Try this small modification to the above code. Compile it, move the mouse like crazy with left button held down. The overall effect is very similar to a skyscape coming towards you.
//// Skyscape effect
GETSCREENSIZE w,h
GRABSPRITE 0,0,0,w,h
WHILE TRUE
ROTOSPRITE 0,0,0,0 // draw current display
MOUSESTATE mx, my, b1, b2 // read da mouse
IF b1=1 // button pressed
FOR a=0 TO 90 STEP 5 // change the "STEP" for different effects
DRAWRECT 300+COS(a)*50+mx,200+SIN(a)*50+my,mx-mx/2,my-my/2,RGB(255/mx+my,mx+my,255)
DRAWRECT 300-COS(a)*50-mx,200+SIN(a)*50-my,mx-mx/2,my-my/2,RGB(255/mx+my,mx+my,255)
DRAWRECT 300+COS(a)*50-mx,200+SIN(a)*50+my,mx-mx/2,my-my/2,RGB(255/mx+my,mx+my,255)
DRAWRECT 300+COS(a)*50+mx,200+SIN(a)*50-my,mx-mx/2,my-my/2,RGB(255/mx+my,mx+my,255)
NEXT
ENDIF
GRABSPRITE 0,0,0,w,h // grab new screen display
SHOWSCREEN
WEND