Static doughnut that appears to expand & contract. I still haven't found an explanation for this #illusion -- perhaps there isn't one yet. (From: https://twitter.com/jagarikin/status/1331409504953540613)
-- #pico8 src
pal({[0]=10,140,134},1)
q=0::_::cls(2)q+=cos(t()/20)/20
for y=-41,41do for x=-41,41do
local d,a,c=x*x+y*y,(atan2(x,y)+q)*2,1if(d>=576)then
if(d<625)then c=(a+.25)%1elseif(d<1600)then c=a%1elseif(d<1681)then
c=(a-.25)%1end end pset(64+x,64+y,c*2)end end
flip()goto _
@zep I'm pretty confident its the lines on the inside and outside. With strongly contrasting colors and the spin rate, our brains stop interpreting them as separate lines, and instead interpret them as motion blurs from the larger color blocks, leading to the perceived effect.
Two tests to confirm this theory:
1) Remove the lines. Does the effect go away?
2) Reverse the orientation of the lines. Does it reverse the perceived effect?
@LouisIngenthron Oh, nice. Yeah -- a single frame of this animation would indeed resemble a few averaged frames of a no-lines version where the donut /was/ actually expanding / contracting. I'll buy that!
(Although, I think those two tests don't say anything about the mechanism -- just that they are necessary for the illusion to function).