.LBM File Format – IFF container

Interleaved bitmap (IFF ILBM)

An ILBM file is built from IFF chunks, as Electronic Arts’ standard EA IFF 85 (Jerry Morrison, 14 January 1985) defines them. The whole file is one FORM chunk, and the chunks inside it hold the picture. All numbers are big-endian, high byte first, as on the Amiga’s 68000 processor. On these pages UBYTE, UWORD and ULONG are unsigned 8-, 16- and 32-bit numbers, and WORD and LONG signed 16- and 32-bit ones.

Chunks

Every chunk has the same 8-byte header, followed by its data:

OffsetSizeTypeNameDescription
044 ASCII charactersckIDThe chunk ID, such as BMHD or BODY.
44ULONGckSizeThe number of data bytes that follow. It does not count this 8-byte header or the pad byte.
8ckSizebytesckDataThe data.
8 + ckSize0 or 1bytepadA pad byte, only when ckSize is odd.
  • An ID is four characters from space (0x20) to ~ (0x7E). It may not start with a space, but may end with spaces, as in DPI and CAT . IDs are compared exactly, upper and lower case being different.
  • A form type, such as ILBM, is stricter: no lower-case letters and no punctuation.
  • The smallest chunk is 8 bytes, a header with a ckSize of 0.
  • A reader skips any chunk it does not know, using ckSize. This is what lets new chunks be added without breaking older readers.

Padding

When a chunk’s size is odd, one pad byte follows its data, and ckSize does not count it. So every chunk starts at an even offset. The pad byte is written as zero, but the standard says “don’t count on that when reading”.

Odd sizes are common. Deluxe Paint’s BODY chunks often have one: in the file on the overview, the BODY holds 6,171 bytes and the pad byte after it is the last byte of the file. A reader that forgets the pad byte reads the next chunk’s ID one byte late.

In some files the FORM’s size leaves out the pad byte of its last chunk. The explorer then shows that byte as a pad byte outside the FORM.

The FORM

The file is one FORM chunk whose data starts with a form type:

OffsetSizeTypeNameDescription
044 ASCII charactersIDFORM (46 4F 52 4D).
44ULONGsizeThe bytes that follow: the form type and every chunk, with their pad bytes. The file is normally this plus 8 bytes long.
844 ASCII charactersform typeILBM (49 4C 42 4D).
12…chunksThe bitmap header, the other chunks and the pixel data.

The ILBM document gives the order of the chunks as

FORM "ILBM" BMHD [CMAP] [GRAB] [DEST] [SPRT] [CAMG] CRNG* CCRT* [BODY]
Square brackets: may be left out. A star: any number, none included.
  • BMHD, the bitmap header, is required. The document says it “must appear first”, and then that the properties “may actually be in any order”; a writer should put it first.
  • Every property chunk, BMHD included, must come before the BODY. If a property appears twice, the last one before the BODY counts.
  • Readers stop at the BODY. Chunks after it are allowed, but many programs ignore them.
  • A file with no BODY is allowed: it holds only a colour map, or other settings.
  • Anything after the end of the FORM is not part of the picture. The explorer reports such bytes; some files have them.
00000000  46 4F 52 4D 00 00 1B FC 49 4C 42 4D 42 4D 48 44
00000010  00 00 00 14 …
FORM, size 7,164, form type ILBM, then the first chunk: BMHD, size 20

Group chunks

EA IFF 85 defines four group chunks, whose data starts with a type and then holds other chunks: FORM, LIST, CAT and PROP.

IDIts type fieldWhat it holds
FORMthe form typeOne object, such as a picture: its property chunks and data chunks. A FORM can also hold other FORMs, LISTs and CAT s.
LISTa contents typeSeveral FORMs that share properties. Its PROP chunks come first.
PROPthe form type the properties are forProperty chunks shared by the FORMs of that type in the LIST; at most one PROP for each form type.
CAT a contents type, only a hintChunks put one after another, with nothing shared. The type may be JJJJ, meaning none.

Reserved IDs, which no ordinary chunk may use: FORM, LIST, PROP, CAT , four spaces (filler), FOR1 to FOR9, LIS1 to LIS9, CAT1 to CAT9, and JJJJ.

A picture file is a single FORM ILBM. An animation file (ANIM) is a FORM holding ILBM forms, and is a different format. The explorer reads only files that start with FORM and ILBM; it does not take pictures out of LIST or CAT files. A group found inside the picture’s FORM is shown with its type and its chunks.

Reading the chunks

  1. Read 12 bytes. Check for FORM and the form type ILBM, and take the end of the form as 8 plus its size.
  2. At each offset until the end of the form, read the 4-byte ID and 4-byte size.
  3. Handle the chunk if you know its ID; otherwise skip it.
  4. Move on by 8 plus the size, plus 1 if the size is odd.
  5. Remember the last BMHD, CMAP and CAMG seen. At the BODY, decode the pixels with them.