ZXASCII
AUTO -1
PROG demos:fractals/reaction diffusion
CHANGED FALSE
10 REM Reaction-diffusion
20 FOR f=0 TO 255: PALETTE hsv f,f,f,f: NEXT f:
 paper 8: cls: screen lock

30 cs=8,w=scrw div cs,h=scrh div cs: dim fg(2,w,h,2)
40 da=0.42,db=.125,f=.0545,k=.062,bf=1

50 for x=1 to w:
    for y=1 to h:
       obf=3-bf:
       if x=1 or y=1 or x=w or y=h then
          fg(obf,x,y,1)=1,fg(obf,x,y,2)=0
       else
          a=fg(bf,x,y,1),b=fg(bf,x,y,2),r=a*b*b,
          difa=(fg(bf,x,y-1,1)+fg(bf,x+1,y,1)+fg(bf,x-1,y,1)+fg(bf,x,y+1,1))/4-a,
          difb=(fg(bf,x,y-1,2)+fg(bf,x+1,y,2)+fg(bf,x-1,y,2)+fg(bf,x,y+1,2))/4-b,
          fg(obf,x,y,1)=a+da*difa-r+(1-a)*f,
          fg(obf,x,y,2)=b+db*difb+r-(f+k)*b:
       end if:
       a=fg(obf,x,y,1),b=fg(obf,x,y,2),
       i=255*(b/(a+b)):
       rectangle ink i;cs*(x-1),cs*(y-1),cs,cs fill:
    next y:
 next x

60 bf=3-bf,b=mousebtn=1:
 if mousebtn<>0 then
    x=int(mousex/cs)+1,y=int(mousey/cs)+1:
    if x>=1 and x<=w and y>=1 and y<=h then
       fg(bf,x,y,1)=1-b,
       fg(bf,x,y,2)=b
70 WAIT SCREEN:
 GO To 50