Re: [FFmpeg-devel] [PATCH 4/8] fftools/ffmpeg_demux: set options on codec parameters rather than decoder

[James Almer <jamrial@gmail.com>]

On 1/5/2024 1:42 PM, Anton Khirnov wrote:
> This avoids the requirement to always have a decoder context.
> ---
>   fftools/ffmpeg_demux.c | 43 +++++++++++++++++++++---------------------
>   1 file changed, 21 insertions(+), 22 deletions(-)
> 
> diff --git a/fftools/ffmpeg_demux.c b/fftools/ffmpeg_demux.c
> index cacdc76a71..892094c512 100644
> --- a/fftools/ffmpeg_demux.c
> +++ b/fftools/ffmpeg_demux.c
> @@ -899,19 +899,18 @@ static int choose_decoder(const OptionsContext *o, AVFormatContext *s, AVStream
>       }
>   }
>   
> -static int guess_input_channel_layout(InputStream *ist, int guess_layout_max)
> +static int guess_input_channel_layout(InputStream *ist, AVCodecParameters *par,
> +                                      int guess_layout_max)
>   {
> -    AVCodecContext *dec = ist->dec_ctx;
> -
> -    if (dec->ch_layout.order == AV_CHANNEL_ORDER_UNSPEC) {
> +    if (par->ch_layout.order == AV_CHANNEL_ORDER_UNSPEC) {
>           char layout_name[256];
>   
> -        if (dec->ch_layout.nb_channels > guess_layout_max)
> +        if (par->ch_layout.nb_channels > guess_layout_max)
>               return 0;
> -        av_channel_layout_default(&dec->ch_layout, dec->ch_layout.nb_channels);
> -        if (dec->ch_layout.order == AV_CHANNEL_ORDER_UNSPEC)
> +        av_channel_layout_default(&par->ch_layout, par->ch_layout.nb_channels);
> +        if (par->ch_layout.order == AV_CHANNEL_ORDER_UNSPEC)
>               return 0;
> -        av_channel_layout_describe(&dec->ch_layout, layout_name, sizeof(layout_name));
> +        av_channel_layout_describe(&par->ch_layout, layout_name, sizeof(layout_name));
>           av_log(ist, AV_LOG_WARNING, "Guessed Channel Layout: %s\n", layout_name);
>       }
>       return 1;
> @@ -1145,16 +1144,6 @@ static int ist_add(const OptionsContext *o, Demuxer *d, AVStream *st)
>           ist->user_set_discard = ist->st->discard;
>       }
>   
> -    ist->dec_ctx = avcodec_alloc_context3(ist->dec);
> -    if (!ist->dec_ctx)
> -        return AVERROR(ENOMEM);
> -
> -    ret = avcodec_parameters_to_context(ist->dec_ctx, par);
> -    if (ret < 0) {
> -        av_log(ist, AV_LOG_ERROR, "Error initializing the decoder context.\n");
> -        return ret;
> -    }
> -
>       if (o->bitexact)
>           av_dict_set(&ist->decoder_opts, "flags", "+bitexact", AV_DICT_MULTIKEY);
>   
> @@ -1181,7 +1170,7 @@ static int ist_add(const OptionsContext *o, Demuxer *d, AVStream *st)
>       case AVMEDIA_TYPE_AUDIO: {
>           int guess_layout_max = INT_MAX;
>           MATCH_PER_STREAM_OPT(guess_layout_max, i, guess_layout_max, ic, st);
> -        guess_input_channel_layout(ist, guess_layout_max);
> +        guess_input_channel_layout(ist, par, guess_layout_max);
>           break;
>       }
>       case AVMEDIA_TYPE_DATA:
> @@ -1190,7 +1179,7 @@ static int ist_add(const OptionsContext *o, Demuxer *d, AVStream *st)
>           MATCH_PER_STREAM_OPT(fix_sub_duration, i, ist->fix_sub_duration, ic, st);
>           MATCH_PER_STREAM_OPT(canvas_sizes, str, canvas_size, ic, st);
>           if (canvas_size) {
> -            ret = av_parse_video_size(&ist->dec_ctx->width, &ist->dec_ctx->height,
> +            ret = av_parse_video_size(&par->width, &par->height,
>                                         canvas_size);
>               if (ret < 0) {
>                   av_log(ist, AV_LOG_FATAL, "Invalid canvas size: %s.\n", canvas_size);
> @@ -1201,8 +1190,8 @@ static int ist_add(const OptionsContext *o, Demuxer *d, AVStream *st)
>           /* Compute the size of the canvas for the subtitles stream.
>              If the subtitles codecpar has set a size, use it. Otherwise use the
>              maximum dimensions of the video streams in the same file. */
> -        ist->sub2video.w = ist->dec_ctx->width;
> -        ist->sub2video.h = ist->dec_ctx->height;
> +        ist->sub2video.w = par->width;
> +        ist->sub2video.h = par->height;
>           if (!(ist->sub2video.w && ist->sub2video.h)) {
>               for (int j = 0; j < ic->nb_streams; j++) {
>                   AVCodecParameters *par1 = ic->streams[j]->codecpar;
> @@ -1226,6 +1215,16 @@ static int ist_add(const OptionsContext *o, Demuxer *d, AVStream *st)
>       default: av_assert0(0);
>       }
>   
> +    ist->dec_ctx = avcodec_alloc_context3(ist->dec);
> +    if (!ist->dec_ctx)
> +        return AVERROR(ENOMEM);
> +
> +    ret = avcodec_parameters_to_context(ist->dec_ctx, par);
> +    if (ret < 0) {
> +        av_log(ist, AV_LOG_ERROR, "Error initializing the decoder context.\n");
> +        return ret;
> +    }
> +
>       ist->par = avcodec_parameters_alloc();
>       if (!ist->par)
>           return AVERROR(ENOMEM);

This does not fix the issue of the guessed channel layout not being 
present on the output stream, but might be a change in the right direction.

before:

> $ ./ffmpeg -i ~/samples/wav/200828-005.wav out.wav
> [aist#0:0/pcm_s16le @ 00000252b1d2d420] Guessed Channel Layout: stereo
> Input #0, wav, from '../samples/wav/200828-005.wav':
>   Duration: 00:00:12.80, bitrate: 1536 kb/s
>   Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 48000 Hz, 2 channels, s16, 1536 kb/s
> Stream mapping:
>   Stream #0:0 -> #0:0 (pcm_s16le (native) -> pcm_s16le (native))
> Press [q] to stop, [?] for help
> Output #0, wav, to 'out.wav':
>   Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 48000 Hz, stereo, s16, 1536 kb/s
> 
> 
> $ ./ffmpeg -i out.wav
> [aist#0:0/pcm_s16le @ 000001d616b1d3e0] Guessed Channel Layout: stereo
>   Duration: 00:00:12.80, bitrate: 1536 kb/s
>   Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 48000 Hz, 2 channels, s16, 1536 kb/s

After:

> $ ./ffmpeg -i ../samples/wav/200828-005.wav out.wav
> [aist#0:0/pcm_s16le @ 0000024603c9d420] Guessed Channel Layout: stereo
> Input #0, wav, from '../samples/wav/200828-005.wav':
>   Duration: 00:00:12.80, bitrate: 1536 kb/
>   Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 48000 Hz, stereo, s16, 1536 kb/s
> Stream mapping:
>   Stream #0:0 -> #0:0 (pcm_s16le (native) -> pcm_s16le (native))
> Press [q] to stop, [?] for help
> Output #0, wav, to 'out.wav':
>   Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 48000 Hz, stereo, s16, 1536 kb/s
> 
> $ ./ffmpeg -i out.wav
> [aist#0:0/pcm_s16le @ 000001a1467ed3e0] Guessed Channel Layout: stereo
>   Duration: 00:00:12.80, bitrate: 1536 kb/s
>   Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 48000 Hz, stereo, s16, 1536 kb/s

So the printed output shows the guessed layout for the input stream, but 
the written output stream is still unspec.
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