Surface Plot

    1 REM  this proqram 6.4
    2 REM draws a surface z=f(x,y)
    3 BRIGHT 1
    4 LET ink=0
    5 LET sx1=0: LET sy1=170
    6 INK 7
    7 REM converted to T/S 2068           by  James N. Jones                  2242 Locust                     Amarillo, Texas 79109
    8 REM from the book by Myers          Microcomputer Graphics
   10 GO TO 40
   20 LET xe=-x*s1+y*c1: LET ye=-x*c1*c2-y*s1*c2+z*s2: LET ze=-x*s2*c1-y*s2*s1-z*c2+rho
   30 LET sx=d*xe/ze+cx: LET sy=cy+d*ye/ze: RETURN 
   40 LET rho=30: LET d=350: LET theta=.3: LET phi=1: LET cx=127: LET cy=87: LET s1=SIN (theta): LET s2=SIN (phi): LET c1=COS (theta): LET c2=COS (phi)
   50 DEF FN z(x)=COS (.1*(x*x+y*y))
   60 REM color
   70 FOR x=10 TO -10 STEP -1
   80 LET fl=0
   90 FOR y=-10 TO 10
  100 LET z=FN z(x): GO SUB 20
  110 IF sx<0 OR sx>255 OR sy<0 OR sy>175 THEN LET fl=0: GO TO 140
  120 IF fl=0 THEN LET l=1: PLOT sx,sy
  130 DRAW (sx1-sx),(sy1-sy)
  135 LET sx1=sx: LET sy1=sy
  136 INK 0
  140 NEXT y
  145 INK 7: LET sx1=0: LET sy1=170
  150 NEXT x
 9998 SAVE "SrfcPlot" LINE 1