image metadata
What is image metadata?
Image metadata is text information pertaining to an image file that is embedded into the file or saved to a separate file that is associated with the image file.
Image metadata includes details relevant to the image itself and to its production. Some metadata is generated automatically by the system capturing or creating the image. Additional metadata can be added manually and edited through dedicated software or general image editing software, such as GIMP or Adobe Photoshop. Metadata can also be added directly on some digital cameras.
Image metadata is often divided into three main categories:
- Technical metadata is mostly generated automatically by the device or software that creates the image. For example, if the image is a photograph taken by a digital camera, the camera typically generates metadata about the camera and the photo's settings, including aperture, resolution, focal length, shutter speed, ISO speed, camera brand and model, date and time when the image was created, and the GPS location where it was created. Cameras usually generate more technical data than devices or software such as scanners, screenshot tools or draw programs.
- Descriptive metadata is mostly added manually using special software such as GIMP or Affinity Photo. The individual who creates or manages the image can use the software to add or edit the descriptive metadata. The metadata might include the name of the image creator, keywords specific to the image, captions, titles, comments or other information. Effective descriptive metadata can make it easier to search for images.
- Administrative metadata is like descriptive metadata. Most of the metadata is added manually using special software. The metadata might include usage and licensing rights, restrictions on reuse, contact information for the image owner or similar types of information.
Image metadata formats
Various standardized formats are used for metadata. The following four formats are the most common ones:
- Exchangeable Image File (EXIF or Exif). The format is used extensively in digital cameras, smartphones and other devices when generating image metadata. EXIF data can include a range of information and often represents the bulk of metadata in an image file. It includes data such as image width and height, aperture value, exposure time, camera model and more.
- Information Interchange Model (IIM). The metadata in IIM provides individuals and organizations with a way to add details to images such as titles, genres, instructions, owners or creators, location and contact information, copyright and attribution specifications, and similar types of information.
- International Color Consortium (ICC). This metadata includes details about the color profile embedded in an image. ICC is an organization that defines and publishes open standards for image color management.
- Extensible Metadata Platform (XMP). The metadata is an XML-based format that can accommodate a wide range of information. This format was created by Adobe but is now an ISO standard that has been adopted by Adobe and other vendors. XMP can incorporate both EXIF and IIM metadata.
Many image editing programs make it possible to view and edit an image's metadata. For example, Figure 1 shows a photograph (sunset.jpg) that was taken with an iPhone. The photo is open in Affinity Photo. Next to the photo is a panel that shows some of the image metadata. Most of the metadata was generated by the phone and is included in the EXIF section, although one of the XMP sections shows a small amount of metadata generated by Affinity Photo.
Metadata is usually added to an image file directly, along with the bits that define the image itself. With an application such as Hex Fiend, you can pick out pieces of the metadata text from the binary data. Figure 2 shows the binary data for the sunset.jpg file. The screenshot starts at the beginning of the file, where the EXIF data has been added. The four bytes that represent the term EXIF are highlighted in the screenshot, along with the matching text.
Image metadata can also be stored in a file separate from the main image file. This type of file is often called a sidecar file and must accompany the image file to ensure that the metadata is available to the image when needed. Sidecar files commonly use XMP to format the metadata. Figure 3 shows part of the contents of an XMP file that was generated for the sunset.jpg image. Notice how the data is presented in an XML-based format.
Image metadata can be useful for cataloging and contextualizing visual information. Many photographers and other visual artists provide information about themselves and their images within the metadata.
Image metadata can also help protect intellectual property. However, including copyright information in the metadata is not adequate protection as it can easily be stripped away. Also, as with other types of content, metadata security can be cumbersome, requiring extra measures to safeguard image metadata and protect it from unauthorized access.