Postcard

A work in progress CLI & library for interacting with .postcard files, and the current reference implementation of the structure of this filetype.

A .postcard file represents a physical postcard digitally; a double-sided image with metadata like: URLs referencing to the sender(s) and receiver(s), the date it was sent on, the lat/long of where it was sent from, transcriptions of any written text on the front/back, and image descriptions of the front/back.

The contained CLI tool, postcards, is able to compile front & back images, and a metadata file into a .postcard file:

brew install dotpostcard/tools/postcards
postcards compile fixtures/hello-front.jpg

Implementation notes

The package at the root of this repo handles the 'steady state' interaction with postcard files — eg. readinging & writing — and have extremely limited dependencies. The packages one level deeper (eg. compile and validate) hold functionality that needs complex dependencies (eg. a capable image processing library), except for internal, which holds packages that are common dependencies.

File format notes

A .postcard file contains 4 sections, organised to provide maximum compatibility with web-browsers:

1. A WebP image file representing the front of the postcard, placed at the beginning of the file, so application sunable to read a .postcard file can process it as a standard WebP file.

• Ideally with a transparent background, if any non-postcard pixels are included.
  • Hopefully the postcarder tool will one day help with auto-removal of the background of scanned postcards

2. The string postcard, followed by three uint8 bytes representing the X.Y.Z of the version number of the library that created the postcard (in that order).
3. (a uint32 defining the length of this section and) JSON for the postcard metadata (see types.go for spec)
4. A WebP image for the back of the postcard (identical in format to 1)

• The physical dimensions should be within 1% of physical dimensions of the front of the postcard. This allows for different resolutions on front & back
• Ideally co-registered, so flipping about the verical or horizontal axis (for homoriented postcards) or about one of the diagonal axes (for heteroriented postcards) have the same or extremely similar outlines