Include 3 filters:

• Color quant (== pattern 0)
• Color bimodal threshold (== pattern 1)
• Color dith (Knuth D.E. dither matrix) (== pattern 2,3,4)
• Color odith 8x8 (Knuth D.E. dither matrix) (== pattern 5)
• Color dots dither 8x8 (== pattern 6)
• Color bayer dither 8x8 (== pattern 7)
• Color d-algorithm dither (== pattern 8)

Convert images to simulate palettes of ancient displays

Launching the plugin Menu > Colors > Map > Color Threshold... == pattern 0

• 3-bit RGB 2 colors per channel 8 colors in total
• 6-bit RGB 4 colors per channel 64 colors in total
• 9-bit RGB 8 colors per channel 512 colors in total
• 12-bit RGB 16 colors per channel 4,096 colors in total
• 15-bit RGB 32 colors per channel 32,768 colors in total
• 18-bit RGB 64 colors per channel 262,144 colors in total
• change any number of colors per channel 2-256 colors

The bimodal threshold is widely used to clean scanned images. This plugin allows you to apply a bimodal threshold to individual parts of the image using the "Color" layer effect.

Launching the plugin Menu > Colors > Map > Color Threshold... == pattern 1

Source image:

Plugin dialog:

Result:

Launching the plugin Menu > Colors > Map > Color Threshold... == pattern 2,3,4
(Knuth D.E. dither matrix)

• Gimp 2.8

• copy threshold-color to your ~/.gimp-2.8/plug-ins directory.

• ogniewski-scaler
• http://gimpbook.com/scripting/
• http://www.gimp.org/docs/python/index.html