2006.10.21 23:10 "[Tiff] Re: Modify XMP", by Phil Harvey
XMP blocks includes a small buffer space for in place modifications If your changes fit in that space, you can modify the XMP data without disturbing the rest of the file.
This is true. The XMP specification recommends a few kB worth of blank lines, which can be used in a TIFF file since there is no checksum done.
If your modifications don¹t fit in that space, you would have to read and rewrite the TIFF, or use a TIFF specific modification to move the XMP block to the end of the file while leaving everything else in place (and an unused space where the old XMP block was).
This is one of the tricks. However, it isn't 100% foolproof since it may result in the loss of information in certain types of trailers (ie. AFCP) which may be appended to the end of a TIFF file.