based on this tutorial.
This is an old school effect which has seen use in a lot of games in the 90s. One of the familiar occurrences for a lot of people is on the torches in the original RuneScape login screen. The update logic for each row, each frame, is based on a blending of the row below it. The bottom-most row has no row below it and uses random noise reads instead. What this blending does is a progressive blurring, which you can see manifests as larger and larger blobs as you travel upwards.