All about Format of GEM bitmap (.IMG)

June 19, 2006 (Press Release) -- Format of GEM bitmap (.IMG)



Overview
The GEM bitmap is the save file format used by GEM Paint and Ventura Publisher. Some programs seem to be aware only of the Ventura version and crash on GEM Paint files, or render Paint images as greyscale.

GEM bitmaps are saved with a .IMG extension. However, so are many other file formats, so don't assume that any file ending .IMG is one of these; it might not be a graphics file at all.

All word values are 16-bit, signed and big-endian. In this listing, "DW" (define word) is used to indicate such a word.

File header
DW version
DW header_size
DW planes
DW pattern_size
DW pixel_width
DW pixel_height
DW image_width
DW image_height
(maybe more words)
These fields are:

version
is (as far as I can tell) always 1.
header_size
is the number of words in the file header. This is 8 for GEM Paint files and 9 for Ventura files.
planes
is 1 for monochrome files and 4 for 16-colour files. Paint Shop Pro can also save an 8-plane greyscale file; presumably this file format is used by Ventura.
pattern_size
is the length in bytes of a pattern (see below), 1-8. GEM Paint uses 2 bytes.
pixel_width,pixel_height
are the size of a pixel in tenths of a millimetre. These give the aspect ratio of the image.
image_width, image_height
are the actual dimensions of the image.
Image data
The image is formed of image_height * planes rows of data. For a multi-plane image, the first planes rows are the colours for the first screen line, the next planes rows are for the second, and so on. A normal four-plane image stores the planes in order R,G,B,I.

A row is formed of one or more records, of three types. Records do not span multiple rows;

RLE
DB rlebyte
This record is a single byte. Bits 6-0 hold the number of repetitions (1-127). If bit 7 is 0 then the byte to be repeated is 0; else it is 0xFF.

Literal string
DB 80h
DB count
DB byte,byte,...
This record contains a number of bytes of literal data. The record size is 2 + count

Pattern
DB 00h
DB number of repetitions
DB byte,byte,...
The pattern is a repeated sequence of bytes. The number of bytes in a pattern is given in the header; hence the record length is 2 + pattern_size.

Repeated rows
DB 00h, 00h, 0FFh
DB number of repetitions
(records for a planes rows)
If a set of planes rows (ie, one line of pixels) is repeated several times, then the 4-byte sequence above is used to reduce it to a single instance.