Re: [FFmpeg-devel] [PATCH] lavc/avcodec.h: extend documentation for avcodec_open2()

[Stefano Sabatini <stefasab@gmail.com>]

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On date Wednesday 2023-03-01 15:04:16 +0800, "zhilizhao(赵志立)" wrote:
> > diff --git a/libavcodec/avcodec.h b/libavcodec/avcodec.h
> > index 9a0fe97cad..fbf1d3d83c 100644
> > --- a/libavcodec/avcodec.h
> > +++ b/libavcodec/avcodec.h
> > @@ -2418,8 +2418,14 @@ int avcodec_parameters_to_context(AVCodecContext *codec,
> >   * avcodec_find_decoder() and avcodec_find_encoder() provide an easy way for
> >   * retrieving a codec.
> >   *
> > - * @note Always call this function before using decoding routines (such as
> > - * @ref avcodec_receive_frame()).
> > + * Depending on the codec, you might need to set options in the codec context
> > + * also for decoding (e.g. width, height, or the pixel or audio sample format in
> > + * case the information is not is not available in the bitstream, e.g. as in
> 
> 'is not is not'
> 
> > + * case of decoding raw audio or video).
> 
> in ’the’ case of.
> 
> > + *
> > + * Options on the codec context can be either set by providing the options
> > + * specified in an AVDictionary, or by setting the values on the context itself,
> > + * directly or by using the av_opt_set() API.
> >   *
> >   * @code
> >   * av_dict_set(&opts, "b", "2.5M", 0);
> > @@ -2433,17 +2439,35 @@ int avcodec_parameters_to_context(AVCodecContext *codec,
> >   *     exit(1);
> >   * @endcode
> >   *
> > + * In case AVCodecParameters are available (e.g. when demuxing a stream using
> > + * libavformat, and accessing the AVStream contained in the demuxer), the codec
> > + * parameters can be copied to the codec context using
> > + * avcodec_parameters_to_context(), as in the following code:
> > + *
> > + * @code
> > + * context = avcodec_alloc_context3(codec);
> > + * if (avcodec_parameters_to_context(*dec_ctx, codecpar) < 0)
> > + *     exit(1);
> > + * if (avcodec_open2(context, codec, NULL) < 0)
> > + *     exit(1);
> 
> What’s the dec_ctx?

Updated, thanks for the feedback.

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From 672de0e5d15ab8a22f3957222680d847f60bced8 Mon Sep 17 00:00:00 2001
From: Stefano Sabatini <stefasab@gmail.com>
Date: Tue, 28 Feb 2023 23:26:43 +0100
Subject: [PATCH 1/2] lavc/avcodec.h: extend documentation for avcodec_open2()

In particular, clarify how to set options in the codec context, and
mention when to use avcodec_parameters_to_context().

Fix trac issues:
http://trac.ffmpeg.org/ticket/5781
http://trac.ffmpeg.org/ticket/5838
---
 libavcodec/avcodec.h | 36 +++++++++++++++++++++++++++++++-----
 1 file changed, 31 insertions(+), 5 deletions(-)

diff --git a/libavcodec/avcodec.h b/libavcodec/avcodec.h
index 87ebee22b1..f630fafe24 100644
--- a/libavcodec/avcodec.h
+++ b/libavcodec/avcodec.h
@@ -2429,8 +2429,15 @@ int avcodec_parameters_to_context(AVCodecContext *codec,
  * avcodec_find_decoder() and avcodec_find_encoder() provide an easy way for
  * retrieving a codec.
  *
- * @note Always call this function before using decoding routines (such as
- * @ref avcodec_receive_frame()).
+ * Depending on the codec, you might need to set options in the codec context
+ * also for decoding (e.g. width, height, or the pixel or audio sample format in
+ * the case the information is not available in the bitstream, as when decoding
+ * raw audio or video).
+ *
+ * Options on the codec context can be set either by setting them in the
+ * provided options AVDictionary, or by setting the values on the context
+ * itself, directly or by using the av_opt_set() API before calling the
+ * function.
  *
  * @code
  * av_dict_set(&opts, "b", "2.5M", 0);
@@ -2444,17 +2451,36 @@ int avcodec_parameters_to_context(AVCodecContext *codec,
  *     exit(1);
  * @endcode
  *
+ * In case AVCodecParameters are available (e.g. when demuxing a stream using
+ * libavformat, and accessing the AVStream contained in the demuxer), the codec
+ * parameters can be copied to the codec context using
+ * avcodec_parameters_to_context(), as in the following code:
+ *
+ * @code
+ * AVStream *stream = ...;
+ * context = avcodec_alloc_context3(codec);
+ * if (avcodec_parameters_to_context(context, stream->codecpar) < 0)
+ *     exit(1);
+ * if (avcodec_open2(context, codec, NULL) < 0)
+ *     exit(1);
+ * @endcode
+ *
+ * @note Always call this function before using decoding routines (such as
+ * @ref avcodec_receive_frame()).
+ *
  * @param avctx The context to initialize.
  * @param codec The codec to open this context for. If a non-NULL codec has been
  *              previously passed to avcodec_alloc_context3() or
  *              for this context, then this parameter MUST be either NULL or
  *              equal to the previously passed codec.
- * @param options A dictionary filled with AVCodecContext and codec-private options.
- *                On return this object will be filled with options that were not found.
+ * @param options A dictionary filled with AVCodecContext and codec-private
+ *                options, which are set on top of the options already set in
+ *                avctx, can be NULL. On return this object will be filled with
+ *                options that were not found in the avctx codec context.
  *
  * @return zero on success, a negative value on error
  * @see avcodec_alloc_context3(), avcodec_find_decoder(), avcodec_find_encoder(),
- *      av_dict_set(), av_opt_find().
+ *      av_dict_set(), av_opt_set(), av_opt_find(), avcodec_parameters_to_context()
  */
 int avcodec_open2(AVCodecContext *avctx, const AVCodec *codec, AVDictionary **options);
 
-- 
2.25.1


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