LBM (Image files)

LBM-Files only contain Splash-Screens. Those files contain chunks which means that those files are parted an every part has an other function.

Count	Size(Bytes)	Information
1	4	"FORM" (File-Identifier)
4	1	Unknown
1	4	"PBM" (Header-Identifier)

after this header different chunk-types follow possibly multiple times:

Count	Size(Bytes)	Information
1	4	"BHMD" (Chunk-Identifier)
1	4	length (Chunk-Length, round up uneven numbers)
1	2	width (Image width)
1	2	height (Image height
4	1	Unknown
1	2	depth (Color depth)
1	2	compression_flag
length - 20	1	Unknown

Count	Size(Bytes)	Information
1	4	"CMAP" (Chunk-Identifier)
1	4	length (Chunk-Length, always 768!)
256	3	RGB (Color information)

Count	Size(Bytes)	Information
1	4	"BODY" (Chunk-Identifier)
1	4	length (Chunk-Length, round up uneven numbers)

Depending on compression_flag the following data are to be distinguished.
compression_flag 0 (uncompressed):

Count	Size(Bytes)	Information
length	1	color (Colorindex in "CMAP"-palette, alpha is always 0xFF

compression_flag 1 (compressed):
The following three parts are read until either length or width and height of the image are reached.

Count	Size(Bytes)	Information
1	1	ctype (compression type)

for "ctype" >= 0:

Count	Size(Bytes)	Information
1	1	count (Number of RGB pixels following)
count	1	color (Colorindex in "CMAP"-palette, alpha is always 0xFF

for "ctype" < 0 (or > 127):

Count	Size(Bytes)	Information
1	1	color (Colorindex in "CMAP"-palette, alpha is always 0xFF

Number of pixels in byte can be calculated that way: ctype < 0 ? (-ctype + 1) : ctype (Fill x to width, then next line).

Other chunk-types are either not used or unknown, those need to be skipped:

Count	Size(Bytes)	Information
1	4	"????" (Chunk-Identifier)
1	4	length (Chunk-Length, round up uneven numbers)
length	1	Unkown

Back