How to Export WBMP in GIMP for Legacy Devices

Wireless Bitmap (WBMP) is a legacy, 1-bit monochrome image format originally developed for resource-constrained mobile phones running the Wireless Application Protocol (WAP). This article explains the export options and image preprocessing controls required in GIMP to generate compliant WBMP files for vintage handsets, microcontrollers, or embedded monochrome screens.

The WBMP Export Dialog in GIMP

When you export an image via File > Export As... and assign the .wbmp extension, GIMP invokes the WBMP file plug-in. Unlike formats such as JPEG or PNG, the WBMP specification does not support compression levels, color profiles, layers, or embedded metadata. Consequently, GIMP's final WBMP export dialog contains virtually no customizable parameters.

Because the format standard strictly mandates a 1-bit black-and-white pixel matrix with a minimal header, the options that control your final WBMP output are determined entirely through GIMP’s color indexing and thresholding tools prior to export.

Converting to a 1-Bit Palette

To control how your source image renders as a WBMP, you must convert the image mode to a 1-bit indexed format before exporting:

  1. Navigate to Image > Mode > Indexed... in the top menu.
  2. Under Colormap, select Use black and white (1-bit) palette.
  3. Under Dithering, configure the Color dithering options to determine how gradients and intermediate tones are rendered:
    • None: Best for crisp line art, sharp UI icons, and readable text. Any color brighter than the threshold becomes white; all darker pixels become black.
    • Floyd-Steinberg (normal): Distributes quantization errors to neighboring pixels, creating a patterned dot matrix that approximates photographic tones and gradients.
    • Floyd-Steinberg (reduced color bleeding): Similar to standard Floyd-Steinberg, but reduces pixel spreading across high-contrast edges.
    • Positioned: Applies a structured, repetitive dither pattern, which can provide a cleaner dot grid on ultra-low-resolution LCD screens.

Controlling the Black/White Split with Threshold

If dithering creates unwanted noise in UI elements or logos, use the Threshold tool before indexing:

  1. Navigate to Colors > Threshold....
  2. Drag the slider to set the exact luminance breakpoint where pixels turn either purely black or purely white.
  3. Once satisfied with the contrast and clarity, convert the image using the 1-bit indexed mode with dithering set to None.

Completing the Export

After setting the pixel distribution and ensuring your canvas dimensions match the target device's display resolution:

  1. Go to File > Export As....
  2. Name your file with the extension .wbmp (for example, graphic.wbmp).
  3. Click Export. GIMP will write the raw header and 1-bit bitmap data directly to the disk without altering the pixel values you configured.