mp4 file changes size during playback

[David Ing]

Here is the AVC1 standard:
https://www.itu.int/ITU-T/recommendations/rec.aspx?rec=7825  (the link was
right there as a reference in the wikipedia article,
https://en.wikipedia.org/wiki/Advanced_Video_Coding#Versions).

Page 31 / section 7.3.2.1 defines the Sequence parameter set RBSP syntax,
which includes:

   - pic_width_in_mbs_minus1
   - pic_height_in_map_units_minus_1

Therefore (unless I am mistaken), it looks like sequence parameter sets
(which contain width / height settings) are allowed in avc1.

On Wed, Dec 4, 2019 at 10:11 AM David Ing <ding at panopto.com> wrote:

> Okay ... I think this is where the versions are defined:
> https://en.wikipedia.org/wiki/Advanced_Video_Coding#Versions
>
> (Still trying to figure out how to view the standards for free.)
>
> On Wed, Dec 4, 2019 at 10:03 AM David Ing <ding at panopto.com> wrote:
>
>> 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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>>>
>>>
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