Re: [FFmpeg-devel] [RFC] Documenting metadata keys, informative (non-copied) metadata

["Tomas Härdin" <git@haerdin.se>]

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tor 2025-02-13 klockan 12:54 +0100 skrev Tomas Härdin:
> Hi
> 
> In the samples_md5 patch discussion Michael wanted the proposed key
> to
> be documented. But it turns out we don't have any documentation for
> metadata keys! So I'm starting this thread to talk about it. I reckon
> we create a new file called doc/metadata_keys.texi with a table
> listing
> keys and where they can appear (format, stream, frames), sorted by
> name, with a brief description of each one. Any keys that require
> more
> detailed documentation we could add sections for.
> 
> Another issue raised was that some metadata keys shouldn't be carried
> over automatically to muxers. In the samples_md5 thread it was
> pointed
> out by Andreas that we don't want to mux that in AIFF. It was also
> pointed out that it stops being valid if the audio is cut. This isn't
> the first time I've come across cases where we don't want metadata to
> be copied, so I'm taking the opportunity to propose informative
> output-
> only metadata could reside in their own namespace. I propose info:
> for
> that, so info:samples_md5 in this specific case, or maybe just
> info:md5. HEVC frames could similarly have such metadata applied.

Attached is a PoC for the latter part. Unlike the previous samples_md5
patch, this one only requires updating a FATE ref concerning a .flac
file.

I have to go, else I'd also post the documentation thing as well.

/Tomas

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From 2930d42c1dba1f608c1058b95ebcff3909695715 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tomas=20H=C3=A4rdin?= <git@haerdin.se>
Date: Tue, 18 Feb 2025 16:45:41 +0100
Subject: [PATCH 1/3] lavu/dict: Add info: namespace and AV_DICT_COPY_INFO

The intent is to enable demuxers to provide informative metadata that is not to be copied to muxers
---
 libavutil/dict.c | 3 +++
 libavutil/dict.h | 1 +
 2 files changed, 4 insertions(+)

diff --git a/libavutil/dict.c b/libavutil/dict.c
index 6fb09399ba..067107b7cd 100644
--- a/libavutil/dict.c
+++ b/libavutil/dict.c
@@ -239,9 +239,12 @@ int av_dict_copy(AVDictionary **dst, const AVDictionary *src, int flags)
     const AVDictionaryEntry *t = NULL;
 
     while ((t = av_dict_iterate(src, t))) {
+        // do not copy info: keys unless explicitly told to do so
+        if ((flags & AV_DICT_COPY_INFO) || strncmp(t->key, "info:", 5)) {
         int ret = av_dict_set(dst, t->key, t->value, flags);
         if (ret < 0)
             return ret;
+        }
     }
 
     return 0;
diff --git a/libavutil/dict.h b/libavutil/dict.h
index 713c9e361a..5012374d26 100644
--- a/libavutil/dict.h
+++ b/libavutil/dict.h
@@ -82,6 +82,7 @@
 #define AV_DICT_APPEND         32   /**< If the entry already exists, append to it.  Note that no
                                          delimiter is added, the strings are simply concatenated. */
 #define AV_DICT_MULTIKEY       64   /**< Allow to store several equal keys in the dictionary */
+#define AV_DICT_COPY_INFO     128   /**< In av_dict_copy(), also copy info: keys */
 /**
  * @}
  */
-- 
2.39.5


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From a381e9e26955cd6b2d105cf0f3bcb3b61997febe Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tomas=20H=C3=A4rdin?= <git@haerdin.se>
Date: Tue, 18 Feb 2025 16:45:59 +0100
Subject: [PATCH 2/3] lavu/dict: Indent

---
 libavutil/dict.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/libavutil/dict.c b/libavutil/dict.c
index 067107b7cd..e53ab4e554 100644
--- a/libavutil/dict.c
+++ b/libavutil/dict.c
@@ -241,9 +241,9 @@ int av_dict_copy(AVDictionary **dst, const AVDictionary *src, int flags)
     while ((t = av_dict_iterate(src, t))) {
         // do not copy info: keys unless explicitly told to do so
         if ((flags & AV_DICT_COPY_INFO) || strncmp(t->key, "info:", 5)) {
-        int ret = av_dict_set(dst, t->key, t->value, flags);
-        if (ret < 0)
-            return ret;
+            int ret = av_dict_set(dst, t->key, t->value, flags);
+            if (ret < 0)
+                return ret;
         }
     }
 
-- 
2.39.5


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From ef53493e059e8f952cdb9faa338d1c70b8cb9856 Mon Sep 17 00:00:00 2001
From: Mattias Wadman <wader@spotify.com>
Date: Mon, 11 Oct 2021 15:38:13 +0200
Subject: [PATCH 3/3] libavformat/flacdec: Export samples md5 as metadata

Will be used by mal to compare metadat md5 with decoded samples md5.

Part of fixing https://jira.spotify.net/browse/GOL-681
---
 libavformat/flacdec.c               | 6 ++++++
 tests/ref/fate/cover-art-flac-remux | 1 +
 2 files changed, 7 insertions(+)

diff --git a/libavformat/flacdec.c b/libavformat/flacdec.c
index 3c317acaee..b4bfbc4f85 100644
--- a/libavformat/flacdec.c
+++ b/libavformat/flacdec.c
@@ -33,6 +33,7 @@
 #include "replaygain.h"
 
 #define SEEKPOINT_SIZE 18
+#define MD5_BYTE_SIZE 16
 
 typedef struct FLACDecContext {
     FFRawDemuxerContext rawctx;
@@ -109,6 +110,7 @@ static int flac_read_header(AVFormatContext *s)
         if (metadata_type == FLAC_METADATA_TYPE_STREAMINFO) {
             uint32_t samplerate;
             uint64_t samples;
+            char md5hex[MD5_BYTE_SIZE*2+1]; // hex representation plus null terminator
 
             /* STREAMINFO can only occur once */
             if (found_streaminfo) {
@@ -133,6 +135,10 @@ static int flac_read_header(AVFormatContext *s)
                 if (samples > 0)
                     st->duration = samples;
             }
+
+            ff_data_to_hex(md5hex, st->codecpar->extradata+18, MD5_BYTE_SIZE, 1 /* lowercase */);
+            md5hex[sizeof(md5hex)-1] = '\0';
+            av_dict_set(&s->metadata, "info:samples_md5", md5hex, 0);
         } else if (metadata_type == FLAC_METADATA_TYPE_CUESHEET) {
             uint8_t isrc[13];
             uint64_t start;
diff --git a/tests/ref/fate/cover-art-flac-remux b/tests/ref/fate/cover-art-flac-remux
index fa91975881..857f1a15ad 100644
--- a/tests/ref/fate/cover-art-flac-remux
+++ b/tests/ref/fate/cover-art-flac-remux
@@ -90,6 +90,7 @@ TAG:comment=Publisher/Studio logotype
 TAG:title=White King Granulated Soap
 [/STREAM]
 [FORMAT]
+TAG:info:samples_md5=496206705f222f9a63bf23dc874d9d71
 TAG:major_brand=M4A 
 TAG:minor_version=0
 TAG:compatible_brands=M4A mp42isom
-- 
2.39.5


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