Export GIMP Indexed Palettes as ACT Files

This guide explains how to convert an image to an indexed color palette in GIMP and export it as an Adobe Color Table (.act) file. Because GIMP natively saves palettes in its proprietary .gpl format, generating a Photoshop-compatible .act file involves preparing the indexed color table and utilizing a dedicated Python export script or converter to write the final binary file.

Step 1: Install the ACT Export Plug-in

GIMP does not support the binary .act format out of the box. To enable native export:

  1. Download a GIMP-compatible ACT export script (commonly distributed as export_act.py or act_export.py).
  2. Open GIMP and navigate to Edit > Preferences > Folders > Plug-ins to locate your local plug-in directory.
  3. Place the script inside the designated plug-ins folder.
  4. Ensure the file has execution permissions (on macOS/Linux: chmod +x export_act.py).
  5. Restart GIMP so the new script is recognized.

Step 2: Convert the Image to Indexed Mode

If your artwork is in RGB mode, you must first generate the indexed color map:

  1. Open your image in GIMP.
  2. Go to Image > Mode > Indexed... in the top menu.
  3. In the Indexed Color Conversion dialog:
    • Select Generate optimum palette.
    • Set the Maximum number of colors (typically up to 256 for standard .act files).
    • Choose your preferred Dithering method if applicable.
  4. Click Convert.

Step 3: Extract the Palette

Once the image is indexed, make the palette accessible for export:

  1. Open the Colormap dialog by navigating to Windows > Dockable Dialogs > Colormap.
  2. Click the colormap menu icon or right-click within the list of swatches.
  3. Select Colormap to Palette to generate a reusable palette entry.
  4. Enter a name for your new palette and click OK.

Step 4: Export to .act Format

With the plug-in installed and the palette created:

  1. Open the Palettes dialog via Windows > Dockable Dialogs > Palettes.
  2. Locate the newly created palette in the list.
  3. Right-click the palette and select the export option added by the script (typically Export as... > Adobe Color Table (.act) or Export ACT).
  4. Choose the destination folder on your drive and save the file.

Alternative Method: Using an External Converter

If you cannot run Python plug-ins in your GIMP installation:

  1. In the Palettes dialog, right-click your palette and choose Export As > Text file or save it as a native GIMP .gpl file.
  2. Use an open-source palette converter or web-based converter tool to transform the .gpl plain text RGB values into a 768-byte binary .act file.