Decoding 1984 Apple MacPaint Bitmaps: Byte-Run Compression to Modern PNG

When Apple unveiled the original Macintosh 128K on January 24, 1984, Steve Jobs showed off two groundbreaking applications written by Apple fellows Bill Atkinson and Andy Hertzfeld: MacWrite and MacPaint.

MacPaint revolutionized consumer graphics. Operating on a monochrome screen with a 1-bit framebuffer, MacPaint allowed users to draw, erase, and dither textures using a mouse.

Every document saved in MacPaint has a fixed canvas size: 576 pixels wide by 720 pixels tall, exactly matching an 8.5 x 11 inch printed page on Apple's ImageWriter dot matrix printer at 72 DPI (with 0.25 inch margins).

Modern image tools, operating systems, and browsers cannot render these vintage .mac, .pic, or .pnt files. This article documents the byte format of MacPaint documents and how our WebAssembly parser resurrects them into high-resolution PNGs.

The MacPaint File Layout

A MacPaint file has a straightforward, predictable structure:

+-----------------------------------------------------------+
| Version / Header: 4 bytes (uint32)                        |
| Usually 0x00000000 or 0x00000002                          |
+-----------------------------------------------------------+
| Pattern Palette: 38 patterns * 8 bytes = 304 bytes        |
| Custom 8x8 dither patterns used in the document           |
+-----------------------------------------------------------+
| Reserved Padding: 204 bytes (all zeros)                   |
+-----------------------------------------------------------+
| Bitmap Data: PackBits compressed scanlines                |
| Exactly 720 lines of 576 pixels (72 bytes per scanline)   |
+-----------------------------------------------------------+

Total header size before image data is exactly 4 + 304 + 204 = 512 bytes.

The PackBits Byte-Run RLE Algorithm

Because the Macintosh 128K had only 128 kilobytes of RAM shared with video memory, Atkinson designed a lightweight run-length encoding compression algorithm called PackBits (later incorporated into Apple's QuickDraw and the TIFF standard).

The uncompressed canvas has 720 lines. Each line is 576 pixels wide, requiring 576 / 8 = 72 bytes of bitmap data. The compressed stream encodes these 72-byte scanlines using signed control bytes:

1. Read a signed byte n (-128 to 127):
   • If n >= 0 and n <= 127: Literal run. Copy the next n + 1 bytes directly to the output scanline buffer.
   • If n >= -127 and n <= -1: Replicate run. Read the next 1 byte, and repeat its value (1 - n) times (i.e. 2 to 128 times).
   • If n == -128: No-op (ignored for word alignment).
2. Continue until exactly 72 bytes have been decoded for the current scanline.
3. Repeat for all 720 scanlines.

Restoring 1-Bit Graphics to Modern Formats

In original Macintosh QuickDraw:

• Bit 0 represented a white pixel (#FFFFFF).
• Bit 1 represented a black pixel (#000000).

To convert MacPaint files into modern PNG images:

1. Decode the 720 PackBits scanlines into a contiguous 576 * 720 = 414,720 pixel buffer.
2. Map each bit to standard 32-bit RGBA pixels ([0, 0, 0, 255] for 1s, [255, 255, 255, 255] for 0s).
3. Encode the RGBA buffer into standard RFC 2083 Deflate-compressed PNG scanlines.

You can preserve and restore historical MacPaint illustrations instantly using convrtr's MacPaint to PNG converter with zero dependencies.