ILBM

ILBM is a subtype of the Interchange File Format (IFF) used for storing picture data. ILBM stands for InterLeaved BitMap which refers to the way the pictures are stored. The image data is stored as a varying number of bitplanes, each storing one bit of data for each pixel in the image. The format supports horizontal and vertical RLE (Run Length Encoding) compression.

Besides supporting TrueColor (24-bit color) with or without an 8-bit alpha channel, the format is capable of closely matching the abilities and expectations of the original Amiga chipsets, of which three major revisions existed.

On the OCS/ECS chipsets up to 6 bitplanes are supported, which would normally limit the number of colors to 64. However only 32 palette registers are available. Usage of one to five bitplanes is straightforward, according to binary exponential progression (powers of 2). The first bitplane provides 2 colors, the second one doubles that to 4 colors, the third again doubles that to 8 colors, the fourth provides 16, and five bitplanes allows for 32 colors. With six bitplanes there are two display modes. Extra Half-Bright mode provides 64 colors, the last 32 being half as bright as the first. The Hold And Modify (or HAM) mode which while only making use of 16 palette colors, allowed the display of 4096 colors with some restrictions.