Turtles

DrScheme supports a turtle graphics interface. These are the turtle functions:

• (turtles b) shows and hides the turtles window based on the boolean b. The parameter b is optional; if it is left out, it toggles the state of the turtles.
• (move n) moves the turtle n pixels.
• (draw n) moves the turtle n pixels and draws a line on that path.
• (erase n) moves the turtle n pixels and erases along that path.

• (move-offset h v), (draw-offset h v), (erase-offset h v) are just like move, draw and erase, except they take a horizontal and vertical offset from the turtle's current position.
• (turn theta) turns the turtle theta degrees counter-clockwise.
• (turn/radians theta) turns the turtle theta radians counter-clockwise.
• (clear) erases the turtles window.
• (save-turtle-bitmap filename filetype) saves the window to a bitmap file. The supported file types are:
  • wx:const-bitmap-type-gif -- save a GIF bitmap file (X Windows, Windows, MacOS)
  • wx:const-bitmap-type-xbm -- save an X bitmap file (X Windows, Windows, MacOS)
  • wx:const-bitmap-type-xpm -- save an XPM bitmap file (X Windows, Windows, MacOS)
  • wx:const-bitmap-type-bmp -- save a Windows bitmap file (X Windows, Windows, MacOS)
  • wx:const-bitmap-type-bmp-resource -- save a Windows bitmap from the resource database (Windows)
  • wx:const-bitmap-type-pict -- save a PICT bitmap file (MacOS)
  • wx:const-bitmap-type-pict-resource -- save a PICT resource (MacOS)

Turtles also adds these syntactic forms:

• (split E) spawns a new turtle where the turtle is currently located. In order to distinguish the two turtles, only the new one evaluates the expression E. For example, if you start with a fresh turtle-window and type:

  (split (turn/radians (/ pi 2)))

  you will have two turtles, pointing at right angles to each other. To see that, try this:

  (draw 100)

  You will see two lines. Now, if you evaluate those two expression again, you will have four turtles, etc

• (split* E...) The form split* is similar to split, except it creates as many turtles as there are expressions and each turtles does one of the expression. For example, to create two turtles, one pointing at and one at , evaluate this:

  (split* (turn/radians (/ pi 3)) (turn/radians (/ pi 2)))

• (tprompt E...)

  tprompt provides a way to limit the splitting of the turtles. Before the expression E is run, the state of the turtles (how many, their positions and headings) is "checkpointed," then E is evaluated and the state of the turtles is restored, but all drawing that may have occured during execution of E remains.

  For example, if you do this:

  (tprompt (draw 100))

  the turtle will move forward 100 pixels, draw a line there and then be immediately put back in it's original position. Also, if you do this:

  (tprompt (split (turn/radians (/ pi 2))))

  the turtle will split into two turtles, one will turn 90 degrees and then the turtles will be put back into their original state - as if the split never took place.

  The fern functions below demonstrate more advanced use of tprompt.

In the file ``turex.ss'' in the ``graphics'' library of your PLT distribution, you will find these functions and values defined, as example turtle programs. (The file is located in the ``graphics'' subdirectory of the ``collects'' subdirectory of the ``mred'' subdirectory of the PLT distribution).

• (regular-poly sides radius) draws a regular poly centered at the turtle with sides sides and with radius radius.
• (regular-polys sides s) draws s regular polys spaced evenly outwards with sides sides.
• (radial-turtles n) places turtles spaced evenly pointing radially outward
• (spaced-turtles n) places turtles pointing in the same direction as the original turtle evenly spaced in a line.
• (spokes) draws some spokes, using raidial-turtles and spaced-turtles
• (spyro-gyra) draws a spyro-grya reminicent shape
• (neato) as the name says...
• (grapics-bexam) draws a fractal that came up on an exam I took.
• serp-size a constant which is a good size for the serp procedures

• (serp serp-size), (serp-nosplit serp-size) draws the Serpinski triangle in two different ways, the first using split heavily. After running the first one, try executing (draw 10).
• koch-size a constant which is a good size for the koch procedures

• (koch-split koch-size),(koch-draw koch-size) draws the same koch snowflake in two different ways.

• (lorenz a b c) watch the lorenz "butterfly" attractor with inital values a b and c.
• (lorenz1) a good setting for the lorenz attractor

• (peano1 peano-size)

  This will draw the Peano space-filling curve, using split.

• (peano2 peano-size)

  This will draw the Peano space-filling curve, without using split.

• fern-size a good size for the fern functions
• (fern1 fern-size) You will probably want to point the turtle up before running this one, with something like:

  (turn/radians (- (/ pi 2)))

• (fern2 fern-size) a fern - you may need to backup a little for this one.