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 similar formats whose type identifier is not "ILBM", and which are not necessarily 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.
Be aware that terms such as interleaved, planar, and contiguous are ambiguous. In ILBM jargon, they tend to mean the exact opposite of what they mean with respect to most other image formats. By most definitions of the terms, ILBM would ironically be considered to be planar, ACBM to be even more planar, and PBM to be contiguous.
When "lost" computer graphics created by Andy Warhol on an Amiga computer were discovered and converted by retrocomputing buffs at the Carnegie Mellon University Computer Club in 2013, some of them were found to be in a slight variant of the ILBM format, with a .pic extension.
The file begins with the ASCII string
FORM, and has a type identifier at offset 8 that is usually
BMHD chunk appears somewhere in the file, usually immediately after
Some variants formats have a "FORM type" that is something other than
ILBM, such as "
The image compression algorithm is indicated by a coded integer. The known compression types are 0 for no compression, 1 for PackBits, and 2 for a special "vertical" RLE format (see VDAT, below).
Some ILBM images are designed to be animated using palette color cycling, as indicated by
CRNG chunks or other mechanisms. Many image viewers and converters don't support this feature.
Many ILBM images were designed for graphics modes having non-square pixels, so they will look squished if displayed directly on a modern computer. Fortunately, there is an "aspect ratio" field (and sometimes other fields, such as a
DPI chunk), which can supply the information needed to unsquish them. Unfortunately, though this field is very often correct and useful, it is incorrect in a significant minority of files. For example, it may contain the reciprocal of the correct value, or it may be based on the shape of the image as a whole, instead of the shape of a pixel. There is probably no perfect solution to this problem.
Some viewers appear to display some ILBM images slightly too dark. For example, the whitest white may be (240,240,240) instead of (255,255,255). This happens because, though the palette in an ILBM file always has 8 bits of precision per sample, some graphics modes did not use all 8. Encoders targeting such modes would sometimes carelessly set the low (usually 4) bits to zero. Some ILBM viewers try to detect this situation, and correct for it.
Special types of ILBM
The plain ILBM format is reasonably well standardized. This section describes some of the other ILBM formats.
Hold-and-Modify (HAM) images are designed to work with the Amiga's oddball Hold-and-Modify graphics modes. There are two main kinds of HAM images: HAM6 (6 bits per pixel) and HAM8 (8 bits per pixel). The term HAM sometimes refers just to HAM6.
It is usually said that HAM6 supports up to 4096 different colors in an image, and HAM8 supports 262,144. This is probably true with regard to the actual Amiga graphics modes, but some not-so-carefully-written ILBM specifications imply that more colors are possible. Not that it matters much, because the real limitation is that, for each pixel, there are at most 64 or 256 available colors.
HAM6 files have bit 11 of the
CAMG chunk set, 6 planes (rarely 5), and 16 palette colors.
Reportedly, some HAM6 files are missing the
CAMG chunk. A file with no
CAMG chunk, 6 bit planes, and 16 palette colors is probably HAM6.
HAM8 files have bit 11 of the
CAMG chunk set, 8 planes (rarely 7), and 64 palette colors.
There are at least three extended HAM formats designed to take advantage of the Amiga's ability to make changes to the palette in the middle of an image:
- SHAM (Sliced HAM) - Uses a
- CTBL (Newtek Dynamic Ham) - Uses
- PCHG - Uses a
PCHG(Palette Changes) chunk.
Extra-Halfbrite (EBH) images are designed to work with the Amiga's Halfbrite graphics mode. They have 6 planes, but only 32 colors in the palette. In effect, the palette should be assumed to have an additional 32 colors that are half as bright as the first 32.
Extra-Halfbrite files are identified by bit 7 of the
CAMG chunk being set.
Non-paletted (for example, 24-bit RGB) ILBM images are sometimes called "deep images". This is not to be confused with IFF-DEEP, a different format.
PBM (Planar BitMap) files have FORM type of "
PBM " instead of
ILBM. Each pixel is stored contiguously, and there is only a single plane. PBM was mainly used on the MS-DOS platform.
This is not to be confused with Netpbm's PBM format.
ACBM (Amiga Contiguous Bitmap) is a variant of ILBM designed to be used efficiently by AmigaBASIC. The image is separated into bit-planes and then rows, instead of rows and then bit-planes.
The "FORM type" is
ACBM (instead of
ILBM), and instead of a
BODY chunk there is an
RGBN and RGB8
RGBN (12-bit RGB) and RGB8 (24-bit RGB) were used by Impulse's Silver and Turbo Silver programs (see also TDDD).
In the ILBM variant we're calling VDAT (reportedly used by "the ST version of DPAINT"), instead of raw image data, the
BODY chunk contains a sequence of
VDAT chunks. It uses compression type 2. The format is similar, but not identical, to that used in Tiny Stuff files.
- "ILBM" IFF Interleaved Bitmap - January 17, 1986 (CRNG data updated Oct, 1988 by Jerry Morrison) (Appendix E added and CAMG data updated Oct, 1988 by Commodore-Amiga, Inc.)
- "ILBM" IFF Interleaved Bitmap, from the AmigaOS Documentation Wiki
- "ILBM" IFF Interleaved Bitmap - January 17, 1986
- ModdingWiki: LBM Format
- ST Picture Formats (This page might be broken -- more info here.)
- Picture format docs (of a number of formats including DeluxePaint LBM/IFF)
- IFF File Format Summary, from the Encyclopedia of Graphics File Formats
- FORM RGBN and FORM RGB8
- libilbm / doc
- ILBM64 - An extension to support 16 bits/sample
- http://cd.textfiles.com/zoom2/graphics/clipart/ ...
- http://cd.textfiles.com/megamedia1/ .../*.IFF
- http://samples.libav.org/image-samples/ASH.LBM - a PBM file
- https://github.com/mtuomi/SecondReality .../*.LBM - PBM files
- pl_pix.lha → Plan_pix.lha → *.pic (ACBM)
- Hardwired1a.lha (Extra-Halfbrite)
- neworld.iff (VDAT)
- http://cd.textfiles.com/fishandmore/More/Graphics/Pictures/Sham/ (SHAM)
- Wikipedia: ILBM
- Intro to Amiga IFF ILBM Files and Amiga Viewmodes
- Wikipedia: Hold-And-Modify
- Wikipedia: Amiga Halfbrite mode
- Ralf Steines' (unreleased?) ilbmtoppm - manpage has useful information
- Previously Unknown Warhol Works Discovered on Floppy Disks from 1985
- Detailed report on the Warhol graphic recovery (PDF)