File Magic: ColoRIX

UPDATE: As of 2023-04-02, this has been contributed upstream. 'file-5.45' or later should include it.

This repository contains a magic definition for the ColoRIX file format, for use with the file and libmagic software.

Usage

$ file -m rix.magic tests/*.testfile
tests/colorix-linear-256-compressed.testfile:      ColoRIX Image, 320 x 200 x 256 (compressed)
tests/colorix-linear-256.testfile:                 ColoRIX Image, 640 x 480 x 256
tests/colorix-planar-fields-16.testfile:           ColoRIX Image, 640 x 200 x 16, Planar
tests/colorix-planar-lines-16-compressed.testfile: ColoRIX Image, 640 x 200 x 16, Planar lines (compressed)
tests/colorix-planar-lines-16.testfile:            ColoRIX Image, 640 x 200 x 16, Planar lines
tests/colorix-slideshow.testfile:                  ColoRIX Slideshow

File format

While this format is throughly obsolete, it does pop up from time to time when doing MS-DOS software archaeology. I've used it myself, and it was used for loading screens in at least one of the original Fallout games.

The original file-extension was "SCx", where the x encoded the display type -- separately from the image dimensions in the file -- with "SCI" used for 320x200 VGA images:

Suffix Columns Lines Colors
 D       320    200    16
 E       640    200    16
 F       640    480   256
 I       320    200   256
 K       320    400   256
 P       640    480    16
 Q       360    480   256
 R       640    350    16
 T       640    400    16

When used in other contexts, the extension "RIX" was sometimes used, due to it appearing first in the file as the magic marker.

Overspecification

The official specification contains a number of features that I have as of yet been unable to verify as supported by any actual software, this includes:

Support for direct color images.
Support for indexed images of bit-depths other than 4 and 8 (16 and 256 colors)
Support for images using the storage types 1 (swizzled planar field) and 3 (text)
Extensions
Encryption

This does NOT mean that there is no software that supports these features. Remember, people used this format in a very 'ad-hoc' manner; just because the official software can't generate a direct color image, doesn't mean nobody ever generated one themselves.

Planar modes explained

There are three different planar modes in the specification. These are specified using the lower nibble of the 'storage byte' (the higher nibble contains flags) at offset 9 in the header.

4 (0100b) - Planar lines (0123): This is the 'default' planar mode. The bits for the palette index are interleaved in lines/rows, starting with a row of bits for bit 0 (the LSB) of the index, then a row for bit 1 and so on. For 16 colors there will be four such rows to make up one line of the image.
2 (0010b) - Planar field (0123): This stores the bits in the more conventional field form, e.g all bit 0 bits come in one block, followed by all 1 bits, and so on for bpp number of fields. COLORIX does not support loading such images, but RIXLATE does.
1 (0001b) - Planar field (0213): Based on the specification, this should be a 'swizzled' version of type 2, but I've found nothing which supports this mode.

Tests made to verify planar modes

Taking a working 16-color "planar lines" file and changing the header to one of the other planar modes results in COLORIX.EXE saying "Incompatible Screen File" when trying to load them.

Curiously, RIXLATE.EXE is capable of reading type 2 images (planar fields). Verified by converting a type 4 to a type 2, which then displays correctly.

I have not tested image widths that are not multiples of 8. Almost certainly any such would need the last byte of each bit-row padded to an even byte.

eloj / rix-magic