How are x/yresolution & resolutionunit stored in MP4?

[forderud]

I'm able to successfully write and read-back x/yresolution & resolutionunit parameters to a MP4 movie with the following command: "exiftool -xresolution=1000 -yresolution=1000 -resolutionunit=cm movie.mp4". All good, but I'm wondering what's going on behind the scenes.

The documentation indicates that ExifTool's QuickTime parser is used for storage and loading of MP4 files. However, when looking at https://exiftool.org/TagNames/QuickTime.html I only see XResolution and YResolution tags listed, whereas I cannot see "ResolutionUnit" anywhere. This seems strange to me, since I'm also able to store and read-back "ResolutionUnit" from MP4 files.

Does anyone know if ExitTool's QuickTime documentation incomplete, or are the tags written in some other way to MP4 files? If so, then which MPEG atoms are used for the metadata?

[Phil Harvey]

This command writes XMP tags to the MP4.

- Phil

[StarGeek]

Does anyone know if ExitTool's QuickTime documentation incomplete, or are the tags written in some other way to MP4 files? If so, then which MPEG atoms are used for the metadata?

The documentation is complete.  The problem in this case is that you are not writing video tags. For example, if you try to force your tags to write to Quicktime tags

Code Select Expand

C:\>exiftool -P -overwrite_original -quicktime:xresolution=1000 -quicktime:yresolution=1000 -quicktime:resolutionunit=cm Y:\!temp\Test4.mp4
Warning: Sorry, quicktime:XResolution doesn't exist or isn't writable
Warning: Sorry, quicktime:YResolution doesn't exist or isn't writable
Warning: Sorry, quicktime:ResolutionUnit doesn't exist or isn't writable
Nothing to do.
If you use the command in FAQ #3, you will see where the tags you are writing are actually located

Code Select Expand

C:\>exiftool -P -overwrite_original -xresolution=1000 -yresolution=1000 -resolutionunit=cm Y:\!temp\Test4.mp4
    1 image files updated

C:\>exiftool -G1 -a -s -*resolution* Y:\!temp\Test4.mp4
[Track1]        XResolution                     : 72
[Track1]        YResolution                     : 72
[XMP-tiff]      ResolutionUnit                  : cm
[XMP-tiff]      XResolution                     : 1000
[XMP-tiff]      YResolution                     : 1000
The tags you wrote are in the XMP group. Note that the resolution tags in the Track are embedded in the actual stream and exiftool doesn't edit stream metadata except in a few specific cases.

[forderud]

Thanks a lot Phil & StarGeek for providing clear answers so quickly!

The problem I'm trying to solve is to embed "pixel spacing" metadata in a MP4 movie. It sounds like XMP tags could be an option. Thank you!

I also need to embed the position and orientation of the top-left pixel relative to a position-sensor in the same room. This is similar to "global" GPS coordinates, but on a "local" scale - with values preferably in millimeters. The orientation could be stored either as a quaternion, rotation angles, or a rotation matrix. Both preferably for every image frame, since the camera can move during imaging. Do you know if there's some standard tags that could be used for this purpose?

[Phil Harvey]

Hi Fredrik,

This is similar to some information stored in Google Panoramas and Spherical images.  Maybe see the GPano Tags.

- Phil