[FFmpeg-devel] [PATCH v2 04/18] avcodec/avcodec: add AVCodecContext.alpha_mode

[Niklas Haas <ffmpeg@haasn.xyz>]

From: Niklas Haas <git@haasn.dev>

Following in the footsteps of the previous commit, this commit adds the
new fields to AVCodecContext so we can start properly setting it on codecs,
as well as limiting the list of supported options to detect a format mismatch
during encode.

This commit also sets up the necessary infrastructure to start using the
newly added field in all codecs.
---
 doc/APIchanges              | 4 ++++
 doc/codecs.texi             | 8 ++++++++
 libavcodec/avcodec.c        | 6 ++++++
 libavcodec/avcodec.h        | 8 ++++++++
 libavcodec/codec_internal.h | 5 +++++
 libavcodec/codec_par.c      | 3 +++
 libavcodec/codec_par.h      | 5 +++++
 libavcodec/decode.c         | 2 ++
 libavcodec/options_table.h  | 5 +++++
 libavcodec/version.h        | 2 +-
 10 files changed, 47 insertions(+), 1 deletion(-)

diff --git a/doc/APIchanges b/doc/APIchanges
index 708325c0bf..c520ed29ad 100644
--- a/doc/APIchanges
+++ b/doc/APIchanges
@@ -2,6 +2,10 @@ The last version increases of all libraries were on 2025-03-28
 
 API changes, most recent first:
 
+2025-07-xx - xxxxxxxxxx - lavc 62.9.100 - avcodec.h codec_par.h
+  Add AVCodecContext.alpha_mode, AVCodecParameters.alpha_mode, and
+  AV_CODEC_CONFIG_ALPHA_MODE.
+
 2025-07-xx - xxxxxxxxxx - lavu 60.7.100 - frame.h pixfmt.h
   Add AVAlphaMode, AVFrame.alpha_mode, av_alpha_mode_name() and
   av_alpha_mode_from_name().
diff --git a/doc/codecs.texi b/doc/codecs.texi
index f6bd50eb71..eec5d8e8c7 100644
--- a/doc/codecs.texi
+++ b/doc/codecs.texi
@@ -913,6 +913,14 @@ Possible values:
 
 @end table
 
+@item alpha_mode @var{integer} (@emph{decoding/encoding,video})
+Possible values:
+@table @samp
+@item premultiplied
+@item straight
+@end table
+
+
 @item log_level_offset @var{integer}
 Set the log level offset.
 
diff --git a/libavcodec/avcodec.c b/libavcodec/avcodec.c
index 834b7ad242..aa92d50ad8 100644
--- a/libavcodec/avcodec.c
+++ b/libavcodec/avcodec.c
@@ -758,6 +758,8 @@ int ff_default_get_supported_config(const AVCodecContext *avctx,
                                     const void **out_configs,
                                     int *out_num_configs)
 {
+    const FFCodec *codec2 = ffcodec(codec);
+
     switch (config) {
 FF_DISABLE_DEPRECATION_WARNINGS
     case AV_CODEC_CONFIG_PIX_FORMAT:
@@ -785,6 +787,10 @@ FF_ENABLE_DEPRECATION_WARNINGS
         if (out_num_configs)
             *out_num_configs = 0;
         return 0;
+
+    case AV_CODEC_CONFIG_ALPHA_MODE:
+        WRAP_CONFIG(AVMEDIA_TYPE_VIDEO, codec2->alpha_modes, enum AVAlphaMode, AVALPHA_MODE_UNSPECIFIED);
+
     default:
         return AVERROR(EINVAL);
     }
diff --git a/libavcodec/avcodec.h b/libavcodec/avcodec.h
index a004cccd2d..803f93dc72 100644
--- a/libavcodec/avcodec.h
+++ b/libavcodec/avcodec.h
@@ -1923,6 +1923,13 @@ typedef struct AVCodecContext {
      */
     AVFrameSideData  **decoded_side_data;
     int             nb_decoded_side_data;
+
+    /**
+     * Indicates how the alpha channel of the video is represented.
+     * - encoding: Set by user
+     * - decoding: Set by libavcodec
+     */
+    enum AVAlphaMode alpha_mode;
 } AVCodecContext;
 
 /**
@@ -2528,6 +2535,7 @@ enum AVCodecConfig {
     AV_CODEC_CONFIG_CHANNEL_LAYOUT, ///< AVChannelLayout, terminated by {0}
     AV_CODEC_CONFIG_COLOR_RANGE,    ///< AVColorRange, terminated by AVCOL_RANGE_UNSPECIFIED
     AV_CODEC_CONFIG_COLOR_SPACE,    ///< AVColorSpace, terminated by AVCOL_SPC_UNSPECIFIED
+    AV_CODEC_CONFIG_ALPHA_MODE,     ///< AVAlphaMode, terminated by AVALPHA_MODE_UNSPECIFIED
 };
 
 /**
diff --git a/libavcodec/codec_internal.h b/libavcodec/codec_internal.h
index 7fa3097aa9..0d13a50fed 100644
--- a/libavcodec/codec_internal.h
+++ b/libavcodec/codec_internal.h
@@ -153,6 +153,11 @@ typedef struct FFCodec {
      */
     unsigned cb_type:3;
 
+    /**
+     * This field determines the alpha modes supported by an encoder.
+     */
+    const enum AVAlphaMode *alpha_modes;
+
     int priv_data_size;
     /**
      * @name Frame-level threading support functions
diff --git a/libavcodec/codec_par.c b/libavcodec/codec_par.c
index 790ea01d10..ddf349ceea 100644
--- a/libavcodec/codec_par.c
+++ b/libavcodec/codec_par.c
@@ -51,6 +51,7 @@ static void codec_parameters_reset(AVCodecParameters *par)
     par->framerate           = (AVRational){ 0, 1 };
     par->profile             = AV_PROFILE_UNKNOWN;
     par->level               = AV_LEVEL_UNKNOWN;
+    par->alpha_mode          = AVALPHA_MODE_UNSPECIFIED;
 }
 
 AVCodecParameters *avcodec_parameters_alloc(void)
@@ -165,6 +166,7 @@ int avcodec_parameters_from_context(AVCodecParameters *par,
         par->sample_aspect_ratio = codec->sample_aspect_ratio;
         par->video_delay         = codec->has_b_frames;
         par->framerate           = codec->framerate;
+        par->alpha_mode          = codec->alpha_mode;
         break;
     case AVMEDIA_TYPE_AUDIO:
         par->format           = codec->sample_fmt;
@@ -229,6 +231,7 @@ int avcodec_parameters_to_context(AVCodecContext *codec,
         codec->sample_aspect_ratio    = par->sample_aspect_ratio;
         codec->has_b_frames           = par->video_delay;
         codec->framerate              = par->framerate;
+        codec->alpha_mode             = par->alpha_mode;
         break;
     case AVMEDIA_TYPE_AUDIO:
         codec->sample_fmt       = par->format;
diff --git a/libavcodec/codec_par.h b/libavcodec/codec_par.h
index f4b9bb5c06..19b1d738dc 100644
--- a/libavcodec/codec_par.h
+++ b/libavcodec/codec_par.h
@@ -212,6 +212,11 @@ typedef struct AVCodecParameters {
      * Audio only. Number of samples to skip after a discontinuity.
      */
     int seek_preroll;
+
+    /**
+     * Video with alpha channel only. Alpha channel handling
+     */
+    enum AVAlphaMode alpha_mode;
 } AVCodecParameters;
 
 /**
diff --git a/libavcodec/decode.c b/libavcodec/decode.c
index 2319e76e4b..76ee1ae31f 100644
--- a/libavcodec/decode.c
+++ b/libavcodec/decode.c
@@ -567,6 +567,8 @@ static int fill_frame_props(const AVCodecContext *avctx, AVFrame *frame)
         frame->color_range = avctx->color_range;
     if (frame->chroma_location == AVCHROMA_LOC_UNSPECIFIED)
         frame->chroma_location = avctx->chroma_sample_location;
+    if (frame->alpha_mode == AVALPHA_MODE_UNSPECIFIED)
+        frame->alpha_mode = avctx->alpha_mode;
 
     if (avctx->codec_type == AVMEDIA_TYPE_VIDEO) {
             if (!frame->sample_aspect_ratio.num)  frame->sample_aspect_ratio = avctx->sample_aspect_ratio;
diff --git a/libavcodec/options_table.h b/libavcodec/options_table.h
index c525cde80a..5c816495f4 100644
--- a/libavcodec/options_table.h
+++ b/libavcodec/options_table.h
@@ -352,6 +352,11 @@ static const AVOption avcodec_options[] = {
 {"bottomleft",  "Bottom-left", 0, AV_OPT_TYPE_CONST, {.i64 = AVCHROMA_LOC_BOTTOMLEFT },  INT_MIN, INT_MAX, V|E|D, .unit = "chroma_sample_location_type"},
 {"bottom",      "Bottom",      0, AV_OPT_TYPE_CONST, {.i64 = AVCHROMA_LOC_BOTTOM },      INT_MIN, INT_MAX, V|E|D, .unit = "chroma_sample_location_type"},
 {"unspecified", "Unspecified", 0, AV_OPT_TYPE_CONST, {.i64 = AVCHROMA_LOC_UNSPECIFIED }, INT_MIN, INT_MAX, V|E|D, .unit = "chroma_sample_location_type"},
+{"alpha_mode", "alpha mode", OFFSET(alpha_mode), AV_OPT_TYPE_INT, {.i64 = AVALPHA_MODE_UNSPECIFIED }, 0, INT_MAX, V|E|D, .unit = "alpha_mode_type"},
+{"unknown",       "Unspecified",   0, AV_OPT_TYPE_CONST, {.i64 = AVALPHA_MODE_UNSPECIFIED   }, 0, 0, V|E|D, .unit = "alpha_mode_type"},
+{"unspecified",   "Unspecified",   0, AV_OPT_TYPE_CONST, {.i64 = AVALPHA_MODE_UNSPECIFIED   }, 0, 0, V|E|D, .unit = "alpha_mode_type"},
+{"premultiplied", "Premultiplied", 0, AV_OPT_TYPE_CONST, {.i64 = AVALPHA_MODE_PREMULTIPLIED }, 0, 0, V|E|D, .unit = "alpha_mode_type"},
+{"straight",      "Straight",      0, AV_OPT_TYPE_CONST, {.i64 = AVALPHA_MODE_STRAIGHT      }, 0, 0, V|E|D, .unit = "alpha_mode_type"},
 {"log_level_offset", "set the log level offset", OFFSET(log_level_offset), AV_OPT_TYPE_INT, {.i64 = 0 }, INT_MIN, INT_MAX },
 {"slices", "set the number of slices, used in parallelized encoding", OFFSET(slices), AV_OPT_TYPE_INT, {.i64 = 0 }, 0, INT_MAX, V|E},
 {"thread_type", "select multithreading type", OFFSET(thread_type), AV_OPT_TYPE_FLAGS, {.i64 = FF_THREAD_SLICE|FF_THREAD_FRAME }, 0, INT_MAX, V|A|E|D, .unit = "thread_type"},
diff --git a/libavcodec/version.h b/libavcodec/version.h
index 37c4c39451..230d5fa13e 100644
--- a/libavcodec/version.h
+++ b/libavcodec/version.h
@@ -29,7 +29,7 @@
 
 #include "version_major.h"
 
-#define LIBAVCODEC_VERSION_MINOR   8
+#define LIBAVCODEC_VERSION_MINOR   9
 #define LIBAVCODEC_VERSION_MICRO 100
 
 #define LIBAVCODEC_VERSION_INT  AV_VERSION_INT(LIBAVCODEC_VERSION_MAJOR, \
-- 
2.50.1

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