keyframe interval on iOS hardware encoder

[h...@highfive.com]

Ran into the following todo:

https://chromium.googlesource.com/external/webrtc/+/master/webrtc/modules/video_coding/codecs/h264/h264_video_toolbox_encoder.cc#441

and it seems like without setting kVTCompressionPropertyKey_MaxKeyFrameInterval the encoder gets to decide how many keyframes to send.

<VideoToolbox/VTCompressionProperties.h>:93

 Video encoders are allowed to generate key frames more frequently if
 this would result in more efficient compression.  
 The default key frame interval is 0, which indicates that the
 video encoder should choose where to place all key frames.

Trying this out on AppRTCDemo with iPhone 6S+ in a loopback call resulted in a generated keyframe approximately every 2 secs. So despite setting kVTCompressionPropertyKey_RealTime the encoder is trying to optimize for seekable content with periodic keyframes, instead of optimizing for latency and bandwidth by sending fewer keyframes. Does it make sense to set the kVTCompressionPropertyKey_MaxKeyFrameInterval to something relatively high so that only things like FIR/PLI triggers a keyframe?

[Peter Boström]

Do you mind filing a bug on this? Getting keyframes every two seconds seems bad.

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[h...@highfive.com]

Filed https://bugs.chromium.org/p/webrtc/issues/detail?id=5815.