Re: [FFmpeg-devel] [PATCH 1/2] avcodec: add null encoders

[James Almer <jamrial@gmail.com>]

On 3/14/2022 8:56 AM, Paul B Mahol wrote:
> Signed-off-by: Paul B Mahol <onemda@gmail.com>
> ---
>   libavcodec/Makefile     |  2 ++
>   libavcodec/allcodecs.c  |  2 ++
>   libavcodec/codec_desc.c | 14 ++++++++++
>   libavcodec/codec_id.h   |  2 ++
>   libavcodec/nullenc.c    | 61 +++++++++++++++++++++++++++++++++++++++++
>   5 files changed, 81 insertions(+)
>   create mode 100644 libavcodec/nullenc.c
> 
> diff --git a/libavcodec/Makefile b/libavcodec/Makefile
> index cd929da8e6..8554b5ee7d 100644
> --- a/libavcodec/Makefile
> +++ b/libavcodec/Makefile
> @@ -542,6 +542,8 @@ OBJS-$(CONFIG_MXPEG_DECODER)           += mxpegdec.o
>   OBJS-$(CONFIG_NELLYMOSER_DECODER)      += nellymoserdec.o nellymoser.o
>   OBJS-$(CONFIG_NELLYMOSER_ENCODER)      += nellymoserenc.o nellymoser.o
>   OBJS-$(CONFIG_NOTCHLC_DECODER)         += notchlc.o
> +OBJS-$(CONFIG_NULL_AUDIO_ENCODER)      += nullenc.o
> +OBJS-$(CONFIG_NULL_VIDEO_ENCODER)      += nullenc.o
>   OBJS-$(CONFIG_NUV_DECODER)             += nuv.o rtjpeg.o
>   OBJS-$(CONFIG_ON2AVC_DECODER)          += on2avc.o on2avcdata.o
>   OBJS-$(CONFIG_OPUS_DECODER)            += opusdec.o opus.o opus_celt.o opus_rc.o \
> diff --git a/libavcodec/allcodecs.c b/libavcodec/allcodecs.c
> index 628d27fd75..a30920bfe2 100644
> --- a/libavcodec/allcodecs.c
> +++ b/libavcodec/allcodecs.c
> @@ -487,6 +487,8 @@ extern const AVCodec ff_mpc8_decoder;
>   extern const AVCodec ff_msnsiren_decoder;
>   extern const AVCodec ff_nellymoser_encoder;
>   extern const AVCodec ff_nellymoser_decoder;
> +extern const AVCodec ff_null_audio_encoder;
> +extern const AVCodec ff_null_video_encoder;
>   extern const AVCodec ff_on2avc_decoder;
>   extern const AVCodec ff_opus_encoder;
>   extern const AVCodec ff_opus_decoder;
> diff --git a/libavcodec/codec_desc.c b/libavcodec/codec_desc.c
> index 81f3b3c640..7d6bfd352c 100644
> --- a/libavcodec/codec_desc.c
> +++ b/libavcodec/codec_desc.c
> @@ -3516,6 +3516,20 @@ static const AVCodecDescriptor codec_descriptors[] = {
>           .long_name = NULL_IF_CONFIG_SMALL("AVFrame to AVPacket passthrough"),
>           .props     = AV_CODEC_PROP_LOSSLESS,
>       },
> +    {
> +        .id        = AV_CODEC_ID_AUDIO_NULL,
> +        .type      = AVMEDIA_TYPE_AUDIO,
> +        .name      = "null_audio",
> +        .long_name = NULL_IF_CONFIG_SMALL("Audio NULL"),
> +        .props     = AV_CODEC_PROP_LOSSY,
> +    },
> +    {
> +        .id        = AV_CODEC_ID_VIDEO_NULL,
> +        .type      = AVMEDIA_TYPE_VIDEO,
> +        .name      = "null_video",
> +        .long_name = NULL_IF_CONFIG_SMALL("Video NULL"),
> +        .props     = AV_CODEC_PROP_LOSSY,

Also AV_CODEC_PROP_INTRA_ONLY on both.

> +    },
>   };
>   
>   static int descriptor_compare(const void *key, const void *member)
> diff --git a/libavcodec/codec_id.h b/libavcodec/codec_id.h
> index 3ffb9bd22e..4822dc7685 100644
> --- a/libavcodec/codec_id.h
> +++ b/libavcodec/codec_id.h
> @@ -571,6 +571,8 @@ enum AVCodecID {
>                                   * stream (only used by libavformat) */
>       AV_CODEC_ID_FFMETADATA = 0x21000,   ///< Dummy codec for streams containing only metadata information.
>       AV_CODEC_ID_WRAPPED_AVFRAME = 0x21001, ///< Passthrough codec, AVFrames wrapped in AVPacket
> +    AV_CODEC_ID_AUDIO_NULL = 0x21002, ///< Null audio codec
> +    AV_CODEC_ID_VIDEO_NULL = 0x21003, ///< Null video codec
>   };
>   
>   /**
> diff --git a/libavcodec/nullenc.c b/libavcodec/nullenc.c
> new file mode 100644
> index 0000000000..8d3553ed33
> --- /dev/null
> +++ b/libavcodec/nullenc.c
> @@ -0,0 +1,61 @@
> +/*
> + * Copyright (c) 2022 The FFmpeg Project
> + *
> + * This file is part of FFmpeg.
> + *
> + * FFmpeg is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU Lesser General Public
> + * License as published by the Free Software Foundation; either
> + * version 2.1 of the License, or (at your option) any later version.
> + *
> + * FFmpeg is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> + * Lesser General Public License for more details.
> + *
> + * You should have received a copy of the GNU Lesser General Public
> + * License along with FFmpeg; if not, write to the Free Software
> + * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
> + */
> +
> +#include "libavutil/internal.h"
> +#include "libavutil/frame.h"
> +#include "libavutil/buffer.h"
> +
> +#include "avcodec.h"
> +#include "internal.h"
> +#include "encode.h"
> +
> +static int null_encoder(AVCodecContext *avctx, AVPacket *pkt,
> +                        const AVFrame *frame, int *got_packet)
> +{
> +    int ret;
> +
> +    pkt->pts = frame->pts;
> +    if (avctx->codec_type == AVMEDIA_TYPE_AUDIO)
> +        pkt->duration = ff_samples_to_time_base(avctx, frame->nb_samples);
> +    pkt->flags |= AV_PKT_FLAG_KEY;
> +    if ((ret = ff_alloc_packet(avctx, pkt, 1)) < 0)

To avoid bogus statistics, could you instead allocate a blank packet of 
size av_samples_get_buffer_size()?
Also, use ff_get_encode_buffer() and set the DR1 codec cap.

> +        return ret;
> +    *got_packet = 1;
> +    return 0;
> +}
> +
> +const AVCodec ff_null_video_encoder = {
> +    .name           = "null_video",
> +    .long_name      = NULL_IF_CONFIG_SMALL("Video NULL encoder"),
> +    .type           = AVMEDIA_TYPE_VIDEO,
> +    .id             = AV_CODEC_ID_VIDEO_NULL,
> +    .encode2        = null_encoder,
> +    .caps_internal  = FF_CODEC_CAP_INIT_THREADSAFE,
> +};
> +
> +const AVCodec ff_null_audio_encoder = {
> +    .name           = "null_audio",
> +    .long_name      = NULL_IF_CONFIG_SMALL("Audio NULL encoder"),
> +    .type           = AVMEDIA_TYPE_AUDIO,
> +    .id             = AV_CODEC_ID_AUDIO_NULL,
> +    .encode2        = null_encoder,
> +    .caps_internal  = FF_CODEC_CAP_INIT_THREADSAFE,
> +    .capabilities   = AV_CODEC_CAP_VARIABLE_FRAME_SIZE,

What's the benefit from this? If the idea is to get rid of 
wrappedavframe, then you also need to fix vapoursync, decklink, kmsgrab, 
opengl, xv, and yuv4mpeg.

I'd rather not add more fake codec ids for this purpose unless it's a 
thorough solution.
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