Png Bot is an image bot which uses the png transparency layer to effectively hide one image in another depending on the background color of the viewer. I got the idea from a tweet by user @taki_bump, but the account has since been deleted (or renamed?). Here is the original image that inspired me to try to make a bot:

The python based bot uses Image Magick to do the image modifications:

1. The two images are converted to grayscale and 8×8 ordered dither is added.
2. The Darker image has the transparency layer set to white.
3. The Lighter image has the transparency layer set to black.
4. The two images are composited.

This works because Twitter (as well as many other sites/ apps) default to a light/ white background in preview and a darker background when you view the full sized image. If you save one of the resulting images on an iPhone you can also see the effect between preview/ full size view.  It used to work better on Twitter before they changed the background colors.
Note: if you’re using “Night Mode” the effect doesn’t work at all..

 

pic.twitter.com/lsinUSHf5s

— Png Bot (@pngbot) October 4, 2016

Launched: May, 2015

Platforms: Twitter, Tumblr

Source Code: github.com/vgan/pngbot