ILBM

File Format
Name	ILBM
Ontology	Electronic File Formats Graphics ILBM
Extension(s)	.iff, .lbm, .ilbm, others
PRONOM	fmt/338, x-fmt/424

ILBM (or IFF-ILBM, or LBM) is a loosely-defined family of raster image file formats that use the IFF container format. ILBM stands for InterLeaved BitMap.

It is also sometimes called IFF or Amiga IFF, though IFF more properly refers to the generic IFF format on which ILBM is based.

As seems to be common practice, we consider ILBM to include some formats that are not actually interleaved. We exclude formats that use incompatible variants of IFF.

ILBM files were widely used on Amiga computers, but are not limited to that platform. The format originated with the Deluxe Paint program from Electronic Arts.

Identification

The IFF chunk identifiers (ASCII strings) FORM and (usually) ILBM appear near the beginning of the file. BMHD appears later in the file, usually immediately after ILBM.

Some files do not contain an ILBM chunk, but instead contain a different chunk such as "PBM ", RGBN, or RGB8.

Special types of ILBM

The plain ILBM format is reasonably well standardized. This section describes some of the other ILBM formats.

PBM

PBM (Planar BitMap) files have a "PBM " chunk instead of an ILBM chunk, and the image data is organized differently.

This is not to be confused with Netpbm-PBM, a much more common format named PBM.

HAM

HAM images are designed to work with the Amiga's Hold-And-Modify display mode. HAM files should contain a CAMG chunk with a particular flag set. Images have a maximum of 4096 colors.

HAM8

HAM8 is a variant of HAM, designed to work with a display mode available on some Amigas. Images have a maximum of 262,144 colors.

HALFBRITE

HALFBRITE images are designed to work with the Amiga's Halfbrite display mode. HALFBRITE files should contain a CAMG chunk with a particular flag set. Images have a maximum of 64 colors.

Software

It is possible that there is no single application that does a good job of decoding the full range of ILBM formats. More research is needed here.

• Netpbm
• XnView
• RECOIL
• Irfanview with FORMATS plugin 4.36 or more recent

Sample files

• ftp://ftp.back2roots.org/pub/amigascne/Gfx/pics/
• http://cd.textfiles.com/megamedia2/IMAGES/CARS/IFF24/
• http://samples.libav.org/image-samples/ASH.LBM - a PBM file
• http://www.fileformat.info/format/iff/sample/index.htm

Resources

• Wikipedia: ILBM
• ILBM spec
• ModdingWiki article
• Intro to Amiga IFF ILBM Files and Amiga Viewmodes
• IFF File Format Summary, from the Encyclopedia of Graphics File Formats
• Wikipedia: Hold-And-Modify
• Wikipedia: Amiga Halfbrite mode
• Ralf Steines' (unreleased?) ilbmtoppm - manpage has useful information
• Picture format docs (of a number of formats including DeluxePaint LBM/IFF)
• FORM RGBN and FORM RGB8