MP4 file created by VLC missing NALU type for user data unregistered SEI message.

Using VLC 2.2.0 Weatherwax

Within an MP4 AVC file created by VLC, the first sample of the first chunk contains a SEI message with payload 05 'user data unregistered' that is missing the first byte of a NAL unit: The NAL unit header (IDC + Type '06').

Please take a look at the attached image, which was taken from 'MP4 Reader' which shows the problem.

This presents a problem, because an MP4 parser that reads the first sample of the first chunk will interpret the first byte of the SEI message as the NALU type. In my example, the first byte of the SEI message is '05' which will be interpreted as a 'Coded slice of an IDR picture'.

MP4 sample problem

Added attachment:

• mdat_first_chunk_first_sample.PNG

As can be seen in the attached image, there is:

• The 4-byte sample size 000002A0
• The SEI message (SEI payload type=05, SEI msg length=255 + 255 + 0x9D)

The NAL unit type '06' is missing after the 4-byte sample size.

assigned to @fcartegnie

added Component::Stream output: Muxers + 1 deleted label

Doesn't look like that's a "muxer" specific problem as NAL splitting is done by x264 encoder. That problem might also appear with other muxers.

static block_t *ConvertFromAnnexB(block_t *p_block)
{
    uint8_t *last = p_block->p_buffer;  /* Assume it starts with 0x00000001 */

I hadn't expected such a fast response. You guys are incredible. Merci François.

00 00 01 06 05 block starts with AnnexB 3 bytes sync and is overwritten with 4 bytes size

ISO/IEC 14496-15:2004(E) extra / es NAL split defined in 5.1

commit 7502a03b Author: Francois Cartegnie fcvlcdev@free.fr Date: Tue Mar 17 14:10:14 2015 +0100

mux: mp4: handle both short and long NALU startcodes (fix [#14185](https://code.videolan.org/videolan/vlc/-/issues/14185))

added Status::fixed label

closed

changed milestone to %3.0.0

Milestone renamed

changed milestone to %3.0 FF

Milestone renamed