Comment by omoikane

This looks like the classic fire effect: generate rising flames by averaging pixels below each output pixel, and randomize the last row.

I remember this effect because there was a competition[1] where every entry was a fire effect in 256 bytes, and I was amazed at the simplicity of the core algorithm.

[1] https://www.pouet.net/party.php?which=1791&when=1996