[Libav-user] Can't get MPEG4 decoder
Michael IV
explomaster at gmail.com
Thu Nov 9 20:05:23 EET 2017
Ok.then why when I configure FFMPEG build to include mp4 demuxer,the
after-config report doesn't show mp4?
I do set that name:
*--enable-demuxer='mov,h264,mp4,ogg,matroska,wav,flac' *
Also,right,I was confused with MPEG4 name. In fact I need h264 codec and my
initial video was with MPEG4 .Now I tried to use a video encoded
with h264 AVC. and
dec = avcodec_find_decoder(dec_ctx->codec_id); //codec_id h264
returns a valid codec. But I have another problem, even though I
call avformat_find_stream_info()
I am getting both streams' codecpar uninitialized:
AVCodecParameters* id0= pFormatCtx->streams[0]->codecpar; //here
should be AAC props
AVCodecParameters* id1 = pFormatCtx->streams[1]->codecpar; //here should be
h264 props
Why that happens?
On Thu, Nov 9, 2017 at 7:56 PM, Dan Haddix <dan6992 at hotmail.com> wrote:
> You don't need to enable codecs explicitly when you build ffmpeg unless
> you want to exclude everything else. Just do a build without any of the
> enable/disable flags and it will include most of the codecs and muxers by
> default.
>
> I think you're a little confused on the terminology.
>
> First off "mov" is part of the "mp4" demuxer. It's a slight variation of
> mp4 tha was developed by Apple before mp4 existed. It's actually what the
> mp4 specification was built from.
>
> Secondly the MPEG-4 spec actually contains two video codecs. MPEG-4 part 2
> is the older one that Divx and Xvid were based on. MPEG-4 part 10 is H.264
> (aka AVC) and is what most people think of these days when you say MPEG-4.
>
> In avlib the MP4 demuxer is "mp4". The MPEG-4 part 2 decoder is "mpeg4",
> the H.264 decoder is "h264".
>
> A little trick for finding the right name. Look for the code file that
> you're trying to use, like h264dec.c for H.264 decoder, then scroll all the
> way to the bottom. There is a little constructor class and one of the
> variables is the .name which is what you're looking for.
>
> Good luck,
> Dan
>
>
> On Nov 9, 2017 at 9:38 AM, <Michael IV <explomaster at gmail.com>> wrote:
>
> But now I see in vlc that the MPEG4 codec is actually named "mp4v" .So
> that's the name I have to specificy in --enable-decoder when doing FFMPEG
> config?
>
>
> On Thu, Nov 9, 2017 at 7:26 PM, Michael IV <explomaster at gmail.com> wrote:
>
>> And testing the actual codecId of the video stream in the file I am also
>> getting NULL for avcodec_find_decoder:
>>
>> ret = av_find_best_stream(pFormatCtx, AVMEDIA_TYPE_VIDEO, -1,
>> -1, NULL, 0);
>> st = pFormatCtx->streams[ret];
>> AVCodecContext *dec_ctx = NULL;
>> AVCodec *dec = NULL;
>> dec_ctx = st->codec;
>> dec = avcodec_find_decoder(dec_ctx->codec_id);
>>
>> * where ec_ctx->codec_id == AV_CODEC_ID_MPEG4*
>>
>> On Thu, Nov 9, 2017 at 7:22 PM, Michael IV <explomaster at gmail.com> wrote:
>>
>>> Hi, thanks for the input. Now I am confused even more. All I need is to
>>> start demuxing an mp4 file. Those codecs test above is not what I do.
>>> I started to check those because I am getting into trouble in this part:
>>>
>>> pFormatCtx = avformat_alloc_context(); //ok
>>> AVDictionary *d = NULL;
>>> av_dict_set(&d, "protocol_whitelist","file", 0); //ok
>>> int ret = avformat_open_input(&pFormatCtx, in_f_name , NULL,
>>> &d); //ok
>>> avformat_find_stream_info(pFormatCtx, NULL) ///also ok
>>>
>>> But once I get to access the codecpar I hit "access violation reading
>>> location:
>>>
>>> *pFormatCtx->streams[i]->codecpar->codec_type==AVMEDIA_TYPE_VIDEO*
>>>
>>> Because my *codecpar *is not initialized. That's what I don't
>>> understand here.
>>>
>>>
>>>
>>>
>>>
>>> On Nov 9, 2017 8:14 PM, "Carl Eugen Hoyos" <ceffmpeg at gmail.com> wrote:
>>>
>>>> Hi!
>>>>
>>>> > Am 09.11.2017 um 17:13 schrieb Michael IV <explomaster at gmail.com>:
>>>> >
>>>> > Hi All. What maybe a reason that avcodec_find_decoder returns NULL ?
>>>> >
>>>> > I enabled in my FFMPEG build MOV and h264 demuxer. Configure report
>>>> clearly
>>>> > states that.
>>>> >
>>>> > But doing this test:
>>>> >
>>>> > av_register_all();
>>>> > avcodec_register_all();
>>>> > avformat_network_init();
>>>> > AVCodec* h264codc = avcodec_find_decoder_by_name("h264");
>>>>
>>>> This is supposed to work.
>>>>
>>>>
>>>> > AVCodec* h264codc1 = avcodec_find_decoder_by_name("MOV");
>>>>
>>>> This is not the name of any decoder, mov is a file format, you will
>>>> find a demuxer (and a muxer) with that name (I think).
>>>>
>>>> > AVCodec* h264codc3 = avcodec_find_decoder_by_name("MPEG4");
>>>>
>>>> There is codec "mpeg4" but that's not h264.
>>>>
>>>> > AVCodec* h264codc4 = avcodec_find_decoder_by_name("MP4");
>>>>
>>>> See above for mov, although the same demuxer handles mov and mp4.
>>>>
>>>> > AVCodec* h264codc5 = avcodec_find_decoder(AV_CODEC_ID_MPEG4);
>>>>
>>>> Again, this codec exists but it's not h264.
>>>>
>>>> > Only h264codc returns a valid codec.
>>>>
>>>>
>>>> > The only thing I didn't do during build config was not to enable
>>>> libx264.
>>>>
>>>> This only means that you have no h264 encoder.
>>>>
>>>> > May this be the source of the problem? I didn't do that because I
>>>> just need to demux mp4 movie and get its elementary stream.
>>>>
>>>> Then you don't care about a decoder, you only need a demuxer (and maybe
>>>> a parser).
>>>>
>>>> Carl Eugen
>>>> _______________________________________________
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>>>>
>>>
>>
>
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