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Figure 7-4. Manipulating image file metadata in Photoshop
Depending on the image format, a wide variety of metadata can be applied, including, but not limited to, title,
author, copyright holder, keywords, orientation, point of view, color space, photographer's contact data, camera data,
origin, advanced metadata, and more.
■ Several types of metadata about the exposition are embedded in JpeG files taken by digital cameras that can
be retrieved in image viewer and processing applications.
Behind the scenes, Adobe Photoshop applies the Extensible Metadata Platform (XMP) standard created by
Adobe Systems. The serialization of XMP can be embedded into several popular file formats, including GIF, JPEG,
JPEG 2000, PNG, and TIFF images, as well as PDF files 9 . GIF supports XMP embedding as an application
extension with the identifier XMP Data and authentication code XMP . In JPEG files, the XMP metadata is embedded at
application segment 1 ( 0xFFE1 ) with segment header http://ns.adobe.com/xap/1.0/\x00 . In JPEG 2000 images, the
XMP metadata can be located at the uuid atom identified by the UID 0xBE7ACFCB97A942E89C71999491E3AFAC . In PNG
files, XMP is embedded inside an iTXt text block with the keyword XML:com.adobe.xmp . XMP is located at Tag 700 in
TIFF images. The Portable Document Format supports XMP embedding in a metadata stream within a PDF object.
The major problem with images from this aspect is that they are binary files (Figure 7-5 ) that need different
approaches to be indexed or searched than text files do. For example, in Google Image Search there are additional
aspects, including the techniques applied for eliminating very small (for example, 1×1 pixel) images, scams, and so
forth. Textual metadata used on top of the metadata stored in the image files themselves always provides an advanced
level of processing and sharing options.
9 External .xmp sidecar files can be provided for all other file formats that do not support embedded XMP metadata.