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    <p>Mp4 files (and many others) use the <a href="http://en.wikipedia.org/wiki/MPEG-4" rel="noreferrer" title="MPEG-4">MPEG-4</a> standard, which arranges the data inside it in little boxes called atoms. You can find a great description of atoms in this <a href="http://atomicparsley.sourceforge.net/mpeg-4files.html" rel="noreferrer" title="page">Page</a>. 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)</p> <p>In particular the atom you are looking for is called "tkhd" (Track Header). You can find a list of atoms <a href="http://www.mp4ra.org/atoms.html" rel="noreferrer">here</a>. </p> <p>Within this atom you will find metadata of the video. The structure of the "tkhd" atom is specified <a href="http://developer.apple.com/library/mac/#documentation/QuickTime/QTFF/QTFFChap2/qtff2.html#//apple_ref/doc/uid/TP40000939-CH204-SW1" rel="noreferrer">here</a></p> <p>Finally the chunk of metadata you need (which is not an atom), is called "Matrix Structure". From <a href="http://developer.apple.com/library/mac/#documentation/QuickTime/QTFF/QTFFChap4/qtff4.html#//apple_ref/doc/uid/TP40000939-CH206-18737" rel="noreferrer">developer.apple.com</a>: </p> <blockquote> <p>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.</p> </blockquote> <p>This is shown in the following image:</p> <p><img src="https://i.stack.imgur.com/tW0De.gif" alt="&quot;Matrix Structure&quot; a transformation matrix"></p> <p>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)</p> <p>SO!</p> <p>After all that, what you need is to modify this matrix. The next parragraph is taken from <a href="http://developer.apple.com/library/mac/#documentation/QuickTime/QTFF/QTFFChap4/qtff4.html#//apple_ref/doc/uid/TP40000939-CH206-18737" rel="noreferrer">developer.apple.com</a>: </p> <blockquote> <p>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.</p> </blockquote> <p>This means that the transformation matrix defines a function, that maps each coordinate into a new one.</p> <p>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.</p> <p>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):</p> <p><strong>0°:</strong> (x', y') = (x, y)<br> 1 0<br> 0 1<br> 0 0</p> <p><strong>90°:</strong> (x', y') = (height - y, x)<br> 0 1<br> -1 0<br> height 0</p> <p><strong>180°:</strong> (x', y') = (widht - x, height - y)<br> -1 0<br> 0 -1<br> width height</p> <p><strong>270°:</strong> (x', y') = (y, width - x)<br> 0 -1<br> 1 0<br> 0 width</p> <p>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).</p> <p>It is quite probable your video metadata contains the 90° Matrix </p> <p>(Thanks to Phil Harvey, creator of <a href="http://www.sno.phy.queensu.ca/~phil/exiftool/" rel="noreferrer">Exiftool</a> for his help and a wonderful software)</p>
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    1. POHow to remove or edit Exif from mp4 video?
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    1. POHow to remove or edit Exif from mp4 video?
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    1. COfix some errors: rotation matrix is defined by the bytes 0, 1, 3, 4. I just learned some 3gp files with hexdump, the width and height maybe useless.
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    2. CO@twinsant You need bytes 6 and 7 to specify the rotation function. For example 180°: (x', y') = (widht - x, height - y) for coordinate (0, 0) (top-left) will be moved to (width, height) (bottom, right)
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