bounce.ps

%!PS
<< /PageSize [320 240] >> setpagedevice

/Courier findfont 12 scalefont setfont

/sleep { 100000 mul {} repeat } def

/x 100 def
/y 50 def
/r 15 def
/x_ 2 def
/y_ 1 def
/f 0 def

{    
    % x bounce
    x r sub 0 lt x r add 320 gt or {
        /x_ x_ -1 mul def
    } if

    % y bounce
    y r sub 0 lt y r add 240 gt or {
        /y_ y_ -1 mul def
    } if
    
    % move circle
    /x x x_ add def
    /y y y_ add def
    
    % clear page
    1 setgray
    0 0 320 240 rectfill

    % draw circle
    newpath
    x y r 0 360 arc
    1 0 1 setrgbcolor
    fill

    10 10 moveto
    0 setgray
    (frame            ) dup 6 f 10 string cvs putinterval show
    
    flushpage

    10 sleep
    
    /f f 1 add def
} loop