Converting Atari ST DEGAS & DEGAS Elite Graphics (.PI1, .PC1) to Modern PNG

Introduced by Atari Corporation in 1985, the Atari ST was a personal computer powerhouse powered by the Motorola 68000 16/32-bit CPU. With built-in MIDI ports, high-resolution graphics, and a colorful graphical user interface (GEM), the ST became the machine of choice for musicians, graphic designers, and demoscene coders throughout the late 1980s and early 1990s.

Among the early creative tools released for the Atari ST, Tom Hudson's DEGAS (Design & Entertainment Graphic Arts System), published in 1985 by Batteries Included and updated to DEGAS Elite in 1986, became the dominant digital painting suite.

Vintage Atari ST artwork created in DEGAS was saved in .pi1, .pi2, .pi3 (uncompressed) or .pc1, .pc2, .pc3 (compressed) files. This guide details how the ST's video memory and bitplane shifter work and how to decode DEGAS files into lossless 32-bit RGBA PNGs using convrtr's Atari ST DEGAS to PNG converter.

Resolution Modes and File Extensions

The Atari ST supported three hardware display modes, each directly mirrored by DEGAS file extensions:

| Mode | Resolution | Colors | Bitplanes | Uncompressed Ext | Compressed Ext | | :--- | :--- | :--- | :--- | :--- | :--- | | Low Resolution | 320 × 200 | 16 | 4 | .pi1 | .pc1 | | Medium Resolution | 640 × 200 | 4 | 2 | .pi2 | .pc2 | | High Resolution | 640 × 400 | 2 (Monochrome) | 1 | .pi3 | .pc3 |

In all modes, the total video memory (VRAM) occupied exactly 32,000 bytes. Uncompressed DEGAS files consist of a 34-byte header followed by the 32,000-byte VRAM dump, yielding an exact file size of 32,034 bytes.

Anatomy of the 34-Byte Header

The first 34 bytes of any DEGAS file contain crucial metadata:

• Resolution Word (2 bytes):
  • 0: Low Resolution
  • 1: Medium Resolution
  • 2: High Resolution
  • For DEGAS Elite files, the highest bit (bit 15) is set if color animation cycles or extended features were recorded.
• Hardware Palette (32 bytes):
  • 16 16-bit big-endian words representing the 16 hardware color registers.
  • The Atari ST uses a 9-bit RGB palette: 3 bits for Red, 3 bits for Green, and 3 bits for Blue.
  • Bits 8–10: Red intensity (0 to 7).
  • Bits 4–6: Green intensity (0 to 7).
  • Bits 0–2: Blue intensity (0 to 7).

To map these 3-bit channels (values 0–7) to modern 8-bit color channels (0–255), each value is scaled: channel8 = Math.round((channel3 / 7) * 255)

Interleaved Bitplane Mechanics

Unlike modern framebuffers where each pixel's color is stored contiguously (e.g. 32-bit RGBA or 24-bit RGB), the Atari ST hardware utilized interleaved bitplanes:

In Low Resolution (320 × 200, 16 colors):

• There are 4 bitplanes, requiring 160 bytes per scanline ((320 * 4) / 8 = 160).
• Every group of 16 horizontal pixels is packed into 4 sequential 16-bit words (8 bytes).
• To decode pixel $p$ (from 0 to 15, left to right):
  • Bit from Word 0 forms bit 0.
  • Bit from Word 1 forms bit 1.
  • Bit from Word 2 forms bit 2.
  • Bit from Word 3 forms bit 3.
  • The resulting 4-bit number ($0 \dots 15$) serves as an index into the 16-color hardware palette.

In Medium Resolution (640 × 200, 4 colors):

• 2 bitplanes are interleaved. Because original Atari monitors displayed this mode at a 640 × 400 physical raster, scanlines must be doubled vertically to maintain the correct native aspect ratio.

PackBits RLE Compression in DEGAS Elite

DEGAS Elite introduced run-length compression (.pc1, .pc2, .pc3) based on the Apple Macintosh PackBits algorithm:

• Read control byte $b$:
  • If $b < 128$: Copy the next $b + 1$ literal bytes.
  • If $b > 128$: Repeat the next byte $257 - b$ times.
  • If $b = 128$: No operation (skip).

Decompressing until 32,000 bytes are restored reconstructs the exact original video memory buffer.

In-Browser Privacy and Integrity

With convrtr's DEGAS to PNG tool, you can drag and drop your vintage Atari ST files directly into your browser:

• Decodes both uncompressed .pi1/.pi2/.pi3 and compressed .pc1/.pc2/.pc3 archives.
• Preserves authentic 9-bit color values and line-doubling.
• Offers 2x nearest-neighbor upscaling for crisp reproduction on high-DPI monitors.
• Operates 100% locally with zero server uploads or tracking.