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RE: V4 comments (Re: Settings API Proposal v3 feedback summary)

From: Travis Leithead <travis.leithead@microsoft.com>
Date: Fri, 5 Oct 2012 18:04:15 +0000
To: Harald Alvestrand <harald@alvestrand.no>, "public-media-capture@w3.org" <public-media-capture@w3.org>
Message-ID: <9768D477C67135458BF978A45BCF9B3853A4C217@TK5EX14MBXW605.wingroup.windeploy.ntdev.microsoft.com>
> From: Harald Alvestrand [mailto:harald@alvestrand.no]
> On 10/03/2012 03:10 AM, Travis Leithead wrote:
> > V4 is now ready. I've posted it on Mercurial for easier reading:
> >
> > http://dvcs.w3.org/hg/dap/raw-file/tip/media-stream-
> capture/proposals/SettingsAPI_proposal_v4.html
> >
> > Thanks! Looking forward to your continued feedback.
> >
> Thanks Travis!
> I like the way this is moving - in particular, I like the idea of
> reducing the number of places where apps have to care what kind of
> stream or track they're handling.
> Some worries persist, however:
> - I really like the idea in section 3 where you model the request
> interface as "construct a constraints object and apply it". This unifies
> the constraints namespace and the namespace for requested attributes
> quite neatly.
> HOWEVER - the namespaces are a bit non-overlapping, in particular the
> control of height and width, where you introduce a new enumerated
> "dimension" namespace. When items in both namespaces collide, do we
> synthesize an union, or do we declare a winner?

My concept of constraints is non-persistent, which is where I think this 
confusion is coming from. You seem to have the view that constraints persist,
and that whatever changes the applications wants to make after an initial 
set of constraints have been established must be layered on top of those
initial constraints. Hence there's the notion of collisions, unions, etc., 
and the need to deal with the complexity that introduces.

In my view (the way I've structured the proposal), constraints are 
non-persistent, and behave conceptually similar to a "picker" or "search"
API. The result of applying constraints produces a single device that has been
suitably configured to match the requested constraints. (Note that getUserMedia
actually allows for at most two [different] devices to be selected, but this 
can be thought of as two separate applications of constraints to each class
of device.) There is no "memory" associated with the previous constraints 
after they have been applied. Having selected and configured a device, the 
previous constraints are forgotten.

Thus when you start a set of "request()" changes to change the settings of
a particular device, this is building up a brand-new set of constraints to
apply to the device (which could contradict a previous set of constraints 
that were applied). If it didn't work this way, then it would be very 
challenging to configure a setting to the minimum, then later to the maximum
and then back to the minimum again, as a particular scenario (or end-user) might 
want to do.

> - It's not clear to me what happens to existing constraints placed on a
> track after a series of "request" calls and subsequent applications; are
> the constraint lists merged into an intersection? are they replaced? or
> is this device and implementation dependent?
> I'm particularly thinking of usage patterns where an implementation uses
> constraints at the getUserMedia phase to get a suitable device, and then
> uses the request interface to manipulate it afterwards; is he allowed to
> set constraints outside the originally specified ones, or isn't he?
> Not clear what the right answer is, but there should be only one.

I hope my previous explanation made the answer to the above questions clear.

> - I've worried about this before and gotten some pushback, but I'm still
> worried about the wisdom of setting specific values rather than
> "acceptable ranges" - for instance, some systems will drop resolution,
> framerate or encoding complexity based on CPU temperature, to avoid
> systems shutting down mid-call under "hot" conditions. Setting specific
> values will seem to take away that freedom; setting a range of "values I
> can live with" would preserve it.

Interesting. My proposal doesn't account for the user agent dynamically 
changing the min/max ranges of the settings. In practice, I don't know how
important this case really is? For example, if the device becomes too hot,
then as an implementer, I'd just turn off the device to let it cool down!

I'm pretty confident that setting specific values is desirable, given the
settings tell the developer exactly what the acceptable value ranges are.
I also envision a use case where these settings are likely to be hooked up
directly to the application's UI, in which case a select control or input 
range control will represent the various settings, and the application just
passes those values directly into the request() API to apply the changes.

> - I wonder if experimentation and flexibility could be served if we
> could generalize the interface, so that we don't have to rev the API
> every time we add a setting - the long list of settings in
> PictureAndVideoSettings could possibly be expressed as
> interface MediaSetting {
>      DOMString name;
>      (MediaSettingsList or MediaSettingsRange) setting;
> }
> interface PictureAndVideoSettings {
>     MediaSetting[] settings;
> }
> I don't know if it is wise, but I'd like to hear others' thoughts on
> such an API:

I definitely thought about this. However, there are many more drawbacks to 
this approach (in my opinion) than otherwise:

1. The settings list makes feature detection much more difficult.
2. The settings list moves the actual settings two levels deeper off of the
   object. (Harder to use.)
3. The settings list does not solve name collisions (it's still a problem that
   would need to be dealt with).
4. The settings list does not make it easier/more flexible to rev the API
   (partial interfaces in WebIDL allow simple extensions for v2 specs)

> - lastly, I think that in some cases, for some constraints, it makes
> sense to offer a media settings interface on *any* media stream track,
> including remote ones. I wonder if we can generalize it to be an
> interface on MediaStream itself without hurting it?

I agree that this hypothetical constraint could exist. If we wanted to add it
we could simply define it in terms of the proposed interface like so:

partial interface MediaStreamTrack {
   readonly attribute MediaSettingsRange? hypotheticalSetting;

However, I think we've gotten to the point where we need to stop talking about
hypotheticals, and talk about actual settings and actual constraints--something that
implementers can start building with confidence. To make progress along those lines
my proposal puts a stake in the ground about what settings and constraints could be
relevant for device capture in v1. We should have a conversation about what track types
might be necessary for RTCPeerConnection, and what settings/constraints could be
exposed on those objects (though it's somewhat orthogonal to completing the Media Capture
and Streams spec IMO).
Received on Friday, 5 October 2012 18:05:02 GMT

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