Working with Markers

General Information

"Metadata" refers to extra information that is stored with an image, but is not necessary for decoding the image. It can contain information about the camera used to take the image, the author, date and time the image was taken, where the image was taken, whether the camera has GPS capabilities, etc. When an image is modified, you should preserve the original information, if possible. Metadata includes markers, tags and comments. This topic will deal primarily with markers. For more information on tags and comments, refer to the following:

Implementing TIFF Comments and Tags

Types of File Comments

JPEG files contain metadata information in user-defined markers. The user-defined markers are in the range 0xE0 to 0xFE. These markers can have a maximum size of 0xFFFD, or 65533 bytes. The markers below 0xE0 are reserved for the encoding of the image and inserting a reserved marker can make a file invalid. Therefore, you should not use reserved markers unless you are well acquainted with markers and image encoding in JPEG files.

TIFF files do not contain markers, however, LEADTOOLS provides a "workaround" for transferring Exif comments from JPEG files to TIFF files, and vice versa. The Exif metadata information from a TIFF file will be loaded as an APP1 marker. Therefore, you can load Exif metadata from an uncompressed file and store it in a compressed Exif file. You can also take metadata information from a compressed Exif file and store it in an uncompressed Exif file. Please note however, that some information cannot be stored inside uncompressed Exif files. For example, audio data is stored in APP2 markers and there is no built-in support for saving APP2 markers in TIFF files. However, you can still save the audio data yourself, using a custom tag. For more information on saving custom tags, refer to Implementing TIFF Comments and Tags.

Handling Markers

LEADTOOLS provides several functions for loading, creating, setting, modifying, getting information from and freeing markers.

To load markers from an existing JPEG file, call LMarker::Load. Although TIFF files do not support markers, LEADTOOLS provides limited support through the use of APP1 Exif markers. When you call LMarker::Load for a TIFF file, an APP1 marker is created and all the TIFF, GPS and Exif comments will be stored in it. In addition, any other tag with an ID greater than 0x8000 (32768) will be stored in the APP1 marker. The other information will be considered as being useful only for the image and will not be loaded into the APP1 marker. The only exception is the resolution information, which will be loaded in the APP1 marker.

The LMarker::Create function lets you create a collection of markers from scratch. You can then insert markers with LMarker::Insert. After you call LMarker::Insert once, you can call LMarker::Enum and insert markers using LMarker::EnumMarkersCallBack.

Markers, like tags and comments can be set so that when a file is saved using the SaveXXX functions, the markers, tags and/or comments that have been set will be saved too. Set the markers to save using the LMarker::SetAsGlobalMarkers function. Once markers have been set using LMarker::SetAsGlobalMarkers, all the save operations from the current thread will use these markers. If you do not want to save any markers, call LMarker::SetAsGlobalMarkers with the hMarkers parameter set to NULL. To get the markers that were set by the last call to LMarker::SetAsGlobalMarkers, call LMarker::GetGlobalMarkers.

Please note that both the LMarker::SetAsGlobalMarkers and LMarker::GetGlobalMarkers functions use copies of the list of markers. Therefore, the resources associated with these lists of markers should be freed by calling LMarker::Free when the markers are no longer needed.

Markers set using LMarker::SetAsGlobalMarkers can also be written to files using the LFile::WriteComment and LFile:WriteTag functions. The comments or tags being written by these two functions will overwrite any similar comments or tags contained in the markers. The LFile::WriteMetaData function can also be used to write markers, tags and comments to a file. To summarize:

Markers are generally modified by deleting all markers of a certain type. To delete all markers of a certain type, use the LMarker::Delete function. To delete a marker at a given index, use LMarker::DeleteIndex. A marker can also be deleted by using LMarker::Enum to delete the specific marker or create a new marker collection using LMarker::Create and insert all markers except the marker to be deleted.

The LMarker::Enum function can be used to enumerate and modify a marker collection. It is a powerful function, used internally by most marker manipulation functions. For each marker enumerated by LMarker::Enum, an LMarker::EnumMarkersCallBack function is called. This callback function lets you delete markers from or insert markers to the marker collection being enumerated. This allows you to modify the marker collection passed to the LMarker::Enum function. To insert a new marker in a marker collection, use LMarker::Insert. You can get a marker by index using the LMarker::GetMarker function.

To determine how many markers are in a collection, call LMarker::GetCount.

Finally, to create a copy of a marker collection and obtain a handle to the new marker collection, use LMarker::Copy.