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Re: Need clarification: Does the spec allow varying the video resolution sent to the sink per-frame?

From: Yuri Wiitala <miu@chromium.org>
Date: Mon, 28 Dec 2015 15:09:32 -0800
Message-ID: <CA+N+EKY9M7x4PipOTWE-QsOUDzzZYHkHKXdtKm8Z86Bf0g=mdQ@mail.gmail.com>
To: Martin Thomson <martin.thomson@gmail.com>
Cc: "public-media-capture@w3.org" <public-media-capture@w3.org>
Thanks for the response, Martin.  I'm wondering if it's worth mentioning
this in the spec; at least, as an example?

On Thu, Dec 10, 2015 at 3:05 PM, Martin Thomson <martin.thomson@gmail.com>
wrote:

> In general, media capture recognizes that sources can change in size
> dynamically.  See
>
> https://w3c.github.io/mediacapture-main/#the-model-sources-sinks-constraints-and-settings
>
> I don't think that there is any implicit assumption about limits on
> this, so per frame changes are perfectly reasonable (in theory,
> changes could be even more frequent than that, but you would never see
> them).
>
> Constraints do establish limits on this, but they can be as loose as
> you like.  There is no need to fix a resolution (or frame rate, or
> aspect ratio, etc...).
>
> On 11 December 2015 at 08:39, Yuri Wiitala <miu@chromium.org> wrote:
> > For screen capturing use cases, such as desktop or tab capture, the
> source
> > content can be resized during a capture session (e.g., the user resizes
> > their browser window).  However, a video stream whose resolution changes
> > dynamically (i.e., possibly per-frame) at the sink is not discussed nor
> in
> > the examples.
> >
> > From what I can tell, the spec seems to always assume one fixed video
> > resolution is chosen at the beginning, within what is allowed by the
> > MediaTrackConstraints.  Then, only by using applyConstraints() can a
> > different fixed resolution be set (by evaluating the new constraints).
> >
> > Or, am I wrong, and because it's not explicitly stated, that there is NO
> > assumption about video resolution to the sink being fixed?  For what it's
> > worth, in the Chrome implementation today, the resolution can vary
> > per-frame.
> >
> > Details on my use case: In Chrome, our tab capture implementation uses
> the
> > min and max width and height constraints to produce video frames of any
> > resolution within range.  It can also provide a fixed aspect ratio, with
> > letter-boxing added where needed, or use any aspect ratio.
> >
> > In addition, video resolution can change, not just because of source
> content
> > resizes, but also depending on system performance (CPU/GPU availability)
> and
> > other external conditions (e.g., network bandwidth when remoting
> content).
> > For example, on a low-end machine, video resolution will be decreased in
> > order to maintain a smooth frame rate.  For further details, please see:
> >
> https://www.chromium.org/developers/design-documents/auto-throttled-screen-capture-and-mirroring
> >
> > Thanks,
> > Yuri
> >
>
Received on Monday, 28 December 2015 23:10:00 UTC

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