| Full Name | CALS Raster |
| Format ID | IG_FORMAT_CAL = 4 |
| File Extension(s) | *.cal, *.ras, *.cals |
| Data Type | Raster image |
| Data Encoding | Binary |
| Color Profile Support | No |
| Multi-Page Support | No |
| Alpha Channel Support | No |
IG_COMPRESSION_CCITT_G4 - Indexed RGB : 1 bpp
IG_COMPRESSION_CCITT_G4 - Indexed RGB : 1 bpp
None
The CALS file format was created as a graphics format specification by The Department of Defense to standardize the data exchange of logistics support operations across the military branches and military contractors. It is mandatory for most military document-handling applications. It is no longer used solely by the military and its contractors; other government agencies and commercial businesses have also adopted this format, including the aerospace, commercial computer, and medical industries.
There are two types of CALS files: Type I, and a newer, significantly more complicated Type II. Type II supports the use of tiles. Sometimes it acts a repository for a group of Type I files. Whether or not a Type II contains Type I images determines what kind of compression scheme is used. Type I is supported by ImageGear.
Type I and Type II files begin with a header that has the same format and size. It includes information about the source and destination documents, as well as image characteristics data. The data storage units under the header are each 128 bytes in length, and are referred to as records. These are written with 7-bit ASCII characters, making it more "human-readable" than most file format headers.
The image data follows the header. In a Type II file, if the data is a series of Type I images, the images are encoded with CCITT Group 4 compression. If they are Type II files, the data may either be uncompressed or encoded with CCITT Group 4. In addition, Type II data may be stored in tiles, wherein some, all, or none of the tiles may be compressed.
The Type II file contains several more substructures than Type I. Between the header and the image data are three groups of document formatting data. Other data preceding each image (or images), are "layout information" and a "Tile Index", that contains the address of each tile stored for the image.
Murray, James D. and William vanRyper. Encyclopedia of Graphics File Formats. Sebastopol, CA: O'Reilly & Associates, Inc., 1994.