Turtle Graphics

[Jim Hurley]

Greetings all,

I have been working on a optics tutorial. It involves the 
manipulation of mirrors, lenses, microscopes, telescopes, focal 
points, light rays, fish, bugs, eyes, etc. I have found it useful to 
modify the traditional Turtle Graphics so that the language may be 
applied to any RR control and not just to the turtle drawing cursor. 
This will not be useful to the majority on this list, those whose 
primary objective is text manipulation. But for those interested in 
educational software or games, you may find TG a useful tool.

Actually there are three flavors, each useful in a different 
circumstance. The first is "Control Graphics." The turtle becomes a 
metaphor for a  control, any control--button, field, image, graphic. 
The Turtle Graphic vocabulary acts on the custom properties assigned 
to each control. These properties are:

px, py, pangle,p PenDown, pPoints

They are in order: The x and y Cartesian coordinates (measured 
relative to the center of the screen) the heading, the pen state (up 
or down--drawing or not) and the graphic points which define the line 
drawn by the control if the pen is down.

For example, in the following handler, a hare moves in a circle 
chased by a fox. (The hare and fox are buttons with the obvious icon.)

on mouseUp
   startTurtle "hare"
   startTurtle "fox"
   put 0 into theta
   put 5 into dTheta

   repeat until theta > 360

     tell "hare"
     setRA 200,theta -- Set the polar coord. radius and angle
     add dTheta to theta
     put xycor() into theHareLocation

     tell "fox"
     setheading direction(theHareLocation)
     forward 8

   end repeat
end mouseUp

The see some example of "Control Graphics" run this in the msg box:

go url "http://home.infostations.net/jhurley/ControlGraphics.rev"

(Richard: Would this be better: go url 
<http://home.infostations.net/jhurley/ControlGraphics.rev>?)


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The second flavor is "Multiple Turtles" which is useful in programing 
graphic lines with different properties (color, line size, etc.) It 
only draws; it does not control the controls.

The syntax is somewhat different from Control Graphics. As an 
example, the following handler draws a pinwheel with different 
colored spokes:

on mouseUP
   put "red,orange,black,green,blue,violet" into colorList

   repeat with i = 1 to 6
     put "Spoke" &i into tName

     startTurtle tName
     set the forecolor of grc tName to item i of colorlist
     set the linesize of grc tName to 8
     setheading i*360/6
     forward 100
     stopturtle tName

   end repeat

end mouseUP

To see some examples run this in the msg box:

go url "http://home.infostations.net/jhurley/MultipleTurtle.rev"

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And lastly, there is the flavor which is most useful in teaching 
science students to program in Transcript. It addresses only the 
turtle graphic (the cursor). It draws an image rather than a graphic. 
(A graphic slows down dramatically for lengthy draws--the graphic 
must repeatedly be redrawn with each additional graphic points since 
this is an *evolving* line.) It is based on the assumption that 
beginning students are more receptive to graphic output rather than 
text. This has the potential for quite sophisticated applications. 
After a short while students learn how to program satellite 
orbits--the satellite being the turtle (cursor).

For some examples, run this in the msg box:

go url "http://home.infostations.net/jhurley/TurtleGraphics.rev"

All three tools are RR 1.1.1 compatible.

Jim