FRAC
!0000000D
: frac go ;
-*-
v
XMAX
!00000027
( Maximum X value)
320 constant xmax
-*-
>
YMAX
!00000013
200 constant ymax
-*-
>
GO
!0000007B
: GO
  cls gcls generate
    99 0 do
      ?term if key drop abort endif
      i 3 mod 1+ i 3 and plot
      loop
;
-*-
v
GCLS
!00000046
: GCLS  <graphics> 4 vmode
        0 0 0 xmax 1- ymax 1- FILLBOX
;
-*-
>
ARRAY
!0000000E
DEFINE ARRAY
-*-
v
DEFINE
!00000088
: DEFINE  CREATE
        16 1024 * ALLOT
        DOES>
        SWAP DUP 16384 U< IF + ELSE ." Out of range, array" ABORT ENDIF

;
-*-
^
>
GENERATE
!0000021A
: GENERATE
        2 0 ARRAY C!
        0 1 ARRAY C!            \ Changing the ARRAY initial values or
        1 2 ARRAY C!            \
        3 TOP !                 \ uncommenting this line and removing the
                                \ trailing +1 changes the pattern.
        11 0 DO                 \  |
             1  TOP @ 1- DO     \  |
                I ARRAY C@      \  V
 (              J 3 AND IF 1+ ELSE 1- THEN 3 AND     ) 1+
                TOP @ ARRAY C! 1 TOP +!
             -1 +LOOP
        LOOP
;
-*-
v
TOP
!0000000E
VARIABLE TOP
-*-
^
>
PLOT
!0000017F
( Color Rotation -> )
: PLOT
        RR !                     ( Save rotation)
        xmax 2/ X ! ymax 2/ Y !
          x @ y @                ( Initial point, color on stack)
                1    array c@ rr @ + 3 and CURR
        512 1 * 2 DO
                I 1- ARRAY C@ RR @ + 3 AND PREV
                I    ARRAY C@ RR @ + 3 AND CURR
        LOOP drop drop drop
;
-*-
v
X
!0000000C
VARIABLE X
-*-
>
Y
!0000000C
VARIABLE Y
-*-
>
RR
!0000000D
variable rr
-*-
>
PREV
!000000AD
: prev

        dup 0 = if  2 y +! drop exit endif
        dup 1 = if -3 x +! drop exit endif
            2 = if -2 y +!      exit endif
                    3 x +!
;
-*-
>
CURR
!0000014C
: CURR
        dup 0 = if drop  2 y +!  L
                         2 y +!  L exit endif
        dup 1 = if drop -3 x +!  L
                        -3 x +!  L exit endif
            2 = if      -2 y +!  L
                        -2 y +!  L exit endif
                         3 x +!  L
                         3 x +!  L
;
-*-
v
L
!000000E7
: L <graphics>
  stack ab|abab ymax u< swap xmax u< and if
    else drop drop X @ Y @ exit endif

  moveto dup X @ Y @

  stack ab|abab ymax u< swap xmax u< and if
     else stack abcde|de exit endif

  lineto moveto?
;
-*-
^
^
^
^