes:  40,
>    video-min-bandwidth:  1.5,
>    video-max-bandwidth:  3.5},
> camera002:{
>    video-min-width:  1600,
>    video-max-width:  1920,
>    video-min-height:  1080,
>    video-max-height:  1200,
>    video-min-aspectratio:  1.33333333333,
>    video-max-aspectratio:  1.77777777777,
>    video-min-framerate:  24,
>    video-max-framerate:  120,
>    video-min-pixelrate:  57.6,
>    video-max-pixelrate:  248,
>    video-min-timebetweenkeyframes;  20,
>    video-max-timebetweenkeyframes:  40,
>    video-min-bandwidth:  8,
>    video-max-bandwidth:  29.4},
> audio001:{
>    audio-min-bandwidth:  1.4,
>    audio-max-bandwidth:  128,
>    audio-min-mos:  2,
>    audio-max-mos:  5,
>    audio-min-codinglatency:  10,
>    audio-max-codinglatency:  50,
>    audio-min-samplingrate:  8000,
>    audio-max-samplingrate:  48000}}
>

As mentioned on the conf call, if this proposal is targeted at 
getUserMedia, I'd like to see how it maps to some of this group's 
original use cases and requirements - especially those documented in [1].

I do think it makes sense to apply characteristics at recording or 
streaming time but I'm unsure as to why I would need to, as a web 
developer, have to go and look up these parameters in a registry and 
then configure a stream object to this level to get some local playback 
going (it's very IANA and web apis typically don't require a manual, a 
steep learning curve on the internals of codecs or registries).

My understanding as of now is that MediaStream is and should remain a 
raw codec-less parameter-less singleton object per input source that is 
shared between all callees to getUserMedia. That allows us to do a great 
deal of code optimizations when doling out multiple MediaStream objects. 
A MediaStream object is then likely to get some parameter refinement if 
or when it is used in a secondary API - recording or streaming.

I expect a valid use case for applying constraints at getUserMedia 
invocation is to allow hardware-based encoding parameters to be applied 
at invocation time. I worry how that scales with multiple calls, each 
with conflicting constraints, to getUserMedia rather than distributing a 
single stream at local invocation time.

What we might want are intent-driven constraints applied at getUserMedia 
invocation, essentially something very much like, or exactly the same as 
[2], with more specific constraints applied further down the RTC 
toolchain at local recording or peer establishment time, assuming in the 
latter case that real meaningful codec/quality parameters cannot be 
established as part of any SDP negotiation between peers.

We may absolutely need constraints and capabilities on getUserMedia but 
it seems this thinking has come a long way from the established use 
cases and requirements.

- Rich

[1] http://www.w3.org/TR/capture-scenarios/

[2] http://lists.w3.org/Archives/Public/public-webrtc/2012Jan/0047.html
Received on Tuesday, 13 March 2012 18:28:59 GMT

This archive was generated by hypermail 2.3.1 : Tuesday, 26 March 2013 16:14:59 GMT