ILBM

ILBM, or Interleaved Bitmap, is a bitmap image format used by the Amiga computer family as part of the IFF (Interchange File Format) standard. It is a common container for storing Amiga graphics and was widely used by image, graphics, and game programs on the platform. ILBM files describe color at the pixel level through a planar representation and can include additional information such as display mode and transparency.

An ILBM file is a FORM type ILBM within IFF and is built from a set of chunks. The most essential chunks are BMHD (Bitmap Header), CMAP (Color Map), and BODY (Bitmap Data). The BMHD chunk contains metadata about the image, including its width and height, the number of bitplanes (nPlanes), masking and transparency options, and the compression method used for the pixel data. The CMAP chunk stores the palette as RGB triples, with a maximum color count determined by the number of bitplanes. The BODY chunk holds the actual bitmap data in a planar, interleaved arrangement: the image’s bitplanes are stored in a way that allows the color of each pixel to be determined by combining bits from the planes.

Optional chunks provide additional information, such as CAMG, which specifies display mode flags for interpreting the image, and other metadata that can assist rendering or editing. The format supports two main data representations: uncompressed and compressed data, with Run-Length Encoding being a common compression method in practice.

ILBM images represent color indices rather than direct RGB values, relying on the palette in CMAP to map indices to colors. With nPlanes colors, the image can display up to 2^nPlanes colors. The format remains influential for archival Amiga graphics and is often encountered in emulation and retro-computing contexts.