ILBM

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.

See also ANIM.

Terminology
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.

Identification
The file begins with the ASCII string, and has a type identifier at offset 8 that is usually. A  chunk appears somewhere in the file, usually immediately after.

Some variants formats have a "FORM type" that is something other than, such as  ,  ,  , or.

Compression
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).

Color cycling
Some ILBM images are designed to be animated using palette color cycling, as indicated by  chunks or other mechanisms. Many image viewers and converters don't support this feature.

Aspect ratio
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  chunk), which can supply the information needed to unsquish them. Unfortunately, though this field is usually correct, 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.

Palette precision
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
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
HAM6 files have bit 11 of the  chunk set, 6 planes (rarely 5), and 16 palette colors.

Reportedly, some HAM6 files are missing the  chunk. A file with no  chunk, 6 bit planes, and 16 palette colors is probably HAM6.

HAM8
HAM8 files have bit 11 of the  chunk set, 8 planes (rarely 7), and 64 palette colors.

Multi-palette HAM
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  chunk.
 * CTBL (Newtek Dynamic Ham) - Uses  and   chunks.
 * PCHG - Uses a  (Palette Changes) chunk.

HAM-E
HAM-E (or HAME) is a format designed to work with particular video hardware from Black Belt Systems. Identifying an ILBM file as HAM-E is difficult; it can apparently only be done by looking for a special signature near the beginning of the pixel data. HAM-E files are structurally valid ILBM files, but will be displayed as garbage by viewers that don't support HAM-E.

Extra-Halfbrite
Extra-Halfbrite (EHB) 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  chunk being set.

Deep images
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
PBM (Planar BitMap) files have FORM type of  instead of. 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
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  (instead of  ), and instead of a   chunk there is an   chunk.

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).

VDAT
In the ILBM variant we're calling VDAT (reportedly used by "the ST version of DPAINT"), instead of raw image data, the  chunk contains a sequence of   chunks. It uses compression type 2. The format is similar, but not identical, to that used in Tiny Stuff files.

DCTV
DCTV is a format designed to work with particular video hardware from Digital Creations. Like HAM-E, identifying an ILBM file as DCTV is difficult; it can apparently only be done by looking for a special signature near the beginning of the pixel data. DCTV files are structurally valid ILBM files, but will be displayed as garbage by viewers that don't support DCTV.

NEOchrome Master
Uses a  chunk. Refer to NEOchrome Master.

Specifications

 * "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
 * Picture format docs (of a number of formats including DeluxePaint LBM/IFF)
 * , from the Encyclopedia of Graphics File Formats
 * FORM RGBN and FORM RGB8
 * libilbm / doc
 * ILBM64 - An extension to support 16 bits/sample
 * ILBM64 - An extension to support 16 bits/sample

Software

 * RECOIL
 * Konvertor
 * Netpbm
 * XnView
 * Irfanview with FORMATS plugin 4.36 or more recent
 * libilbm
 * FFmpeg
 * ImageMagick
 * abydos
 * iff-convert

Sample files

 * ftp://ftp.back2roots.org/pub/amigascne/Gfx/pics/
 * .../*.IFF
 * https://samples.libav.org/image-samples/ASH.LBM - a PBM file
 * https://github.com/mtuomi/SecondReality .../*.LBM - PBM files
 * http://www.fileformat.info/format/iff/sample/index.htm
 * pl_pix.lha → Plan_pix.lha → *.pic (ACBM)
 * Hardwired1a.lha (Extra-Halfbrite)
 * (VDAT)
 * (SHAM)
 * https://telparia.com/fileFormatSamples/image/iffILBM/
 * https://telparia.com/fileFormatSamples/image/iffRGBN/
 * https://telparia.com/fileFormatSamples/image/iffILBM/
 * https://telparia.com/fileFormatSamples/image/iffRGBN/

Links

 * Wikipedia: ILBM
 * Intro to Amiga IFF ILBM Files and Amiga Viewmodes
 * Wikipedia: Hold-And-Modify
 * Wikipedia: Amiga Halfbrite mode
 * Previously Unknown Warhol Works Discovered on Floppy Disks from 1985
 * Detailed report on the Warhol graphic recovery (PDF)