[FFmpeg-devel] [PATCH] WIP: [RFC] avformat/a64: add codec tag list (PR #21205)

[FFmpeg-devel] [PATCH] WIP: [RFC] avformat/a64: add codec tag list (PR #21205)

[Gyan Doshi via ffmpeg-devel]

PR #21205 opened by Gyan Doshi (GyanD)
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21205
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21205.patch

I'm writing a py script which prepares

1) for each muxer, a list of the codec IDs it accepts.
2) for each codec ID, a list of the muxers which accept it.

Out of 184 muxers in git master, I've identified 65 where neither `.p.codec_tag` nor `.query_codec` is populated and neither is the flag `FF_OFMT_FLAG_ONLY_DEFAULT_CODECS` set i.e. the muxer accepts multiple codecs but there is no standardized data structure containing that list. Codec acceptance is tested usually via a switch or if-else block in init or write_header.

So, I'm about to start the process of adding codec tag lists for all such muxers.
But before I start, this is a RFC patch for this project, for the base idea as well as implementation.


>From 3a44733e2e5df161972515d37a8e95047456c97a Mon Sep 17 00:00:00 2001
From: Gyan Doshi <ffmpeg@gyani.pro>
Date: Mon, 15 Dec 2025 17:41:02 +0530
Subject: [PATCH] avformat/a64: add codec tag list

---
 libavformat/a64.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/libavformat/a64.c b/libavformat/a64.c
index 6e722c7e9f..f0db77d513 100644
--- a/libavformat/a64.c
+++ b/libavformat/a64.c
@@ -60,6 +60,12 @@ static int a64_write_header(AVFormatContext *s)
     return 0;
 }
 
+static const AVCodecTag codec_a64_tags[] = {
+    { AV_CODEC_ID_A64_MULTI,  0 },
+    { AV_CODEC_ID_A64_MULTI5, 0 },
+    { AV_CODEC_ID_NONE, 0 }
+};
+
 const FFOutputFormat ff_a64_muxer = {
     .p.name         = "a64",
     .p.long_name    = NULL_IF_CONFIG_SMALL("a64 - video for Commodore 64"),
@@ -67,6 +73,7 @@ const FFOutputFormat ff_a64_muxer = {
     .p.video_codec  = AV_CODEC_ID_A64_MULTI,
     .p.audio_codec    = AV_CODEC_ID_NONE,
     .p.subtitle_codec = AV_CODEC_ID_NONE,
+    .p.codec_tag      = (const AVCodecTag* const []){ codec_a64_tags, 0 },
     .flags_internal   = FF_OFMT_FLAG_MAX_ONE_OF_EACH,
     .write_header   = a64_write_header,
     .write_packet   = ff_raw_write_packet,
-- 
2.49.1

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