jamtool

jamtool is a Rust-based library and command-line utility for manipulating .JAM texture files, used by the game Grand Prix 2. It also includes an interactive WebAssembly-based web editor for editing textures pixel-by-pixel.

Disclaimer: this is not the classic early 2000 hero-level take on reverse engineering that made Grand Prix 2 a legend, but my mid-2026 personal excuse to learn Rust with help from generative AI on something I've never dared to do before. Because of renewed interest, I hope this can be useful to someone :).

Try it out at jamtool.playlinux.net!

Installation

Prerequisites

• Rust (latest stable)
• wasm-pack (for building the web editor)

Building the CLI

cargo build --release

The binary will be available at target/release/jamtool.

Building the Web Editor

wasm-pack build --target web

This will generate the WASM and JavaScript glue code in the pkg/ directory.

Usage

CLI

Exporting to PNG

./target/release/jamtool <INPUT.JAM> <OUTPUT_DIR>

This extracts all textures into the specified directory as PNGs and creates a .jammeta.txt file. Output also includes images using the haze palettes, though they are not explicitly included in the JAM file and are ignored when encoding back to JAM.

Importing from PNG

./target/release/jamtool --encode <INPUT_DIR> <OUTPUT.JAM>

This takes the PNGs and the .jammeta.txt file from the directory and encodes them back into a single JAM file.

Web Editor Features

• Pixel painting — select any GP2 palette color and draw on textures click-by-click or drag
• Zoom & pan — slider, CTRL + scroll wheel, and fit-to-canvas for precise editing
• Paste images from clipboard — paste any image, position it by dragging, resize freely with corner handles, and commit with Enter — preview is quantized to GP2 colors in real time
• Load & save .JAM — open existing files and save modified ones
• Import/Export ZIP — roundtrip to indexed PNGs with metadata for external editing
• Texture browser — sidebar with thumbnails and detail panel showing all header fields
• Pixel inspector — hover any pixel to see its global palette index and RGB color

Running Locally

1. Build the WASM package (see above).
2. Serve the project root using a web server (browsers block WASM via file://):

Python:

python3 -m http.server

Node.js:

npx serve .

3. Open http://localhost:8000 (or the provided port) in your browser.

References

I'm doing nothing new here, none of this would have been possible without the following resources:

• JAM Structure
• GP2JAM Source Code

License

This project is licensed under the MIT License - see the LICENSE file for details.