MFCreateFMPEG4MediaSink does not generate MSE-compatible MP4

I'm attempting to stream a H.264 video feed to a web browser. Media Foundation is used for encoding a fragmented MPEG4 stream (MFCreateFMPEG4MediaSink with MFTranscodeContainerType_FMPEG4, MF_LOW_LATENCY and MF_READWRITE_ENABLE_HARDWARE_TRANSFORMS enabled). The stream is then connected to a web server through IMFByteStream.

Streaming of the H.264 video works fine when it's being consumed by a <video src=".."/> tag. However, the resulting latency is ~2sec, which is too much for the application in question. My suspicion is that client-side buffering causes most of the latency. Therefore, I'm experimenting with Media Source Extensions (MSE) for programmatic control over the in-browser streaming. Chrome does, however, fail with the following error when consuming the same MPEG4 stream through MSE:

Failure parsing MP4: TFHD base-data-offset not allowed by MSE. See https://www.w3.org/TR/mse-byte-stream-format-isobmff/#movie-fragment-relative-addressing

mp4dump of a moof/mdat fragment in the MPEG4 stream. This clearly shows that the TFHD contains an "illegal" base data offset parameter:

[moof] size=8+200
  [mfhd] size=12+4
    sequence number = 3
  [traf] size=8+176
    [tfhd] size=12+16, flags=1
      track ID = 1
      base data offset = 36690
    [trun] size=12+136, version=1, flags=f01
      sample count = 8
      data offset = 0
[mdat] size=8+1624

I'm using Chrome 65.0.3325.181 (Official Build) (32-bit), running on Win10 version 1709 (16299.309).

Is there any way of generating a MSE-compatible H.264/MPEG4 video stream using Media Foundation?

Status Update:

Based on roman-r advise, I managed to fix the problem myself by intercepting the generated MPEG4 stream and perform the following modifications:

Modify Track Fragment Header Box (tfhd):

remove base_data_offset parameter (reduces stream size by 8bytes)

set default-base-is-moof flag

Add missing Track Fragment Decode Time (tfdt) (increases stream size by 20bytes)

set baseMediaDecodeTime parameter

Modify Track fragment Run box (trun):

adjust data_offset parameter

The field descriptions are documented in https://www.iso.org/standard/68960.html (free download).

Switching to MSE-based video streaming reduced the latency from ~2.0 to 0.7 sec. The latency was furthermore reduced to 0-1 frames by calling IMFSinkWriter::NotifyEndOfSegment after each IMFSinkWriter::WriteSample call.

There's a sample implementation available on https://github.com/forderud/AppWebStream

Solution

The mentioned 0.7 sec latency (in your Status Update) is caused by the Media Foundation's MFTranscodeContainerType_FMPEG4 containterizer which gathers and outputs each roughly 1/3 seconds (from unknown reason) of frames in one MP4 moof/mdat box pair. This means that you need to wait 19 frames before getting any output from MFTranscodeContainerType_FMPEG4 at 60 FPS.

To output single MP4 moof/mdat per each frame, simply lie that MF_MT_FRAME_RATE is 1 FPS (or anything higher than 1/3 sec). To play the video at the correct speed, use Media Source Extensions' <video>.playbackRate or rather update timescale (i.e. multiply by real FPS) of mvhd and mdhd boxes in your MP4 stream interceptor to get the correctly timed MP4 stream.

Doing that, the latency can be squeezed to under 20 ms. This is barely recognizable when you see the output side by side on localhost in chains such as Unity (research) -> NvEnc -> MFTranscodeContainerType_FMPEG4 -> WebSocket -> Chrome Media Source Extensions display.

Note that MFTranscodeContainerType_FMPEG4 still introduces 1 frame delay (1st frame in, no output, 2nd frame in, 1st frame out, ...), hence the 20 ms latency at 60 FPS. The only solution to that seems to be writing own FMPEG4 containerizer. But that is order of magnitude more complex than intercepting of Media Foundation's MP4 streams.