add new and non defined metadata to a mp4 file

Question

I have been trying to understand how to add some additional metadata to a mp4 file. I understand how to add metadata, like this:

ffmpeg -i bb.mp4 -metadata title="my title" bb2.mp4

but what I am looking for is how to add some new tags i.e.

ffmpeg -i bb.mp4 -metadata newTag="newTag" bb2.mp4

newTag might be something like author's birthday, anything new not already existing!

Is this even possible?

Answer 1

By default, FFmpeg supports the limited number of iTunes tags in the MP4 format. These are listed below. But custom tags can be written if -movflags use_metadata_tags is added. This applies both for adding new tags or carrying over custom global tags from the input.

iTunes tags supported in MP4:

"title"    
"artist"   
"album_artist"
"composer" 
"album"    
"date"     
"encoding_tool"
"comment"  
"genre"    
"copyright"
"grouping" 
"lyrics"   
"description"
"synopsis" 
"show"     
"episode_id"
"network"  
"episode_sort"
"season_number"
"media_type"
"hd_video"
"gapless_playback"
"compilation"

---

The Matroska format accepts custom tags.

Answer 2

I've found that for my needs I wanted the exif metadata and the exiftool was the right solution.

You can copy metadata between video files: https://unix.stackexchange.com/a/492338/83370

There's a trick to copy all metadata using the option -all:all>all:all: https://exiftool.org/forum/index.php?topic=3440.0