Converting C64 Advanced Art Studio (.art) to PNG: Commodore 64 Bitmap Recovery

In 1986, Oxford Computer Systems published Advanced Art Studio for the Commodore 64. Following the massive success of The Complete Graphic System and The Artist, Advanced Art Studio became one of the most capable graphic design suites on the C64.

Images authored in Advanced Art Studio use the .art extension. Unlike KoalaPainter (.koa), which was locked exclusively into the C64's 160x200 Multicolor mode, Advanced Art Studio gave artists the freedom to paint in either High-Resolution (Hires) 320x200 monochrome mode or Multicolor 160x200 mode.

This dual-mode architecture made .art a preferred format for creating intricate game loading screens, demoscene title art, and detailed digital illustrations across the 1980s.

This guide explains how Commodore 64 VIC-II bitmap data is organized in .art files, how Hires and Multicolor bitplane packing functions, and how convrtr's ART to PNG converter renders vintage C64 paintings into crystal-clear 32-bit RGBA PNG graphics directly in your browser.

Memory Maps and PRG Structure

Commodore 64 graphic formats are structured as direct snapshots of the computer's video RAM. A .art file contains:

1. Load Address (2 bytes): Typically 0x00 0x20 ($2000) or 0x00 0x60 ($6000) in little-endian format.
2. Bitmap Data (8,000 bytes): 1,000 character cells (40 columns $\times$ 25 rows), where each $8 \times 8$ cell occupies 8 sequential bytes.
3. Screen RAM (1,000 bytes): Color attribute definitions for each of the 1,000 cells.
4. Color RAM (1,000 bytes, Multicolor only): Extended color data mapped to hardware address $D800.
5. Background Color Byte (1 byte, Multicolor only): Color register $D021.

File Size Differentiation

Depending on the mode selected by the artist, the file size varies:

• Hires Mode: 9,002 bytes (2-byte header + 8,000 bytes bitmap + 1,000 bytes screen RAM).
• Multicolor Mode: 10,002 or 10,003 bytes (including Color RAM and background register).

Decoding High-Resolution (Hires) Mode

In Hires mode, the Commodore 64 delivers a full $320 \times 200$ resolution with single-width square pixels. However, the hardware imposes an 8-bit attribute limit: each $8 \times 8$ pixel cell can only use 2 colors simultaneously.

For each character cell ($Y \in [0, 24], X \in [0, 39]$):

1. Color Decoding: The cell's color attribute byte is fetched from Screen RAM:
   • Foreground Color: High nibble (screenByte >> 4) & 0x0F
   • Background Color: Low nibble screenByte & 0x0F
2. Bitplane Mapping: For each of the 8 scanlines in the cell, 1 byte is fetched from Bitmap RAM:
   • Each bit represents one pixel from left to right (bit 7 down to bit 0).
   • If the bit is 1, the pixel is rendered in the cell's Foreground Color.
   • If the bit is 0, the pixel is rendered in the cell's Background Color.

Decoding Multicolor Mode

In Multicolor mode, horizontal resolution is halved to $160 \times 200$, creating double-wide rectangular pixels, but allowing 4 simultaneous colors per $8 \times 8$ cell:

1. Color Selection:
   • Color 00: Universal background color register ($D021)
   • Color 01: High nibble of Screen RAM for the cell
   • Color 10: Low nibble of Screen RAM for the cell
   • Color 11: Low nibble of Color RAM ($D800) for the cell
2. Pixel Bit Pairs: Each byte in Bitmap RAM contains 4 double-wide pixels (2 bits each):
   • Bits 7-6: Pixel 0
   • Bits 5-4: Pixel 1
   • Bits 3-2: Pixel 2
   • Bits 1-0: Pixel 3

The converter doubles each pixel horizontally to reconstruct the full $320 \times 200$ framebuffer.

The 16-Color VIC-II Palette

The Commodore 64's VIC-II graphics chip generated analog composite video with 16 fixed colors. convrtr supports both the industry-standard Pepto palette (calibrated to authentic CRT phosphor decay) and the Colodore palette (color-calibrated to modern D65 sRGB displays):

| Index | Color Name | Pepto sRGB | Colodore sRGB | | :--- | :--- | :--- | :--- | | 0 | Black | #000000 | #000000 | | 1 | White | #FFFFFF | #FFFFFF | | 2 | Red | #880000 | #813338 | | 3 | Cyan | #AAFFEE | #75CEC8 | | 4 | Purple | #CC44CC | #8E3C97 | | 5 | Green | #00CC55 | #56AC4D | | 6 | Blue | #0000AA | #2E2C9B | | 7 | Yellow | #EEEE77 | #EDF171 | | 8 | Orange | #DD8855 | #8E5029 | | 9 | Brown | #664400 | #553800 | | 10 | Light Red | #FF7777 | #C46C71 | | 11 | Dark Grey | #333333 | #4A4A4A | | 12 | Medium Grey | #777777 | #7B7B7B | | 13 | Light Green | #AAFF66 | #A9FF9F | | 14 | Light Blue | #0088FF | #706DEB | | 15 | Light Grey | #BBBBBB | #B2B2B2 |

Preserving Retro Art Locally

With convrtr's Art Studio decoder, vintage Commodore 64 .art files decode entirely in browser memory. Choose your preferred palette and scaling factor (up to 8x for sharp pixel scaling) to export authentic 1980s 8-bit digital artwork into high-resolution PNG with zero cloud uploads.