Re: [FFmpeg-devel] [PATCH] avformat/flvenc: Specify codec tag with MKTAG

[Zhao Zhili <quinkblack-at-foxmail.com@ffmpeg.org>]

> On May 17, 2025, at 01:10, Zhao Zhili <quinkblack-at-foxmail.com@ffmpeg.org> wrote:
> 
> 
> 
>> On May 17, 2025, at 00:27, Timo Rothenpieler <timo@rothenpieler.org> wrote:
>> 
>> On 16.05.2025 17:59, Zhao Zhili wrote:
>>>> On May 16, 2025, at 22:52, Timo Rothenpieler <timo@rothenpieler.org> wrote:
>>>> 
>>>> On 16/05/2025 16:24, Zhao Zhili wrote:
>>>>> From: Zhao Zhili <zhilizhao@tencent.com <mailto:zhilizhao@tencent.com>>
>>>>> ffmpeg -i input.mp4 -c copy -tag:v av01 output.flv
>>>>> [flv @ 0x143204080] Tag av01 incompatible with output codec id '225' (10va)
>>>> 
>>>> I don't quite understand what causes this.
>>>> Is this an issue when running on big endian architectures?
>>>> I'm pretty sure I tested all combinations of codecs with muxing and demuxing, and never ran into that error.
>>> The key point is when specify tag via command line, e.g., -tag:v av01, it’s
>>> passed to AVCodecParameters codec_tag in little endian.
>>> You didn’t see the error because without specify the tag explicitly, codec_tag is copied
>>> from AVOutputFormat codec_tag to AVCodecParameters codec_tag, so they are
>>> the same.
>>> Another example is codec_mp4_tags.
>> 
>> This still irks me as wrong.
>> There is _a lot_ of places all over flvenv.c, in all kinds of functions, that hard-depend on the values in par->codec_tag being from the _codec_ids tables at the top of the file.
>> Like, they contain flv specific audio and video codec IDs for the pre-ext-flv codecs, those would all also break.
>> 
>> So there seems to be a deeper issue there if those values can be overridden from the commandline. The encoder clearly does not expect that.
>> 
>> Looking at the code this error comes from:
>> https://github.com/FFmpeg/FFmpeg/blob/master/libavformat/mux.c#L314
>> 
>> It looks to to me like it's working exactly as intended and required by flvenc, protecting it from invalid tags.
>> 
>> So, when the user provides a custom tag that is invalid, isn't that kinda on the user?
>> The check you're running into does what it's supposed to:
>> It detects that the provided tag is invalid for this codec in this container.
>> 
>> Why do you want to override it anyway? There is only exactly one valid tag for each codec.
> 
> A user shows the error message to me. Because he know there are -tag option for mp4, and
> enhanced-rtmp use fourcc for extended codecs, so he thought it should work.
> 
> The -tag:v av01 is redundant, it should be a NOP, not trigger error. The strings “av01”
> is the right order of fourcc in spec. The endian issue should be limited to the internal.
> Current error message is confusing, because it shows 10va instead of av01.

Doc from Microsoft shows fourcc use small endian
https://learn.microsoft.com/en-us/windows/win32/directshow/fourcc-codes

While wiki and enhanced rtmp spec says it’s big endian
https://en.wikipedia.org/wiki/FourCC

It’s clear in av_fourcc_make_string
that we use small endian.

For normal codec id in flv, e.g, 7 for H.264, it’s not a big issue, since they’re not fourcc.

> 
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