Function h264_frame_type 

pub fn h264_frame_type(au: &[u8]) -> u8

Wire frame kind of one encoded H.264 access unit, read from the bitstream itself: IDR = key, intra-only non-IDR = intra, predicted = delta. The stripe header labels real decode entry points, so the NAL and slice types the encoder actually produced decide, never what was requested of it. An access unit whose slices cannot be parsed is labeled delta: a false delta costs one skipped entry point, a false key breaks the decoder.