Amiga ACBM vs IFF-ILBM: Continuous vs Interleaved Planar Bitmaps Compared
Technical comparison between Amiga Continuous Bitmap (.acbm) and standard IFF-ILBM (.iff / .ilbm). Compare planar memory layouts, Amiga Blitter DMA performance, and chunk structures.
Both ACBM and ILBM are raster graphics formats built upon Electronic Arts' Interchange File Format (IFF) for the Commodore Amiga. However, while ILBM interleaves bitplanes scanline-by-scanline to match Amiga copper display timing, ACBM stores each bitplane as a contiguous, uninterrupted block of memory to optimize fast Blitter DMA copying.
| FEATURE | AMIGA ACBM (.ACBM) | AMIGA IFF-ILBM (.IFF / .ILBM) |
|---|---|---|
| IFF Subtype ID | ACBM | ILBM or PBM |
| Planar Organization | Continuous (all scanlines for plane 0, then plane 1...) | Interleaved (scanline 0 across all planes, then scanline 1...) |
| Primary Image Chunk | ABMP (Amiga Continuous BitMap) | BODY |
| Hardware Target | Amiga Blitter DMA and offscreen chip RAM buffers | Amiga Denise / AGA video display hardware |
| Compression Algorithm | ByteRun1 RLE (or uncompressed) | ByteRun1 RLE (or uncompressed) |
- Continuous bitplane layout allows an entire plane to be transferred in a single Blitter DMA operation
- Eliminates CPU overhead of de-interleaving scanlines when streaming graphics to chip RAM
- Standardized IFF container with familiar BMHD, CMAP, and ABMP chunks
- Ideal for game background tiles, sprite sheets, and raw offscreen graphics buffers
- Universal Commodore Amiga standard used by Deluxe Paint and almost all commercial Amiga titles
- Interleaved scanlines (row 0 plane 0, row 0 plane 1...) match display hardware fetch order directly
- Full support for advanced Amiga display modes: Extra Half-Brite (EHB 64-color) and Hold-And-Modify (HAM6)
- Immense demoscene and retro gaming catalog preserved across tens of thousands of disks
ILBM is the quintessential Amiga graphic format for displaying art directly with Denise, while ACBM was engineered for blitting speed without scanline reorganization. Convert both ACBM and ILBM planar graphics to 32-bit RGBA PNG for flawless reproduction on modern screens.
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