{"id":2028,"date":"2009-12-21T20:34:55","date_gmt":"2009-12-22T04:34:55","guid":{"rendered":"http:\/\/multimedia.cx\/eggs\/?p=2028"},"modified":"2010-07-16T07:21:42","modified_gmt":"2010-07-16T14:21:42","slug":"supplying-ffmpeg-with-metadata","status":"publish","type":"post","link":"https:\/\/multimedia.cx\/eggs\/supplying-ffmpeg-with-metadata\/","title":{"rendered":"Supplying FFmpeg With Metadata"},"content":{"rendered":"

UPDATE, 2010-06-17: This information is now maintained via the FFmpeg Metadata page on the MultimediaWiki<\/a>.<\/strong><\/p>\n

While creating my automated game archiving solution<\/a>, I wanted to automatically tag ALAC\/M4A files with appropriate metadata while encoding using FFmpeg<\/a>. Then I realized I didn’t know how to do that. I do remember that FFmpeg recently supplanted a series of specific metadata command line options (like -title and -album) with a highly generalized metadata framework that is accessed by the option ‘-metadata <key>=<value>’. The official documentation<\/a> provides this lonely (and, I came to realize, useless NO-OP; more on why later) example of the option’s usage:<\/p>\n

\r\n    `-metadata key=value'\r\n\r\n      Set a metadata key\/value pair. For example, \r\n      for setting the title in the output file:\r\n\r\n      ffmpeg -i in.avi -metadata title=\"my title\" out.flv\r\n<\/pre>\n
So this option allows you to specify absolutely any key\/value metadata pair you can imagine. However, a specific muxer won’t necessarily recognize it. How can I know the specific keys that, e.g., the MOV\/MP4 muxer honors? 2 methods spring to mind: Trial and error (worked out well for me at first) and reading the code (which was made a good deal easier when I already knew a few of the keys that worked).<\/p>\n
I think it would be a good idea to document the specific metadata keys that each muxer in FFmpeg recognizes. This blog post will lead by example. This data comes from SVN revision 20910, current as of 2009-12-21.<\/p>\n
QuickTime\/MOV\/MP4\/M4A\/et al.<\/strong><\/p>\n
I have constructed the following table of the metadata keys that libavformat\/movenc.c honors. The low-level identifier column lists the atom name that the format uses to encode the data on disc, which is not interesting to most readers. For the interested but uninitiated, the notation, e.g., ‘\\251nam’ indicates a 4-byte code consisting of the byte A9 in hexadecimal (or 251 in octal) followed by the ASCII characters ‘n’, ‘a’, and ‘m’.<\/p>\n
<\/p>\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
Key<\/th>\niTunes field<\/th>\nLow-level identifier<\/th>\n<\/tr>\n
“title”<\/td>\nName<\/td>\n‘\\251nam’<\/td>\n<\/tr>\n
“author”<\/td>\nArtist<\/td>\n‘\\251ART’<\/td>\n<\/tr>\n
“composer”<\/td>\nComposer<\/td>\n‘\\251wrt’<\/td>\n<\/tr>\n
“album”<\/td>\nAlbum<\/td>\n‘\\251alb’<\/td>\n<\/tr>\n
“year”<\/td>\nYear<\/td>\n‘\\251day’<\/td>\n<\/tr>\n
“track”<\/td>\nTrack Number<\/td>\n‘trkn’<\/td>\n<\/tr>\n
“comment”<\/td>\nComments<\/td>\n‘\\251cmt’<\/td>\n<\/tr>\n
“genre”<\/td>\nGenre<\/td>\n‘\\251gen’<\/td>\n<\/tr>\n
“copyright”<\/td>\n??<\/td>\n‘\\251cpy’<\/td>\n<\/tr>\n
“description”<\/td>\nDescription<\/td>\n‘desc’<\/td>\n<\/tr>\n
“synopsis”<\/td>\nInformation dialog when selecting “Show Description” in context menu<\/td>\n‘ldes’<\/td>\n<\/tr>\n
“show”<\/td>\nShow<\/td>\n‘tvsh’<\/td>\n<\/tr>\n
“episode_id”<\/td>\nEpisode ID<\/td>\n‘tven’<\/td>\n<\/tr>\n
“network”<\/td>\n??<\/td>\n‘tvnn’<\/td>\n<\/tr>\n<\/table>\n
Further, the MOV muxer encodes libavformat version string into the ‘\\251too’ field. Here’s an example that ties together all of the presently supported metadata options, even a few video options that don’t make sense for an audio file:<\/p>\n
\r\n  ffmpeg -i track05.wav \\\r\n    -metadata title=\"Track #5\" \\\r\n    -metadata author=\"Unknown Artist\" \\\r\n    -metadata composer=\"Composer Unknown\" \\\r\n    -metadata album=\"Tracer Video Game Soundtrack\" \\\r\n    -metadata year=\"1996\" \\\r\n    -metadata track=\"5\" \\\r\n    -metadata comment=\"This is redbook CD audio track #5 from the Windows 95 game \\\"Tracer\\\"\" \\\r\n    -metadata genre=\"Game Soundtrack\" \\\r\n    -metadata copyright=\"Copyright 1996 Future Endeavors, Inc.\" \\\r\n    -metadata description=\"Nifty techno background tune for a futuristic video game\" \\\r\n    -metadata synopsis=\"Hey, is this thing on? This is where the 'synopsis' field shows up.\" \\\r\n    -metadata show=\"Tracer\" \\\r\n    -metadata episode_id=\"102\" \\\r\n    -metadata network=\"Some network\" \\\r\n    -acodec alac \\\r\n    -y track05.m4a\r\n<\/pre>\n
The lavf version shows up in the Summary tab of the informational dialog:<\/p>\n

\n\"iTunes
\n<\/center><\/p>\n
This is the Info tab of the same dialog:<\/p>\n

\n\"iTunes
\n<\/center><\/p>\n
Here is the video tab:<\/p>\n

\n\"iTunes
\n<\/center><\/p>\n
And this is where the “synopsis” key shows up (selecting “Show Description” from the context menu):<\/p>\n

\n\"iTunes
\n<\/center><\/p>\n
I don’t know where the “copyright” or “network” strings appear in various iTunes dialogs. But the strings are definitely encoded in the file.<\/p>\n
ASF\/WMV\/WMA<\/strong><\/p>\n
FFmpeg’s ASF muxer honors the following metadata keys:<\/p>\n