Questions about H.264 encoding settings with low-latency rate control

Hi everyone,

I noticed that Apple recently added a few new beta sample codes related to video encoding:

Encoding video for low-latency conferencing Encoding video for live streaming

While experimenting with H.264 encoding, I came across some questions regarding certain configurations:

When I enable kVTVideoEncoderSpecification_EnableLowLatencyRateControl, is it still possible to use kVTCompressionPropertyKey_VariableBitRate? In my tests, I get an error.

It also seems that kVTVideoEncoderSpecification_EnableLowLatencyRateControl cannot be used together with kVTCompressionPropertyKey_ConstantBitRate when encoding H264. Is that expected?

When using kVTCompressionPropertyKey_ConstantBitRate with kVTCompressionPropertyKey_MaxKeyFrameInterval set to 2, the encoder outputs only keyframes, and the frame size keeps increasing, which doesn’t seem like the intended behavior.

Regarding the following code from the sample:

let byteLimit = (Double(bitrate) / 8) * 1.5 as CFNumber
let secLimit = Double(1.0) as CFNumber
let limitsArray = [ byteLimit, secLimit ] as CFArray // Each 1 second limit byte as bitrate
err = VTSessionSetProperty(session, key: kVTCompressionPropertyKey_DataRateLimits, value: limitsArray)

This DataRateLimits setting doesn’t seem to have any effect in my tests. Whether I set it or not, the values remain unchanged.

Since the documentation on developer.apple.com/documentation doesn’t clearly explain these cases, I’d like to ask if anyone has insights or recommendations about the proper usage of these settings.

Thanks in advance!