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

[Zhao Zhili]

> 在 2025年5月17日，上午1:39，Timo Rothenpieler <timo at rothenpieler.org> 写道：
> 
> On 16.05.2025 19:24, Zhao Zhili wrote:
>>>> On May 17, 2025, at 01:10, Zhao Zhili <quinkblack-at-foxmail.com at ffmpeg.org> wrote:
>>> 
>>> 
>>> 
>>>> On May 17, 2025, at 00:27, Timo Rothenpieler <timo at rothenpieler.org> wrote:
>>>> 
>>>> On 16.05.2025 17:59, Zhao Zhili wrote:
>>>>>> On May 16, 2025, at 22:52, Timo Rothenpieler <timo at rothenpieler.org> wrote:
>>>>>> 
>>>>>> On 16/05/2025 16:24, Zhao Zhili wrote:
>>>>>>> From: Zhao Zhili <zhilizhao at tencent.com <mailto:zhilizhao at 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.
> 
> This patch just fixes it for a very small subset of codecs in flv though.
> There's nothing indicating that -tag:v is needed or sensibly supported in flvenc.
> If you'd want to pass h264 or aac as fourcc, it'd be flat out impossible, and no easy fix is available.
> 
> I'd rather not complicate flvenc, even if just a little bit, just to swap around the endianness of the few codecs that do use a fourcc based tag.
> 
> flvenv should probably just completely ignore -tag:v, since the option makes no sense for it anyway.

I can remove setting tag list to AVOutputFormat, does that works for you?

> And maybe print a warning when it's used only.
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