.LBM File Format

Introduction

ILBM, the Interleaved Bitmap, is the picture format of the Amiga. It is one form of IFF, the Interchange File Format that Electronic Arts published in January 1985, and Jerry Morrison of Electronic Arts described it in January 1986. Electronic Arts’ Deluxe Paint wrote it from its first release in 1985, on the Amiga and later on the PC and the Atari ST, and most Amiga software reads and writes it.

The Amiga’s display hardware drew pictures from bitplanes: plane 0 holds the lowest bit of every pixel’s colour number, plane 1 the next bit, and so on. ILBM stores the pixels the same way, a row at a time, with one line of each plane in turn. Pictures can have from 1 to 8 planes and a colour map, use the Amiga’s hold-and-modify and Extra Half-Brite modes, or have 24 or 32 planes of direct colour.

Every later version has only added to the format: new display mode bits, more planes, new optional chunks and a new compression value. So every ILBM file is one format, and a reader that skips the chunks it does not know can read them all. Commodore revised the document in 1988, and it is still published in the AmigaOS documentation.

Files on the Amiga often have no extension, or .iff, which is shared with every other IFF form. On the PC Deluxe Paint used .lbm, which it also used for its PBM pictures; .ilbm is rarer. A file dropped on any page of this site opens in the explorer, which shows every chunk and every row, with the picture, its palette and its colour ranges.

Structure

An ILBM file is one IFF FORM chunk with the form type ILBM. Every chunk inside it is a four-letter ID, a 4-byte big-endian length and the data, padded to an even length. The bitmap header comes first and the pixel data last; the chunks in between can come in any order.

The ILBM document gives the chunks as FORM "ILBM" BMHD [CMAP] [GRAB] [DEST] [SPRT] [CAMG] CRNG* CCRT* [BODY]: square brackets mark a chunk that may be left out, and a star one that may be repeated. Only BMHD is required. Later chunks, such as Deluxe Paint’s DPPV, fit in among the optional ones.

00000000  46 4F 52 4D 00 00 1B FC 49 4C 42 4D 42 4D 48 44
00000010  00 00 00 14 01 40 00 C8 00 00 00 00 08 00 01 00
00000020  00 87 05 06 01 40 00 C8 43 4D 41 50 00 00 03 00

Telling it apart

The explorer’s parser for this format checks that the file starts with FORM and that the form type, the four bytes at offset 8, is ILBM (49 4C 42 4D). It turns the file down if there is no BMHD chunk, if the BMHD is shorter than 20 bytes, or if the BODY comes before the BMHD, and the explorer then tries the parsers for the other formats of the family.

A Deluxe Paint PC bitmap has the form type PBM and an Amiga contiguous bitmap ACBM. Other IFF files, such as 8SVX sound or ANIM animations, have other form types and are other formats.

Variants

• Interleaved bitmap (IFF ILBM) – the Amiga’s format: interleaved bitplanes, with HAM, EHB and 24-bit pictures
• Deluxe Paint PC bitmap (IFF PBM) – Deluxe Paint for MS-DOS: one byte a pixel
• Amiga contiguous bitmap (IFF ACBM) – AmigaBASIC’s form: each bitplane stored whole

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