One document matched: draft-alvestrand-constraints-resolution-02.xml

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<rfc category="std" docName="draft-alvestrand-constraints-resolution-02"
     ipr="trust200902">
  <front>
    <title abbrev="Resolution Constraints">Resolution Constraints in Web Real
    Time Communications</title>

    <author fullname="Harald Alvestrand" initials="H. T." surname="Alvestrand">
      <organization>Google</organization>

      <address>
        <email>harald@alvestrand.no</email>
      </address>
    </author>

    <date day="25" month="February" year="2013" />

    <abstract>
      <t>This document specifies the constraints necessary for a Javascript
      application to successfully indicate to a browser that supports WebRTC
      what resolutions it desires on a video stream.</t>

      <t>It also discusses the possible use of SDP to carry that information
      between browsers.</t>
    </abstract>

    <note title="Requirements Language">
      <t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
      "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
      document are to be interpreted as described in <xref
      target="RFC2119">RFC 2119</xref>.</t>
    </note>
  </front>

  <middle>
    <section title="Introduction">
      <t>There are a number of scenarios where it's useful for a WebRTC
      application to indicate to the WebRTC implementation in the supported
      browser what the desired characteristics of a video stream are. These
      include, but are not limited to:</t>

      <t><list style="symbols">
          <t>Specifying a minimum desired resolution for a given application,
          in order to control the user experience or resource tradeoffs made
          by the browser to favour a particular stream</t>

          <t>Specifying a maximum desired resolution for a given stream, in
          order to save some resource (bandwidth, CPU....), possibly outside
          of the browser where the browser can't tell that it's exceeding a
          constraint</t>

          <t>Specifying resolutions that are a reasonable fit for the current
          usage of the video stream, for instance fitting with the number of
          pixels available on the part of a device's display surface that is
          devoted to displaying this video stream</t>

          <t>Specifying the shape of a video stream, in order to fit the video
          onto a display surface without the need for black bars or image
          distortion</t>
        </list>Similar considerations apply for framerate.</t>

      <t></t>

      <section title="Disposition of this text">
        <t>This draft is written in order to get something specific out to
        refer to during spec-writing and implementation. The text may
        eventually get merged into the JSEP specification, <xref
        target="I-D.ietf-rtcweb-jsep"></xref>.</t>
      </section>
    </section>

    <section title="Usage Scenarios">
      <t>Consider the following (simplified) model of a video stream through a
      WebRTC application:</t>

      <figure>
        <artwork><![CDATA[
   |<-------------- Browser A -------------------->|
  Camera ---> MediaStream A ---> Peerconnection A ------+  
          |<------- Application A ---------->|          |
                          v  ^                          v
                 Signalling channel              Internet (media)
                          v  ^                          |
          |<------- Application B ---------->|          |
  <video> tag <-- MediaStream B <--- Peerconnection B --+
   |<-------------Browser B ----------------------->|


]]></artwork>
      </figure>

      <t></t>

      <t>Both applications are running in browsers, with Application A
      connected to a camera that is able to deliver video streams up to HD
      quality (1280x720).</t>

      <t></t>

      <t></t>

      <section title="Scenario: Resolution change">
        <t>At one particular moment in time, the <video> tag in
        Application B is rendered as a thumbnail, and video is flowing to it
        in a 160x100 resolution; there is no need to send any more data, since
        no more pixels are available for its display anyway.</t>

        <t>Then the user of Application B hits the "full-screen" button. There
        are now 1600x1200 pixels available for display.</t>

        <t>Initially, Application B will splay the 160x100 image across the
        larger surface, because there is no other choice, but it will desire
        to have as many pixels as possible available to provide a high quality
        image.</t>
      </section>

      <section title="Scenario: Constrained bandwidth">
        <t>At one particular moment in time, the camera is generating
        1280x720, resulting in a 2 Mbits/second data flow from A to B.
        Congestion control signals that this data rate is no longer available;
        rather than letting the browser reduce the bandwidth of some flow of
        its choice, Application A decides that the high definition video is
        the feature that is least valuable. It can then apply a new constraint
        to Mediastream A, specifying that resolution should be at most
        640x360; browser A is then responsible for making sure this decision
        is communicated to browser B (if it needs to be).</t>
      </section>

      <section title="Scenario: Limited processing capacity">
        <t>If application B is running on a slow machine (2000-class PC or
        2010-class mobile phone), the maximum capacity of the video decoder
        may be 320x200 - Application B may then wish to indicate that
        application A should limit the stream sent across the network to that
        resolution - sending more bits isn't useful, because the receiver
        doesn't have enough capacity to decode and downscale the video
        stream.</t>
      </section>
    </section>

    <section title="Models for resolution manipulation">
      <t>As specified in the "v6 Settings model" being developed in the Media
      Capture Task Force (snapshot:
      http://dvcs.w3.org/hg/dap/raw-file/tip/media-stream-capture/proposals/SettingsAPI_proposal_v6.html),
      the consumer of a video track in a MediaStream will have a "native
      resolution", which indicates what size video it's useful to push to it.
      The application can also set (and change) constraints on the video
      MediaStreamTrack, indicating which range of properties it sees useful
      for the purposes of the application.</t>

      <t>In SDP, the "a=imageattr" attribute is available to provide
      information on the resolution of video streams described by an SDP
      m-line; a proposal <xref
      target="I-D.lennox-mmusic-sdp-source-selection"></xref> has been floated
      to provide similar information on a per-SSRC basis.</t>

      <t>If both mechanisms are available, the choices available to the writer
      of application B in the "increase screen area" above are:</t>

      <t><list style="symbols">
          <t>Signal (by non-standard means) to Application A that more pixels
          are needed. Application A will then modify the constraints on
          Mediastream A to say that the desired (not mandatory) min resolution
          is 1600x1200; Browser A will then reconfigure the camera to generate
          the closest available resolution, which is 1280x720.</t>

          <t>Apply a new constraint set to Mediastream B's video track, saying
          that the desired resolution is now 1600x1200. Browser B will then
          have to figure out that this is an incoming track via Peerconnection
          B, and that the resolution needs to be signalled; it will then fire
          a NegotationNeeded event at Application B, which will then
          renegotiate the desired resolutions using an SDP exchange with
          Browser A; Browser A will then figure out from the SDP that it's
          useful to generate a higher resolution video stream, and reconfigure
          the camera as above.</t>

          <t>Execute a renegotiation with Application A, adding a=remote-ssrc:
          attributes as described in <xref target="s-examples"></xref> by
          modifying the SDP generated by CreateOffer, and triggering the
          behaviour in the previous alternative inside Browser A. API-wise,
          this is perhaps the most complex method.</t>
        </list></t>

      <t>The advantage of the first method is that it does not require any SDP
      parsing or generation.</t>

      <t>The advantage of the second method is that it will work when
      appliation A and application B are different applications; there is no
      need for them to have any private agreement on how to set bitrate. It
      does require both the implementation of constraints and that browser B
      has the ability to generate the proper constraints in the SDP.</t>

      <t>The third method requires SDP parsing in browser A, but not SDP
      generation in browser B. It does require SDP manipulation in Javascript
      at application B.</t>
    </section>

    <section title="Constraints for specifying resolution">
      <t>In order to have either of the approaches to work, the specific
      constraints to use have to be defined. This section defines the
      constraints for resolution and framerate needed.</t>

      <t>These constraints are usable in several places:</t>

      <t><list style="symbols">
          <t>As constraints to the getUserMedia call <xref
          target="W3C.WD-mediacapture-streams-20120628"></xref>, where they
          serve to guide the configuration of the camera obtained, and may
          influence the choice of camera.</t>

          <t>As constraints to the addStream call on a PeerConnection <xref
          target="W3C.WD-webrtc-20120821"></xref>, where they serve to guide
          the configuration of the codec that encodes the video content for
          transmission.</t>

          <t>As constraints applied to an existing local video stream using
          the "change constraints" API, where it may cause the video engine to
          reconfigure the device or codec for that particular stream.</t>

          <t>As constraints applied to an incoming video stream using the
          "change constrains" API on a MediaStreamTrack, where it serves to
          inform the video engine about the desirable properties of the video
          track, which may lead to the video engine choosing to reencode the
          video and/or signal a remote video source that it wishes certain
          constraints to be put in place.</t>
        </list>All of the constraints may be meaningful in both "mandatory"
      and "optional" forms.</t>
    </section>

    <section anchor="s-examples" title="Syntax and Mapping Examples">
      <t>See <xref target="IANA"></xref> for the actual definition of the
      constraints used here.</t>

      <section title="Examples with GetUserMedia">
        <t>A constraint saying that we absolutely must have a minimum
        resolution of 1024x768:</t>

        <figure>
          <artwork><![CDATA[
getUserMedia({
   video: { mandatory: { minWidth: 1024, minHeight: 768 } }
}, successCallback, errorCallback);


]]></artwork>
        </figure>

        <t>A constraint saying that we'd prefer 60 frames per second, if
        available, and if we can get that, we'd like to limit the max
        resolution, but in all cases, the screen must be clamped to a 4:3
        aspect ratio - 16:9 or odd aspect ratios are not acceptable to this
        application:</t>

        <figure>
          <artwork><![CDATA[
getUserMedia({
   video: {
     mandatory: { minAspectRatio: 1.333, maxAspectRatio: 1.334 },
     optional [
       { minFrameRate: 60 },
       { maxWidth: 640 },
       { maxHeigth: 480 }
     ]
   }
}, successCallback, errorCallback);
]]></artwork>
        </figure>

        <t></t>
      </section>

      <section title="SDP mappings">
        <t>This document does not specify the exact mapping of constraints
        into imageattr values; this will have to be done before this mechanism
        can be depended on.</t>

        <t>The examples below are thought exercises, based on <xref
        target="I-D.lennox-mmusic-sdp-source-selection"></xref> and <xref
        target="I-D.alvestrand-rtcweb-resolution"></xref>.</t>

        <t>An optional constraint has been applied to an incoming stream where
        both upper and lower are constrained to 320x200. The stream has been
        assigned to a hardware video decoder that can decode most resolutions
        up to 1024x768, in any aspect ratio, but only if all divisions are
        divisible by 4. The incoming stream has SSRC 1234.</t>

        <t>Escaped line breaks are added for readability.</t>

        <figure>
          <artwork><![CDATA[
m=video
a=remote-ssrc:1234 imageattr:* [x=320,y=200,q=1.0] \
                  [x=[120:4:1024],y=[100:4:768],q=0.2]



]]></artwork>
        </figure>

        <t></t>
      </section>
    </section>

    <section anchor="IANA" title="IANA Considerations">
      <t>This document requests IANA to register constraints in the "RTCWeb
      Media Constraints" registry created by <xref
      target="I-D.burnett-rtcweb-constraints-registry"></xref>. NOTE: The
      registrations assume that this document is updated to no longer have
      "video" as part of the name, but have "video" as a field-of-use in the
      registration.</t>

      <t>The definitions of width, height and aspect ratio are taken from
      <xref target="RFC6236"></xref>.</t>

      <t><list style="symbols">
          <t>minWidth - valid for video. Corresponds to the "x" value (pixel
          count) from RFC 6236. Only integer values are valid.</t>

          <t>maxWidth - valid for video. Definition as for minWidth.</t>

          <t>minHeight - valid for video. Corresponds to the "y" value (pixel
          count) from RFC 6236. Only integer values are valid.</t>

          <t>maxHeight - valid for video. Definition as for minHeight.</t>

          <t>minAspectRatio - valid for video. Corresponds to the "par"
          (picture aspect ratio), with "sar" set to 1.0. A 4:3 format display
          corresponds to an AspectRatio of 1.3333. Floating point values are
          valid.</t>

          <t>maxAspectRatio - valid for video. Definition as for
          minAspectRatio.</t>

          <t>minFramerate - valid for video. Corresponds to the framerate
          defined in <xref target="RFC4566"></xref>, the "a=framerate"
          attribute.</t>

          <t>maxFramerate - valid for video. Definition as for
          minFramerate.</t>
        </list></t>

      <t>The contact person is Harald Alvestrand <hta@google.com>.</t>

      <t>Change control for the registration is with the IETF, as designated
      by the IESG.</t>

      <t>Note that minFramerate defines a lower bound for the a=framerate
      attribute, which is itself defined as an upper limit; this means that
      even if a high framerate is negotiated, the actual framerate used may be
      lower due to temporary considerations (for instance CPU or bandwidth, or
      simply lack of movement in the picture).</t>
    </section>

    <section anchor="Security" title="Security Considerations">
      <t>No security considerations particular to these specific constraints
      have so far been identified.</t>
    </section>

    <section anchor="Acknowledgements" title="Acknowledgements">
      <t>Special thanks are given to Dan Burnett, Cullen Jennings, the IETF
      RTCWEB WG and the W3C WEBRTC WG for strongly influencing this memo, and
      to Per Kjellander for being the first to implement the constraints in
      getUserMedia.</t>
    </section>
  </middle>

  <back>
    <references title="Normative References">
      <?rfc include="reference.RFC.2119"?>

      <?rfc include='reference.RFC.4566'?>

      <?rfc include='reference.RFC.6236'?>

      <?rfc include='reference.I-D.burnett-rtcweb-constraints-registry'?>
    </references>

    <references title="Informative References">
      <?rfc include='reference.I-D.lennox-mmusic-sdp-source-selection'?>

      <?rfc include='reference.I-D.alvestrand-rtcweb-resolution'?>

      <?rfc include='reference.I-D.ietf-rtcweb-jsep'?>

      <?rfc include='reference.W3C.WD-mediacapture-streams-20120628'?>

      <?rfc include='reference.W3C.WD-webrtc-20120821'?>
    </references>

    <section title="Changes from -00 to -01">
      <t>Added the "Usage Scenarios" chapter.</t>

      <t>Repointed the eventual target to be incorporation in the JSEP
      draft.</t>

      <t>Made sure the constraints are consistently spelled in camelCase, with
      a small initial letter.</t>
    </section>

    <section title="Changes from -01 to -02">
      <t>Moved a bit of the text around between sections, and referred to the
      "settings API" proposal from the Media Capture task force.</t>
    </section>
  </back>
</rfc>