How to add "dmb1" four char code in ffmpeg?

Question

I'm trying to stream a video from webcam to the local computer. The stream has resolution of 3840x2160 and 30fps. Computer I'm using is Mac Pro. However when I run it with next command:

ffmpeg -f avfoundation -framerate 30 -video_size 3840x2160 -pix_fmt nv12 -probesize "50M" -i "0" -pix_fmt nv12 -preset ultrafast -vcodec libx264 -tune zerolatency -f mpegts udp://192.168.1.5:5100/mystream

it has a latency of 3-4 seconds. This problem is not present in Chromium, when using MediaStream API stream is displayed in realtime.

I believe that's because Chromium has "dmb1" four char code supported:

+ (media::VideoPixelFormat)FourCCToChromiumPixelFormat:(FourCharCode)code {
  switch (code) {
    case kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange:
      return media::PIXEL_FORMAT_NV12;  // Mac fourcc: "420v".
    case kCVPixelFormatType_422YpCbCr8:
      return media::PIXEL_FORMAT_UYVY;  // Mac fourcc: "2vuy".
    case kCMPixelFormat_422YpCbCr8_yuvs:
      return media::PIXEL_FORMAT_YUY2;
    case kCMVideoCodecType_JPEG_OpenDML:
      return media::PIXEL_FORMAT_MJPEG;  // Mac fourcc: "dmb1".
    default:
      return media::PIXEL_FORMAT_UNKNOWN;
  }
}

To set pixel format Chromium is using next piece of code:

NSDictionary* videoSettingsDictionary = @{
    (id)kCVPixelBufferWidthKey : @(width),
    (id)kCVPixelBufferHeightKey : @(height),
    (id)kCVPixelBufferPixelFormatTypeKey : @(best_fourcc),
    AVVideoScalingModeKey : AVVideoScalingModeResizeAspectFill
  };
  [_captureVideoDataOutput setVideoSettings:videoSettingsDictionary];

I tried doing the same thing in ffmpeg by changing avfoundation.m file. First I added new pixel format AV_PIX_FMT_MJPEG:

static const struct AVFPixelFormatSpec avf_pixel_formats[] = {
    { AV_PIX_FMT_MONOBLACK,    kCVPixelFormatType_1Monochrome },
    { AV_PIX_FMT_RGB555BE,     kCVPixelFormatType_16BE555 },
    { AV_PIX_FMT_RGB555LE,     kCVPixelFormatType_16LE555 },
    { AV_PIX_FMT_RGB565BE,     kCVPixelFormatType_16BE565 },
    { AV_PIX_FMT_RGB565LE,     kCVPixelFormatType_16LE565 },
    { AV_PIX_FMT_RGB24,        kCVPixelFormatType_24RGB },
    { AV_PIX_FMT_BGR24,        kCVPixelFormatType_24BGR },
    { AV_PIX_FMT_0RGB,         kCVPixelFormatType_32ARGB },
    { AV_PIX_FMT_BGR0,         kCVPixelFormatType_32BGRA },
    { AV_PIX_FMT_0BGR,         kCVPixelFormatType_32ABGR },
    { AV_PIX_FMT_RGB0,         kCVPixelFormatType_32RGBA },
    { AV_PIX_FMT_BGR48BE,      kCVPixelFormatType_48RGB },
    { AV_PIX_FMT_UYVY422,      kCVPixelFormatType_422YpCbCr8 },
    { AV_PIX_FMT_YUVA444P,     kCVPixelFormatType_4444YpCbCrA8R },
    { AV_PIX_FMT_YUVA444P16LE, kCVPixelFormatType_4444AYpCbCr16 },
    { AV_PIX_FMT_YUV444P,      kCVPixelFormatType_444YpCbCr8 },
    { AV_PIX_FMT_YUV422P16,    kCVPixelFormatType_422YpCbCr16 },
    { AV_PIX_FMT_YUV422P10,    kCVPixelFormatType_422YpCbCr10 },
    { AV_PIX_FMT_YUV444P10,    kCVPixelFormatType_444YpCbCr10 },
    { AV_PIX_FMT_YUV420P,      kCVPixelFormatType_420YpCbCr8Planar },
    { AV_PIX_FMT_NV12,         kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange },
    { AV_PIX_FMT_YUYV422,      kCVPixelFormatType_422YpCbCr8_yuvs },
    { AV_PIX_FMT_MJPEG,        kCMVideoCodecType_JPEG_OpenDML }, //dmb1
#if !TARGET_OS_IPHONE && __MAC_OS_X_VERSION_MIN_REQUIRED >= 1080
    { AV_PIX_FMT_GRAY8,        kCVPixelFormatType_OneComponent8 },
#endif
    { AV_PIX_FMT_NONE, 0 }
};

After that I tried to hardcode it:

        pxl_fmt_spec = avf_pixel_formats[22];
        ctx->pixel_format = pxl_fmt_spec.ff_id;

        pixel_format = [NSNumber numberWithUnsignedInt:pxl_fmt_spec.avf_id];

        capture_dict = [NSDictionary dictionaryWithObject:pixel_format
                                                   forKey:(id)kCVPixelBufferPixelFormatTypeKey];

        [ctx->video_output setVideoSettings:capture_dict];

Code compiles and builds successfully, but when I run it with above command, without -pix_fmt specified, program enters infinite loop in get_video_config function:

    while (ctx->frames_captured < 1) {
        CFRunLoopRunInMode(kCFRunLoopDefaultMode, 0.1, YES);
    }

It looks obvious that ffmpeg is not able to load first frame. My camera is more than capable of supporting this pixel and stream formats. I proved it with this piece of code which comes after ffmpeg selects which format to use for specified width, height and fps:

            FourCharCode fcc = CMFormatDescriptionGetMediaSubType([selected_format formatDescription]);
            char fcc_string[5] = { 0, 0, 0, 0, '\0'};

            fcc_string[0] = (char) (fcc >> 24);
            fcc_string[1] = (char) (fcc >> 16);
            fcc_string[2] = (char) (fcc >> 8);
            fcc_string[3] = (char) fcc;

            av_log(s, AV_LOG_ERROR, "Selected format: %s\n", fcc_string);

Above code prints "Selected format: dmb1".

Can someone tell me why ffmpeg can't load first frame and how to add new pixel format in this library?

Also, any suggestion on how to resolve input latency of 3 seconds in some other way is more than welcome.

EDIT:

If you try setting any other pixel format in Chromium other then MJPEG there is a latency of 2 seconds. When I say "setting" I mean changing Chromium source code and recompiling it. I am pretty sure that the problem is in pixel format, because camera is sending dmb1 and ffmpeg doesn't know about that format.

Also latency is only present on MacOS.