mp4 file changes size during playback

[David Ing]

Mathieu --

Please excuse the inadequacy of my google-fu.  I am having trouble finding
any kind of authoritative reference document which describes avc1, 2, 3, 4,
... (and the subtle differences between them)

   - My best guess is that avc1 refers to the first amendment:
   https://www.iso.org/standard/67318.html
   - My best guess is that avc3 refers to the third amendment:
   https://www.iso.org/standard/69728.html

Also -- I am a bit surprised that we need to pay in order to view documents
like this.  Does anyone know if there is a way to view this stuff for free?

On Wed, Dec 4, 2019 at 9:14 AM Mathieu Duponchelle <mathieu at centricular.com>
wrote:

> That's an interesting file, but I'm not sure it is a a compliant one:  the
> resolution change is
> advertised through SPS in the bitstream, but the MP4 header says "avc1",
> which as far as I remembe
> means the H264 bitstream should not contain any SPS (please double check
> this, haven't looked at the spec).
>
> Ideally, the tool you produced this file with would use an "avc3" stream
> format, and not hold
> any parameter sets in its MP4 header. That way, you could detect
> resolution changes by simply
> plugging a demuxer and a parser together and looking for new caps.
> Currently you can only do that
> either by also plugging a decoder afterwards and looking for caps changes,
> or analysing the SPS
> manually :)
>
> Best,
>
> --
> Mathieu Duponchelle · https://www.centricular.com
>
>
> On 12/3/19 10:50 PM, David Ing wrote:
>
> I have an mp4 file which changes size during playback.  But I noticed that
> gst-discoverer-1.0 does not report the fact that the size changes.
>
> I have two questions.
>
>    1. In theory, what is the fastest way to scan the file to determine
>    where the size changes?
>    2. In practice, what is the easiest way (given the current state of
>    gstreamer) to scan the file and determine where the size changes?
>
> Here is the file which changes size during playback:
>
>
> https://drive.google.com/drive/folders/1f4pGNDhzCvsg1xXWj4y9DS3vXkaglOvZ?usp=sharing
>
> Here is the gst-discoverer-1.0 output:
>
> $ *gst-discoverer-1.0 -v screencap.m4v *
> Analyzing file:///home/ding/mess/repro/screencap.m4v
> Done discovering file:///home/ding/mess/repro/screencap.m4v
>
> Topology:
>   container: video/quicktime, variant=(string)iso
>     video: video/x-h264, stream-format=(string)avc, alignment=(string)au,
> level=(string)4.2, profile=(string)main,
> codec_data=(buffer)014d402affe1002b674d402a965402a835f35602d40404050000030001000003003ce060007d0000030177009a9503b4244d4001000468ce3520,
> width=(int)996, height=(int)632, pixel-aspect-ratio=(fraction)1/1,
> framerate=(fraction)30/1, interlace-mode=(string)progressive,
> chroma-format=(string)4:2:0, bit-depth-luma=(uint)8,
> bit-depth-chroma=(uint)8, parsed=(boolean)true
>       Tags:
>         video codec: H.264 / AVC
>         bitrate: 309812
>         datetime: 2019-11-01T17:40:33Z
>         title: n/a
>         container format: ISO MP4/M4A
>
>       Codec:
>         video/x-h264, stream-format=(string)avc, alignment=(string)au,
> level=(string)4.2, profile=(string)main,
> codec_data=(buffer)014d402affe1002b674d402a965402a835f35602d40404050000030001000003003ce060007d0000030177009a9503b4244d4001000468ce3520,
> width=(int)996, height=(int)632, pixel-aspect-ratio=(fraction)1/1,
> framerate=(fraction)30/1, interlace-mode=(string)progressive,
> chroma-format=(string)4:2:0, bit-depth-luma=(uint)8,
> bit-depth-chroma=(uint)8, parsed=(boolean)true
>       Additional info:
>         None
>       Stream ID:
> 14c5bd3783191c8af25f30b00a879f8f7bdaa4367c8ecbd954b38e5367507175/001
>       Width: 1350
>       Height: 846
>       Depth: 24
>       Frame rate: 30/1
>       Pixel aspect ratio: 1/1
>       Interlaced: false
>       Bitrate: 309812
>       Max bitrate: 0
>
> Properties:
>   Duration: 0:00:34.861322222
>   Seekable: yes
>   Live: no
>   Tags:
>       video codec: H.264 / AVC
>       bitrate: 309812
>       datetime: 2019-11-01T17:40:33Z
>       title: n/a
>       container format: ISO MP4/M4A
>
>
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