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

[Stefano Sabatini <stefasab@gmail.com>]

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On date Tuesday 2023-02-28 23:28:56 +0100, Stefano Sabatini wrote:
> In particular, clarify how to set options in the codec context, and mention when to use
> avcodec_parameters_to_context().
> 
> Fix trac issue http://trac.ffmpeg.org/ticket/5781.
> ---
>  libavcodec/avcodec.h | 29 ++++++++++++++++++++++++-----
>  1 file changed, 24 insertions(+), 5 deletions(-)

Updated with some more information, after I realized there was another
related issue.

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From e4e5622d1c4eac5dde4065b260523e4fd9ada85c Mon Sep 17 00:00:00 2001
From: Stefano Sabatini <stefasab@gmail.com>
Date: Tue, 28 Feb 2023 23:26:43 +0100
Subject: [PATCH] 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 | 34 +++++++++++++++++++++++++++++-----
 1 file changed, 29 insertions(+), 5 deletions(-)

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
+ * case of decoding raw audio or video).
+ *
+ * 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);
+ * @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 avctx.
  *
  * @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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