The effect you saw scrolling down, implemented in osu! by replacing the cursor-ripple.png
This has been a fun project I've spend around 100 hours on, experimenting with different uses of making ridiculously large cursor-ripple images ever since I read a post pointing out osu doesn't have a upper load limit. At first making images by hand, but later taking the time to make more intricate images using code.
But what is a moire effect?
Simply put, it's what happens when a pattern containing more pixels than can be shown moves on screen. In real life, you might have seen it on those horrible looking shirts (google "moire shirt"). In osu, the movement is achieved in two ways. The first is the zooming of the cursor-ripple image. The second stronger effect comes from pattern interference when multiple ripples are close in time and space, creating interference patterns and sometimes the illusion of motion. This happens during streams or bursts, when new ripples are added before the old ones have fully faded. The effect is somewhat chaotic, but in general look likes magnetic swirls (sort of like the effect seen scrolling down the page).
With how the game scales ripple-images, they need to be really big to cover the entire screen. And also because images "spawn" small and then quickly zoom out, full-screen effects look better / flicker less with very larger images. I think around 15000x15000 pixels look good, but that creates an issue:
It's very gpu intensive to use large cursor-ripples, so much so these effects won't even run or might even crash osu on below average gpu.
But why tho?
I know, it's totally unpractical. Ruins frame-rate. But I think the effects are cool looking, even if it's bizarre and distracting to actually play with. If anyone want to check out the effects, links below, instructions included with the downloads.
Minimal download; 13 mb:
https://mega.nz/file/l1MHGQoC#ZfgIfzmj6DmNzusrskjOsBXGkFiuqCBgZeAJMs0hrY0
Full download; 375 mb:
https://mega.nz/file/g9tSWB7Q#WqI5cj1BvCtmu0YIVertv44CvmYXoMzF0HdtwoVTw_o