Make a dark mode illusion

What is a dark mode illusion image?

A dark mode illusion image is a single PNG that shows one picture to viewers in light mode and a completely different picture to viewers in dark mode. Nothing in the file changes and no script runs. The image carries a partly transparent alpha channel, and every app composites it over its own theme background — white in light mode, black in dark mode — so identical bytes resolve into two different pictures.

It isn't an animation, it isn't two files being swapped, and it doesn't detect your device. It's the same reason a paper cut-out looks different on a white table and a black one — except the cut-out is carrying two whole pictures at once.

• What is a dark mode illusion image?The definition, what it isn't, and the vocabulary.
• How to make oneSeven steps and the one rule that decides whether it works.
• Where it worksX, Slack, Discord, iMessage — and why Instagram never will.
• Why it's everywhere this weekViral now — and older than most write-ups admit.