I like Low Tech Magazine's use of dithering to reduce bandwidth and storage consumption. I'd played around with dithering before and come up with a set of options I really liked... but I didn't record them anywhere, so now it's lost.
For the grayscale images in this wiki, I'm using:
convert in.jpg -resize 640x480 -colorspace Gray -separate -average -ordered-dither o8x8,6 out.png
This command usually turns the greater-than 500kb JPEGs from my phone's camera into PNGs smaller than 30kb.
For the color images in this wiki, aside from the images on the the camera pages (as those are meant to accurately represent the output of their respective cameras), I'm using:
convert in.jpg -resize 640x480 -ordered-dither o8x8,3 out.png
I've also achieved a good monochromatic look for a photograph of my pottery, resulting in the same size image as the grayscale version, using this command:
convert in.jpg -resize 640x480 -average -ordered-dither o8x8,6 -colors 4 out.png
After using this technique for a while, though, it seems to not work so well outside of images that were already fairly monochrome to begin with. I've learned a new technique, demonstrated below, which builds on a dithered grayscale PNG prepared using the previously shown technique.
$ magick -size 10x100 gradient:'#322110'-'#f0e0d0' gradient-brown.png $ magick 2025-mug-6.gray.png gradient-brown.png -clut 2025-mug-6.brown.png
The first command creates a gradient reference image, and the second command uses that gradient as a lookup table for converting the shades in the grayscale image into shades along the gradient. This produces a nice monochrome version of the grayscale image which is no bigger than the original.
