CALS — CALS Type 1

Driver short name

CALS

New in version 2.1.

CALS Type 1 rasters are untiled back and white rasters made of a 2048-byte text header in the MIL-STD-1840 standard, followed by a single datastream compressed with CCITT/ITU-T Recommendation 6, aka Group 6, aka CCITT FAX 4. CALS Type 1 rasters are one of the 4 types of formats described by MIL-PRF-28002C (this standard is now deprecated). Other types are not handled by this driver.

This driver supports reading and creation of CALS Type 1 rasters. Update of existing rasters is not supported.

A CALS dataset is exposed by the driver as a single-band 1-bit raster with a 2-entry color table. The first entry (0) is white (RGB=255,255,255) and the second entry (1) is black (RGB=0,0,0).

Driver capabilities

Supports CreateCopy()

This driver supports the GDALDriver::CreateCopy() operation

Supports VirtualIO

This driver supports virtual I/O operations (/vsimem/, etc.)

Metadata

The following metadata items might be exposed by the driver (or read from the input dataset to generate a cor

  • PIXEL_PATH: First value of the “rorient” header field, measured in degrees counterclockwise from the positive horizontal axis (east). The pel path value represents the number of degrees the image would have to be rotated counterclockwise in order to display the image with proper viewing orientation. The permissible values for the pixel path direction shall be “0”, “90”, “180”, or “270”. 0 is the typical value. If PIXEL_PATH=0 and LINE_PROGRESSION=270, neither are reported.

  • LINE_PROGRESSION: Second value of the “rorient” header field. The line progression direction is measured in degrees counterclockwise from the pel path direction. The permissible values for the line progression direction shall be “90” or “270”. 270 is the typical value. If PIXEL_PATH=0 and LINE_PROGRESSION=270, neither are reported.

  • TIFFTAG_XRESOLUTION: Scan X resolution in dot per inch, from the “rdensty” header field. TIFFTAG_XRESOLUTION and TIFFTAG_YRESOLUTION are always equal in CALS.

  • TIFFTAG_YRESOLUTION: Scan Y resolution in dot per inch, from the “rdensty” header field. TIFFTAG_XRESOLUTION and TIFFTAG_YRESOLUTION are always equal in CALS.

Creation issues

Only single band 1-bit rasters are valid input to create a new CALS file. If the input raster has no color table, 0 is assumed to be black and 1 to be white. If the input raster has a (2 entries) color table, the value for the black and white color will be determined from the color table.