.LBM File Format

Introduction

An .lbm or .iff file is a picture saved in one of the bitmap forms of IFF, the Interchange File Format that Electronic Arts published in January 1985. IFF files are built from chunks: a four-letter ID, a length and the data. A picture is one FORM chunk holding a bitmap header, a colour map, the pixels and any number of optional chunks. The format was made popular by Electronic Arts’ paint program Deluxe Paint, first on the Amiga, where it became the standard picture format, and later on the PC and the Atari ST. Deluxe Paint is no longer sold.

Three forms hold bitmaps in this way, and the form type, four letters at offset 8, says which one a file is. The Interleaved Bitmap (ILBM) stores pixels as bitplanes, as the Amiga’s display hardware drew them. Deluxe Paint for MS-DOS saved its 256-colour pictures as PBM, with one byte a pixel. Commodore’s ACBM, for AmigaBASIC, keeps each bitplane whole. The three share most of their chunks, but an ILBM reader cannot read the pixels of the other two.

Versions that only add share a format; a version that changes existing structures is a separate format. Every change to ILBM since 1985 has only added: new display modes, more bitplanes, new chunks and a new compression value. So every ILBM is one format, and PBM and ACBM, which store the pixels differently, are separate formats.

A file dropped on any page of this site opens in the explorer, which shows its structure chunk by chunk and row by row, with the picture, its palette and any thumbnail it carries.

Variations

Every file of the family starts with FORM (46 4F 52 4D), a 4-byte big-endian size, and the form type. The explorer reads only these three form types; any other FORM, and LIST and CAT files, are other formats.

Versions

The releases that matter for these files, with the format each writes. Deluxe Paint’s dates are those usually given for its releases.

Both PBM and ILBM files with Deluxe Paint II Enhanced’s chunks and its PC aspect ratio were seen, so it writes both; which it saves by default, and whether the 1989 release or a later one first wrote PBM, is not confirmed. Deep ILBMs, with 24 or 32 bitplanes and no colour map, are described in the ILBM document; which program first wrote them, and when, is not recorded in the sources used here. Which Deluxe Paint version first wrote the TINY thumbnail and the DPPS chunk is not known.

What is documented

IFF, ILBM and ACBM are open: Electronic Arts published EA IFF 85 and the ILBM document, Commodore revised the ILBM document and published ACBM, and all three are still available in the AmigaOS documentation. Deluxe Paint’s own chunks CRNG and DPPV, and Deluxe Paint IV’s DRNG, are described in the ILBM document’s appendices.

Some parts were never published and are known only from files and from the programs that read them:

• PBM itself: no specification has been found. Its layout comes from files and from FFmpeg, Netpbm and libilbm, which read it.
• The DPPS chunk of Deluxe Paint for the PC: no description exists; it seems to hold perspective settings, but that is an inference.
• The TINY thumbnail: described in later write-ups, not in Electronic Arts’ or Commodore’s documents.
• Compression 2, Deluxe Paint ST’s vertical runs: named in later write-ups; the only full description of the bytes is FFmpeg’s decoder.

Other IFF files

The .iff extension is shared with every IFF form: 8SVX sound, ANIM animations (a FORM of ILBM frames), SMUS music, FTXT text, PREF AmigaOS preferences and others. Apple’s AIFF sound is built the same way. Other image forms exist that are not part of this family: RGBN and RGB8 from Impulse, TVPaint’s DEEP, YUVN and others. They have different form types and different chunks.

Links