for a simple tunnel effect you'd want to do something like this:
x=f(angle relative to center)
y=g(distance from center)
Where f and g are 2 appropriate functions. Below is an example that works:
r=atan2(y,x);
d=sqrt(sqr(x)+sqr(y));
x=abs(r*4/$pi); //could also just be r but this looks better...
y=1/d;
Then to avoid the aliasing noise near the center you might want to set alpha to some function of d.
