WPB (openCanvas)
From Just Solve the File Format Problem
WPB is the native image format of some versions (at least v1.1) of openCanvas. It is a layered raster graphics format.
openCanvas is a graphics editor for Windows, used mainly in Japan. Though its name may suggest otherwise, it is not particularly open. Versions through 1.1 are freeware, and later versions are commercial/demoware.
Identification
WPB files apparently begin with the ASCII signature "WPB".
