How to remove or edit Exif from mp4 video?

Question

I recorded a Full HD video with Samsung Galaxy II, when I uploaded it to YouTube I found that it turned to 90 degrees like Portrait layout 1080x1920 NOT 1920x1080. I found the cause of the problem:

YouTube is reading video metadata and rotate video acording Exif orientation before encoding

This is ExifTool report (please see last tag "Rotation"):

ExifTool Version Number         : 8.61
File Name                       : video.mp4
Directory                       : .
File Size                       : 217 MB
File Modification Date/Time     : 2011:08:11 00:47:23+04:00
File Permissions                : rw-rw-rw-
File Type                       : 3GP
MIME Type                       : video/3gpp
Major Brand                     : 3GPP Media (.3GP) Release 4
Minor Version                   : 0.3.0
Compatible Brands               : 3gp4, 3gp6
Movie Data Size                 : 227471371
Movie Header Version            : 0
Create Date                     : 1900:01:00 00:00:00
Modify Date                     : 1900:01:00 00:00:00
Time Scale                      : 1000
Duration                        : 0:01:46
Preferred Rate                  : 1
Preferred Volume                : 100.00%
Preview Time                    : 0 s
Preview Duration                : 0 s
Poster Time                     : 0 s
Selection Time                  : 0 s
Selection Duration              : 0 s
Current Time                    : 0 s
Next Track ID                   : 3
Track Header Version            : 0
Track Create Date               : 1900:01:00 00:00:00
Track Modify Date               : 1900:01:00 00:00:00
Track ID                        : 1
Track Duration                  : 0:01:46
Track Layer                     : 0
Track Volume                    : 0.00%
Image Width                     : 1920
Image Height                    : 1080
Graphics Mode                   : srcCopy
Op Color                        : 0 0 0
Compressor ID                   : avc1
Source Image Width              : 1920
Source Image Height             : 1080
X Resolution                    : 72
Y Resolution                    : 72
Bit Depth                       : 24
Video Frame Rate                : 30.023
Matrix Structure                : 1 0 0 0 1 0 0 0 1
Media Header Version            : 0
Media Create Date               : 1900:01:00 00:00:00
Media Modify Date               : 1900:01:00 00:00:00
Media Time Scale                : 16000
Media Duration                  : 0:01:46
Handler Type                    : Audio Track
Handler Description             : SoundHandler
Balance                         : 0
Audio Format                    : mp4a
Audio Channels                  : 1
Audio Bits Per Sample           : 16
Audio Sample Rate               : 16000
Play Mode                       : SEQ_PLAY
Avg Bitrate                     : 17.1 Mbps
Image Size                      : 1920x1080
Rotation                        : 90

How do I remove whole Exif data or just edit Rotation property?

Answer 1

Mp4 files (and many others) use the MPEG-4 standard, which arranges the data inside it in little boxes called atoms. You can find a great description of atoms in this Page. In short, atoms are organized in a tree like structure, where an atom can be either the parent of other atoms or a container of data, but not both (although some people break this rule)

In particular the atom you are looking for is called "tkhd" (Track Header). You can find a list of atoms here.

Within this atom you will find metadata of the video. The structure of the "tkhd" atom is specified here

Finally the chunk of metadata you need (which is not an atom), is called "Matrix Structure". From developer.apple.com:

All values in the matrix are 32-bit fixed-point numbers divided as 16.16, except for the {u, v, w} column, which contains 32-bit fixed-point numbers divided as 2.30.

This is shown in the following image:

The 9 byte matrix starts in byte 48 of the "tkhd" atom. An example of a "matrix structure" for an orientation of 0° would be 1 0 0 0 1 0 0 0 1 (the identity matrix)

SO!

After all that, what you need is to modify this matrix. The next parragraph is taken from developer.apple.com:

A transformation matrix defines how to map points from one coordinate space into another coordinate space. By modifying the contents of a transformation matrix, you can perform several standard graphics display operations, including translation, rotation, and scaling. The matrix used to accomplish two-dimensional transformations is described mathematically by a 3-by-3 matrix.

This means that the transformation matrix defines a function, that maps each coordinate into a new one.

Since you only need to rotate the image, simply modify the left most 2 x 3 matrix, which is defined by the bytes 0, 1, 3, 4, 6 and 7.

Here are the 2 x 3 matrices I use to represent each orientation (values 0, 1, 3, 4, 6 and 7 of the 3x3 matrix):

0°: (x', y') = (x, y)
1 0
0 1
0 0

90°: (x', y') = (height - y, x)
0 1
-1 0
height 0

180°: (x', y') = (widht - x, height - y)
-1 0
0 -1
width height

270°: (x', y') = (y, width - x)
0 -1
1 0
0 width

If you don't have them, the width and height can be obtained just after the matrix structure. They are also fixed point numbers of 4 bytes (16.16).

It is quite probable your video metadata contains the 90° Matrix

(Thanks to Phil Harvey, creator of Exiftool for his help and a wonderful software)