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Kevin Altis
altis@semi-retired.com
2001-06-10
---
BYTE magazine
November 1982
p. 282 - 288
I haven't found a comprehensive list of turtle graphic commands online yet, so any pointers would be appreciated. The problem with turtle graphic commands is that at least back in 1982, there wasn't a standard, so some names differed and in some cases the functionality differed as well between implementations. In addition, other turtle graphics libraries done for other languages expanded or created their own command variations.
BACKGROUND - returns the color number of the current background. (SETBG) Also sets the background to a given color.
BACKWARD (BACK, BK) - moves the turtle backward
CLEAN - clears the screen, but doesn't move the turtle
CLEARSCREEN (CLS, DRAW) - clears screen and initializes turtle
COLOR - returns the color number of the sprite currently being talked to
DOT - puts a dot at the indicated point on the turtle graphics screen
FENCE (NOWRAP) - tells the turtle to draw without wraparound - gives error message if turtle tries to plot offscreen
FORWARD (FD) - moves the turtle forward
HEADING - returns the heading number of the active sprite or turtle
HIDETURTLE (HT) - makes the triangular turtle shape disappear
HOME - tells active turtle or sprite to go to the center of screen
LEFT (LT) - turns the turtle or sprite left a given number of degrees
PEN - returns values for pen type and color
PENCOLOR - returns value of pen color. (SETPC) Also sets the current turtle pen color.
PENDOWN (PD) - makes the turtle pen ready to draw a line
PENERASE - makes the turtle pen ready to erase a line
PENREVERSE - makes the turtle pen ready to reverse a line (draws if line isn't there, erases if it is; Terrapin/Krell Logo used PENCOLOR 6 to get the same effect)
PENUP (PU) - makes the turtle pen inactive
POS (WHERE) - returns the position of the turtle
RIGHT (RT) - turns the turtle or sprite right a given number of degrees
SETHEADING - gives the active sprite or turtle a given heading
SETPEN - sets color and type of turtle pen
SETPOS (SETXY, SXY) - moves the turtle to a new position
SETX (SX), SETY (SY) - moves turtle to given x- or y-coordinate, other coordinate unchanged
SHOWTURTLE (ST) - makes the triangular turtle shape appear
TOWARDs - returns heading value turtle would have if it were pointing toward a given position
TURTLESTATE - returns pen position, turtle status, background color, and pen color
WINDOW - allows the turtle to plot offscreen (although the plotting can never be seen)
WRAP - causes the turtle to appear on the opposite side of the screen if it attempts to go offscreen
XCOR, YCOR - returns the x- or y-coordinate of the active sprite or turtle
XVEL, YVEL - returns the x- or y-velocity of the active sprite
Coordinate System
Most turtle graphics systems put the point 0,0 at the center of the screen rather than the top left corner of the screen. In addition, most graphic displays attribute positive values to x and y as they go down and right across the screen, so the top-left corner would be (0,0) and the bottom-right point on a 640x480 pixel display might be (639, 479). On a typical Logo turtle graphics screen the top-left on a 640x480 display would be something like (-320, 240) and the bottom-right point would be (319, -239). The zero degrees (heading = 0) for a turtle is typically pointing right along the x-axis with positive values heading left and up towards the y-axis.
Math
The biggest issue to be aware of is that most Logo programs I've seen expect arguments to cos, sin, tan, etc. to be in degrees, rather than radians. If you're translating a Logo program and see something like sin(180 / n) then in Python that would become sin(pi / n).
Nongraphics commands
If you're translating a turtle graphics program from Logo, you'll likely run into nongraphics commands that will need to be translated into something equivelant in Python. Here are some of the more common commands you are likely to see, along with some example translations.
CLEARTEXT - clears all text from screen and puts cursor at beginning of first text line
COUNT - returns the number of elements in a list
END - signals the end of a procedure
LOCAL - declares a variable as local to the enclosing procedure without making it the argument of the procedure
MAKE - assigns a name to a value
RANDOM - generates a random number between 0 and n
REPEAT - executes a list a given number of times
STOP - stops a procedure
Here's a typical recursive procedure, which I translated in the bytedesign.txt script.
TO PENTL :SIDE :ANG
IF :SIDE < 2 [STOP]
FD :SIDE
LT :ANG
PENTL :SIDE - .38 :ANG
END
in Python using my turtle graphics library and pentl defined as a class method, this becomes:
def pentl(self, side, ang):
if side < 2: return
self.fd(side)
self.lt(ang)
self.pentl(side - .38, ang)
Note that instead of STOP we use return. Python doesn't require the END to signal the end of a function (method). Logo uses lists to get most of its work done, so a typical loop looks like:
REPEAT 5 [FD 64.65 PD WHEEL POS PU BK 64.65 RT 72]
which in Python becomes:
for i in range(5):
self.fd(64.65)
self.pd()
self.wheel(self.getXY())
self.pu()
self.bk(64.65)
self.rt(72)
Note the use of getXY() instead of POS in the original Logo program. Local variables might look something like this:
LOCAL "OLDH MAKE "OLDH HEADING
in Python this becomes
oldh = self.getHeading()
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