.LBM File Format

Introduction

PBM is the picture format of Deluxe Paint for MS-DOS. Deluxe Paint II Enhanced, Electronic Arts’ paint program for VGA, saved its 256-colour pictures and brushes in it; whether the first release, in 1989, already did is not confirmed. The files usually have the extension .lbm, and brushes sometimes as .bbm. Many MS-DOS games kept their artwork in these files.

A PBM file is an IFF file, built from the same chunks as the Amiga’s ILBM: a bitmap header, a colour map, Deluxe Paint’s colour ranges. The pixels are stored differently: one byte a pixel, the colour number, instead of bitplanes. The form type is the three letters PBM and a space. Later write-ups expand PBM as “Planar Bitmap”, although the pixels are not planar; what Electronic Arts meant by it is not recorded.

Electronic Arts never published a description of PBM. The author of libilbm, one of the libraries that read it, wrote that he “couldn’t find any specification”. What is known comes from files and from the programs that read them: FFmpeg, Netpbm and libilbm. The parts it shares with ILBM are documented in Electronic Arts’ IFF and ILBM documents; the PBM BODY, the TINY thumbnail and the DPPS chunk are not.

A file dropped on any page of this site opens in the explorer, which shows every chunk and every row, with the picture, its palette, its colour ranges and its thumbnail.

Structure

A PBM file is one IFF FORM chunk with the form type PBM . Every chunk inside it is a four-letter ID, a 4-byte big-endian length and the data, padded to an even length. In the files Deluxe Paint writes the chunks come in this order:

A reader should not depend on that order: as in any IFF file, the bitmap header comes first and the pixel data last, and the chunks in between can come in any order or be left out.

00000000  46 4F 52 4D 00 00 0F 2C 50 42 4D 20 42 4D 48 44
00000010  00 00 00 14 01 40 00 37 00 00 00 00 08 00 01 00
00000020  00 00 05 06 01 40 00 C8 43 4D 41 50 00 00 03 00

Telling it apart

The explorer’s parser for this format checks that the file starts with FORM and that the form type, the four bytes at offset 8, is PBM (50 42 4D 20), with the space. It turns the file down if there is no BMHD chunk, if the BMHD is shorter than 20 bytes, or if the BODY comes before the BMHD, and the explorer then tries the parsers for the other formats of the family.

Deluxe Paint for the PC also wrote ILBM files, with the same extension and nearly the same chunks; only the form type tells them apart. Netpbm’s .pbm, the portable bitmap, is a different format with nothing in common but the letters.

Variants

• Interleaved bitmap (IFF ILBM) – the Amiga’s format: interleaved bitplanes, with HAM, EHB and 24-bit pictures
• Deluxe Paint PC bitmap (IFF PBM) – Deluxe Paint for MS-DOS: one byte a pixel
• Amiga contiguous bitmap (IFF ACBM) – AmigaBASIC’s form: each bitplane stored whole

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