XMP Metadata#

Postcard metadata can be stored in XMP format. Usually this is stored inside the images produced with the web output format.

WARNING

Some features of metadata (particularly the location of secrets) assumes that the image being described is laid out like a Web format postcard — the front above the back, and the back rotated as in the Web format.

TIP

When output directly with -f xmp a file {name}-meta.xmp will be produced — you may need to rename it to match the name of the postcard file ({name}.postcard.xmp) for other tools to recognise the association.

Structure#

The following XMP fields are used to store Postcard metadata. Some of them don't match perfectly semantically (eg. the time the postcard was sent being represented by the tag usually used for when a photo was taken, or the GPS coordinates for the location the postcard references) — but they're close enough for human use. Postcard-specific machine uses should convert them according to the following guide:

Note that the TIFF schema fields (relating to image size) are omitted for XMP output directly (with postcards -f xmp), as image size information is only relevant when attached to a specific image.

XMP field	Schema	Postcard metadata	Use
ImageWidth	TIFF	(from source images)	The pixel width of the combined image.
ImageLength	TIFF	(from source images)	The pixel height of the combined image
XResolution	TIFF	(from source images)	The cm width of the combined image (always the width of the front of the postcard)
YResolution	TIFF	(from source images)	The cm height of the combined image (always twice the height of the front of the postcard)
ResolutionUnit	TIFF	(from source images)	Always "3", the indicator for centimetres
Description	DC	-	Always "Both sides of a postcard, stored in the '.postcard' format (https://dotpostcard.org)"
DateTimeOriginal	Exif	sentOn	The date the postcard was sent
GPSAreaInformation	Exif	location.name	The name of the location the postcard references
GPSLatitude	Exif	location.latitude	The latitude of that location
GPSLongitude	Exif	location.longitude	The longitude of that location
AltTextAccessibility	IPTC4 XMP Core	front.description, back.transcription	Generated text suitable to be used as alt text for the postcard
Transcript	IPTC4 XMP Ext	back.transcription, front.transcription	The transcript of any writing on the the postcard. A § character will divide the back and the front (in that order), if needed
ImageRegionName	IPTC4 XMP Ext	-	Always "Private information" for secrets
ImageRegionBoundaryVertices	IPTC4 XMP Ext	front.secrets, back.secrets	The (normalized) x, y positions of the edges of the secret region
ImageRegionBoundaryUnit	IPTC4 XMP Ext	-	Always "relative" (the vertex values are normalized to the width and height of the image, not the side)
Flip	Postcard	flip	Which way the postcard should flip (book, calendar, left-hand, right-hand). This field should be the one used to detect a postcard image
Sender	Postcard	sender	The name (and possibly URL) of the sender of the postcard
Recipient	Postcard	recipient	The name (and possibly URL) of the recipient of the postcard
Context	Postcard	context.description	Any context provided about the postcard. Always has `xml:lang` attribute, which is the `locale` of the metadata for the postcard.
ContextAuthor	Postcard	context.author	The name (and possibly URL) of the author of the context
ThicknessMM	Postcard	physical.thickness_mm	The thickness of the postcard, if different to the standard 0.4mm
DescriptionFront	Postcard	front.description	An alt-text style description of the front of the postcard
DescriptionBack	Postcard	back.description	An alt-text style description of the back of the postcard
TranscriptionFront	Postcard	front.transcription	The JSON blob representing the transcription of the front of the postcard (with annotations)
TranscriptionBack	Postcard	back.transcription	The JSON blob representing the transcription of the back of the postcard (with annotations)