Re: [FFmpeg-devel] [PATCH 1/4] avutil: add generic side data for video coding info

[Lynne <dev@lynne.ee>]

On 18/07/2025 19:30, Timothée Regaud wrote:
> From: Timothee Regaud <timothee.informatique@regaud-chapuy.fr>
> 
> Adds the generic data structures to libavutil. The design is recursive to support other codecs, even though the implementation is only for H.264 for now.
> 
> Signed-off-by: Timothee Regaud <timothee.informatique@regaud-chapuy.fr>
> ---
>   libavutil/Makefile            |   1 +
>   libavutil/frame.h             |   7 ++
>   libavutil/side_data.c         |   1 +
>   libavutil/video_coding_info.h | 163 ++++++++++++++++++++++++++++++++++
>   4 files changed, 172 insertions(+)
>   create mode 100644 libavutil/video_coding_info.h
> 
> diff --git a/libavutil/Makefile b/libavutil/Makefile
> index 94a56bb72f..44e51ab7ae 100644
> --- a/libavutil/Makefile
> +++ b/libavutil/Makefile
> @@ -93,6 +93,7 @@ HEADERS = adler32.h                                                     \
>             tree.h                                                        \
>             twofish.h                                                     \
>             uuid.h                                                        \
> +          video_coding_info.h                                           \
>             version.h                                                     \
>             video_enc_params.h                                            \
>             xtea.h                                                        \
> diff --git a/libavutil/frame.h b/libavutil/frame.h
> index c50cd263d9..f4404472a0 100644
> --- a/libavutil/frame.h
> +++ b/libavutil/frame.h
> @@ -254,6 +254,13 @@ enum AVFrameSideDataType {
>        * libavutil/tdrdi.h.
>        */
>       AV_FRAME_DATA_3D_REFERENCE_DISPLAYS,
> +
> +    /**
> +     * Detailed block-level coding information. The data is an AVVideoCodingInfo
> +     * structure. This is exported by video decoders and can be used by filters
> +     * for analysis and visualization.
> +     */
> +    AV_FRAME_DATA_VIDEO_CODING_INFO,
>   };
>   
>   enum AVActiveFormatDescription {
> diff --git a/libavutil/side_data.c b/libavutil/side_data.c
> index fa2a2c2a13..b938ef6f52 100644
> --- a/libavutil/side_data.c
> +++ b/libavutil/side_data.c
> @@ -56,6 +56,7 @@ static const AVSideDataDescriptor sd_props[] = {
>       [AV_FRAME_DATA_SEI_UNREGISTERED]            = { "H.26[45] User Data Unregistered SEI message",  AV_SIDE_DATA_PROP_MULTI },
>       [AV_FRAME_DATA_VIDEO_HINT]                  = { "Encoding video hint",                          AV_SIDE_DATA_PROP_SIZE_DEPENDENT },
>       [AV_FRAME_DATA_3D_REFERENCE_DISPLAYS]       = { "3D Reference Displays Information",            AV_SIDE_DATA_PROP_GLOBAL },
> +    [AV_FRAME_DATA_VIDEO_CODING_INFO]           = { "Video Coding Info",                            AV_SIDE_DATA_PROP_SIZE_DEPENDENT },
>   };
>   
>   const AVSideDataDescriptor *av_frame_side_data_desc(enum AVFrameSideDataType type)
> diff --git a/libavutil/video_coding_info.h b/libavutil/video_coding_info.h
> new file mode 100644
> index 0000000000..17e9345892
> --- /dev/null
> +++ b/libavutil/video_coding_info.h
> @@ -0,0 +1,163 @@
> +/*
> + * 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
> + */
> +
> +#ifndef AVUTIL_VIDEO_CODING_INFO_H
> +#define AVUTIL_VIDEO_CODING_INFO_H
> +
> +#include <stdint.h>
> +#include <stddef.h>
> +
> +/**
> + * @file
> + * @ingroup lavu_frame
> + * Structures for describing block-level video coding information.
> + */
> +
> +/**
> + * @defgroup lavu_video_coding_info Video Coding Info
> + * @ingroup lavu_frame
> + *
> + * @{
> + * Structures for describing block-level video coding information, to be
> + * attached to an AVFrame as side data.
> + *
> + * All pointer-like members in these structures are offsets relative to the
> + * start of the AVVideoCodingInfo struct to ensure the side data is
> + * self-contained and relocatable. This is critical as the underlying buffer
> + * may be moved in memory.
> + */
> +
> +/**
> + * Structure to hold inter-prediction information for a block.
> + */
> +typedef struct AVBlockInterInfo {
> +    /**
> +     * Offsets to motion vectors for list 0 and list 1, relative to the
> +     * start of the AVVideoCodingInfo struct.
> +     * The data for each list is an array of [x, y] pairs of int16_t.
> +     * The number of vectors is given by num_mv.
> +     * An offset of 0 indicates this data is not present.
> +     */
> +    size_t mv_offset[2];
> +
> +    /**
> +     * Offsets to reference indices for list 0 and list 1, relative to the
> +     * start of the AVVideoCodingInfo struct.
> +     * The data is an array of int8_t. A value of -1 indicates the reference
> +     * is not used for a specific partition.
> +     * An offset of 0 indicates this data is not present.
> +     */
> +    size_t ref_idx_offset[2];
> +    /**
> +     * Number of motion vectors for list 0 and list 1.
> +     */
> +    uint8_t num_mv[2];
> +} AVBlockInterInfo;
> +
> +/**
> + * Structure to hold intra-prediction information for a block.
> + */
> +typedef struct AVBlockIntraInfo {
> +    /**
> +     * Offset to an array of intra prediction modes, relative to the
> +     * start of the AVVideoCodingInfo struct.
> +     * The number of modes is given by num_pred_modes.
> +     */
> +    size_t pred_mode_offset;
> +
> +    /**
> +     * Number of intra prediction modes.
> +     */
> +    uint8_t num_pred_modes;
> +
> +    /**
> +     * Chroma intra prediction mode.
> +     */
> +    uint8_t chroma_pred_mode;
> +} AVBlockIntraInfo;
> +
> +/**
> + * Main structure for a single coding block.
> + * This structure can be recursive for codecs that use tree-based partitioning.
> + */
> +typedef struct AVVideoCodingInfoBlock {
> +    /**
> +     * Position (x, y) and size (w, h) of the block, in pixels,
> +     * relative to the top-left corner of the frame.
> +     */
> +    int16_t x, y;
> +    uint8_t w, h;
> +
> +    /**
> +     * Flag indicating if the block is intra-coded.
> +     * 1 if intra, 0 if inter.
> +     */
> +    uint8_t is_intra;
> +
> +    /**
> +     * The original, codec-specific type of this block or macroblock.
> +     * This allows a filter to have codec-specific logic for interpreting
> +     * the generic prediction information based on the source codec.
> +     * For example, for H.264, this would store the MB type flags (MB_TYPE_*).
> +     */
> +    uint32_t codec_specific_type;
> +
> +    union {
> +        AVBlockIntraInfo intra;
> +        AVBlockInterInfo inter;
> +    };
> +
> +    /**
> +     * Number of child blocks this block is partitioned into.
> +     * If 0, this is a leaf node in the partition tree.
> +     */
> +    uint8_t num_children;
> +
> +    /**
> +     * Offset to an array of child AVVideoCodingInfoBlock structures, relative
> +     * to the start of the AVVideoCodingInfo struct.
> +     * This allows for recursive representation of coding structures.
> +     * An offset of 0 indicates there are no children.
> +     */
> +    size_t children_offset;
> +} AVVideoCodingInfoBlock;
> +
> +/**
> + * Top-level structure to be attached to an AVFrame as side data.
> + * It contains an array of the highest-level coding blocks (e.g., CTUs or MBs).
> + */
> +typedef struct AVVideoCodingInfo {
> +    /**
> +     * Number of top-level blocks in the frame.
> +     */
> +    uint32_t nb_blocks;
> +
> +    /**
> +     * Offset to an array of top-level blocks, relative to the start of the
> +     * AVVideoCodingInfo struct.
> +     * The actual data for these blocks, and any child blocks or sub-data,
> +     * is stored contiguously in the AVBufferRef attached to the side data.
> +     */
> +    size_t blocks_offset;
> +} AVVideoCodingInfo;
> +
> +/**
> + * @}
> + */
> +
> +#endif /* AVUTIL_VIDEO_CODING_INFO_H */

Absolutely not.
Use and extend libavutil/video_enc_params.h instead.
And if at all possible, don't implement an inspection tool in ffmpeg 
*just because you want to*. Parsing a bitstream and displaying it is not 
a very complicated thing, but exposing an API very much is a very 
complicated thing.
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