starttime=timer

10 GOTO 40
20 XE = - X * S1 + Y * C1 : YE = -X * C1 * C2 - Y* S1 * C2 + Z * S2 : ZE = - X * S2 * C1 - Y *S2 * S1 + Z * C2 + RHO
30 SX = D * XE / ZE + CX : SY = CY - D * YE / ZE :RETURN
40 RHO = 30 : D = 350 : THETA = .1 : PHI = 1 : CX =140 : CY = 96 : S1 = SIN (THETA) : S2 = SIN(PHI) : C1 = COS (THETA) : C2 = COS (PHI)

' 50 DEF FN Z(X) = COS (.1 * (X * X + Y * Y))
FUNCTION FNZ(X)
 FNZ = COS (.1 * (X * X + Y * Y))
END FUNCTION

'60 HGR2 : HCOLOR= 3

70 FOR X = -20 TO 20 STEP .3
80 FL = 0
90 FOR Y = - 20 TO 20 STEP .07
100 Z = FNZ(X): GOSUB 20
110 IF SX < 0 OR SX > 279 OR SY < 0 OR SY > 191 THEN FL = 0 : GOTO 140
120 IF FL = 0 THEN FL = 1 : PIXEL SX,SY
'130 HCOLOR=3:HPLOT SX,SY:HCOLOR=0:HPLOT SX,SY+1 TO SX,191
PIXEL SX,SY : LINE SX,SY+1, SX,191,,255 ' Colour 255 is blue, so I can see where it's drawing.
140 NEXT Y,X
'150 PRINT CHR$(7):GET Q$:TEXT
print @(0,200) "Time: ";timer-starttime