[FFmpeg-devel] [RFC] Support multiple frames in a singe AVPacket in avcodec_decode_subtitle2

Marton Balint cus at passwd.hu
Wed Oct 16 23:41:37 CEST 2013

On Wed, 16 Oct 2013, wm4 wrote:

> On Wed, 16 Oct 2013 14:15:51 -0400
> "Ronald S. Bultje" <rsbultje at gmail.com> wrote:
>> Hi,
>> On Wed, Oct 16, 2013 at 1:38 PM, wm4 <nfxjfg at googlemail.com> wrote:
>>> On Tue, 1 Oct 2013 23:16:01 +0200 (CEST)
>>> Marton Balint <cus at passwd.hu> wrote:
>>>> Hi,
>>>> When I implemented the DVB teletext decoder, I faced a problem: If
>>>> multiple teletext pages are in a single teletext packet, the decoder has
>>>> no way to return multiple AVSubtitles. So the current decoder only return
>>>> one AVSubtitle in that case, an AVSubtitle containing the first decoded
>>>> page from the packet.
>>>> This is not a problem if the user wants to decode only a single teletext
>>>> page (subtitle page), because the same page is not sent twice in a single
>>>> packet. However, if somebody wants to decode all pages, he probably won't
>>>> be able to do so without losing a page here or there.
>>>> I could have split the teletext PES packets (usually around 1472 bytes)
>>> at
>>>> the demuxer level to 46-byte packets to overcome this, but I thought it
>>>> would be much better to extend the API the same way it is used now for
>>>> audio decoding, where a single packet can contain multiple frames.
>>>> If I combine this with CODEC_CAP_DELAY, the teletext decoder can store
>>> the
>>>> remaining pages of a teletext packet (unfortunately libzvbi parses all
>>>> pages in the packet in a single pass), and return them to the user on the
>>>> next call to avcodec_decode_subtitle2. In that case the decoder obviously
>>>> would not consume anything from the next packet until its buffer
>>>> containing teletext pages from the previous packet is not empty.
>>>> If we do this, we will have to make sure that the current subtitle
>>>> decoders will always return the full buffer size as the number of
>>> consumed
>>>> bytes. I've checked, and it seems that only 3 decoders are problematic,
>>>> but they only need a one-line patch to fix them. Movtext (patch is
>>> already
>>>> on the mailing list), srtdec and dvbsub are the three.
>>>> So, what do you think?
>>> Sounds like a bad idea.
>>> First, this kind of partial packet decoding seems to be in decline in
>>> ffmpeg. Video doesn't use it anymore, audio uses it only for some
>>> obscure formats (hopefully one day it won't require this anymore). It's
>>> also additional pain for the user to keep around a packet and to slice
>>> it. This is pretty unintuitive API and increases the amount of
>>> boilerplate needed to decode something. It's also not entirely robust
>>> and foolproof. And now you want to introduce a new API which uses this
>>> API anti-pattern?
>>> Second, the API is in need for a better design. AVSubtitle still sucks,
>>> and I'm very doubtful about how subtitle->ASS conversion is done. I
>>> think the next iteration of the subtitle API should fix this, and not
>>> just be another shot in the dark just to make teletext work for now.
>>> Are you sure there's no better way to shoehorn proper teletext decoding
>>> into ffmpeg?
>> Video and audio are different in that the subpackets for e.g. voice audio
>> are in the realm of several tens of bytes (e.g. 50 byte), which means the
>> (memory/cpu cycle) overhead of giving each packet its own AVPacket
>> container would be highly disproportionate. For video, packet size is
>> several orders of magnitude more than that, so the tradeoff is entirely
>> different between the two - hence the expected optimal (and therefore
>> proposed) solution is different.
> True. Though it seems that often audio is split into subpackets by
> libavformat (or even the container) anyway.
> Is there any reason why avcodec_decode_audio4 can't decode all
> subpackets at once, instead of having the user do repeated calls? Since
> each decode call produce an AVFrame, I figure this would be more
> efficient in general (for the same reasons as you cited).

Well, I don't know, if there is a real-world example for this, but 
subframes may use different sample rate, or may have different metadata, 
or may have different decode_error_flags.

>> This is why back when we introduced this, we chose to have voice codecs
>> implement the design that you call an "anti-pattern", but keep things
>> as-they-were for video codecs.
>> So where does text fit in here? I'd say it's closer to audio, so it makes
>> more sense to use the audio approach.
> Text is even lower bandwidth (unless I'm underestimating teletext and
> dvb), so just directly wrapping each subpacket as separate AVPacket
> might not be too bad.

Actually the DVB teletext packet has 1 byte data identifier, and at most 
31*46 bytes of subpackets. Let's just forget we may need the 1 byte data 
identifier, if we look at the subpackets, for 25 fps content it means 
31*25 = 775 46-byte AVPackets per second, which is roughly 285 kbps.

But even if we split the packets, libzvbi has a nice feature, it only 
decodes teletext pages after it received all the subpackets, all the VBI 
lines from a frame, and it knows that is the case, when it receives a 
subpacket belonging to a VBI line which is already buffered.

So in practice libzvbi outputs all teletext pages of a single frame at 
once. Yes, you can say that the libzvbi is braindead, but tell me now, 
that you will never need to support a codec, where a single packet will 
mean the end of more than one frame.

That is why I still believe that this approach makes the API future-proof. 
Yes, it is less simple this way. I think what you miss is a decoder helper 
API (on top of the existing one), which works somewhat like how 
filtering/buffersrc/buffersink works now. You push one AVPacket into the 
decoder, and you call the decode function until EAGAIN is returned, so you 
will know that you have to push the next AVPacket.

> Also, unlike audio, subtitles aren't really
> "continuous", and you can't just append the decoded data. I'm not sure
> how teletext is supposed to work here: is each AVSubtitle a separate
> page?

Yes, exactly.

> Or do you have to display all AVSubtitles at the same time?
> Depending on what it is, the API might actually be more fundamentally
> inappropriate, and avcodec_decode_subtitle3 would merely solve the
> technical problem of having to deal with subpackets. I keep wondering:
> how are applications supposed to use this at all?

It's for the users/applications to decide. Teletext based subtitling works 
the same way as any subtitling in ffmpeg, definitely a useful feature for 
everybody. Adding support for other kinds of pages to the decoder was no 
big deal, and may be useful for somebody, even if it is not the best way 
to add teletext support to your application at the moment.

Multi-page decoding is a bit broken at the moment. Even if it is rarely 
used, or rarely will be used, it's worth fixing IMHO, and we can only do 
that by extending the API.


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