Exif Utility Library Overview

Purpose

Exif is an informal standard (CP-3451) from JEIDA (Japanese Electronic Industries Development Association) for image metadata. Exif enhances the JPEG and TIFF specifications with the addition of metadata tags to hold information such as the camera settings used to take the picture. Although not a formal standard, Exif is used by most digital cameras and many image editing and display applications.

Exit Utility Library Details

The DLL that provides the functionality and the library to which your code must link is identified below.

Architectural Relationship

Exif is a member of the Imaging Frameworks component. It is used by several image framework libraries, including:

• Image Display - CImageDisplay Handles the image decoding operation, providing scope for accessing Exif metadata on a read-only basis.

• Image Transformation - CImageTransform Handles image transform operations such as image resizing and rotation.

• Exif - Provides Exif metadata access through TExifReaderUtility and TExifWriterUtility.

Description

Exif defines metadata tags allowing information about an image to be stored with the data for image file. JPEG and TIFF images include a section where this information can be stored in an easily identifiable way enabling applications to read from and write to these metadata tags.

Accessing Exif Metadata

Exif provides access to metadata in JPEG and TIFF images while encoding, decoding, viewing or editing the image data.

Encoding images

To write Exif metadata to an image file while it is being encoded. (MExifMetadataWriter)

Decoding images

To read Exif metadata from an image file while it is being decoded. (MExifMetadataReader)

Viewing images

To display Exif metadata about the image, such as a description or image title, while viewing the image on screen. (MExifMetadataReader)

Editing images

When an image is opened for resizing or rotation or some other editing operation the Exif tags can be read from or written to. (MExifMetadataWriter)

Key Exif Utility Classes

The two main classes provided by the Exif Utility library are:

• MExifMetadataReader

  Used to read the tags from the Exif Metadata. You are expected to provide the tag id and the primary IFD for the tags to be read.

• MExifMetadataWriter

  Used to write or set the tags of the Exif metadata. You are expected to render the key inputs like tag id and IFD for the data that needs to be written or modified.

It is worth reminding you that Exif Utility is only one of the mechanisms for accessing Exif metadata in the Symbian platform. The other, and much easier, is through the JPEG Exif Plugin, which already knows about many of the common Exif tags and provides an easy way to access those tags.

Using Exif Utility

Exif Utility is used for the following:

• Adding Metadata when encoding an image

• Reading Metadata when decoding an image

• Reading or writing metadata when editing an image

• Reading metadata when displaying an image to screen

The Exif utility is used to access Exif metadata in a JPEG or TIFF image as was previously discussed. Please see the Exif Utility Guide for useful code snippets.