H264 codec: Check profile-level-id when matching

webrtc/media/base/mediaconstants.cc

Index: webrtc/media/base/mediaconstants.cc
diff --git a/webrtc/media/base/mediaconstants.cc b/webrtc/media/base/mediaconstants.cc
index 0f31864ede12081b777477d582398463245c711f..e8ed4fd0c63885b5eb5581cec62228db1aef3083 100644
--- a/webrtc/media/base/mediaconstants.cc
+++ b/webrtc/media/base/mediaconstants.cc
@@ -97,6 +97,9 @@ const char kH264CodecName[] = "H264";
 
 // RFC 6184 RTP Payload Format for H.264 video
 const char kH264FmtpProfileLevelId[] = "profile-level-id";
+// If no profile-level-id is present as a parameter, the Baseline Profile
+// without additional constraints at Level 1 is implied.
+const char kH264FmtpDefaultProfileLevelId[] = "66000a";

[hta-webrtc, 2016/09/19 06:38:36]

I don't think this is baseline.

Remember that the bits in the middle are *restrictive* bits - you turn them on
to restrict the profile to this functionality set only.

Also, our default profile is *constrained* baseline, not just baseline.

My general advice when trying to define profile-level-id values is to use only
values you have seen in the wild.

[magjed_webrtc, 2016/09/19 12:40:30]

Sorry, it should be 66 in decimal, i.e. 0x42 in hex.

This is the default when no profile-level-id is specified. We still use
Constrained Baseline in cricket::DefaultVideoCodecList(), that logic has not
been touched. 

This default is not something I made up, I'm trying to base it on the spec. This
is from https://www.iana.org/assignments/media-types/video/H264-SVC: "If no
profile-level-id is present, the Baseline Profile without additional constraints
at Level 1 MUST be implied." I interpreted this as the constraints set flags
should all be zero, but please correct me if I'm wrong.

I guess we could still set constraint_set0_flag, even though it's redundant.
This is from your message with Mo Zanaty:
"constraint_set0_flag: baseline profile features only (which includes
ASO/FMO/RS)
...
It is a bit redundant to signal constraint set 0 (BP) when using profile=XX=42
(BP), which is still ok, but this signals full/unconstrained BP unless
constraint set 1 and/or 2 are also set to signal CBP."

[hta-webrtc, 2016/10/05 11:56:29]

Yes, I think we should set those 2 flags. They are a bit illogical to my mind,
because setting them to false means that the profile is not constrained, while
setting them to true means that the profile is constrained - I'd think that the
opposite was logical (true = more permitted), but that's not what the spec says.

 const char kH264FmtpLevelAsymmetryAllowed[] = "level-asymmetry-allowed";
 const char kH264FmtpPacketizationMode[] = "packetization-mode";
 const char kH264ProfileLevelConstrainedBaseline[] = "42e01f";