Velocity Reviews > Ruby > [QUIZ] Turtle Graphics (#104)

# [QUIZ] Turtle Graphics (#104)

Ruby Quiz
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 12-01-2006
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-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

by Morton Goldberg

http://rubyquiz.com/turtle.zip

--JEG2]

Turtle Graphics
===============

Turtle graphics is a form of computer graphics based on the ideas of turtle
geometry, a formulation of local (coordinate-free) geometry. As a brief
introduction to turtle graphics, I quote from [1]:

Imagine that you have control of a little creature called a turtle
that exists in a mathematical plane or, better yet, on a computer
display screen. The turtle can respond to a few simple commands:
FORWARD moves the turtle in the direction it is facing some
number of units. RIGHT rotates it clockwise in its place some
number of degrees. BACK and LEFT cause the opposite movements. ...
The turtle can leave a trace of the places it has been: [its
movements] can cause lines to appear on the screen. This is
controlled by the commands PENUP and PENDOWN. When the pen is
down, the turtle draws lines.

For example, the turtle commands to draw a square, 100 units on a side, can be
written (in a Ruby-ized form) as:

pen_down
4.times { forward 100; right 90 }

This quiz is a bit different from most. If the usual Ruby quiz can be likened to
an essay exam, this one is a fill-in-the-blanks test. I'm supplying you with a
complete turtle graphics package, except -- to give you something to do -- I've
removed the method bodies from the key file, lib/turtle.rb. Your job is to
repair the damage I've done and make the package work again.

Turtle Commands
===============

There are quite a few turtle commands, but that doesn't mean you have to write a
lot of code to solve this quiz. Most of the commands can be implemented in a
couple of lines. It took me a lot longer to write a description of the commands
than it did for me to implement and test all of them.

I use the following format to describe turtle commands:

long_name | short_name <arg>
description ...
Example: ...

All turtle commands take either one argument or none, and not all turtle
commands have both a long name and a short name.

Required Commands
-----------------

These commands are required in the sense that they are needed to reproduce the
sample designs. Actually, you could get away without implementing 'back' and
'left', but implementing them is far easier than trying to write turtle code
without them.

pen_up | pu
Raises the turtle's pen. The turtle doesn't draw (lay down a visible
track) when its pen is up.

pen_down | pd
Lowers the turtle's pen. The turtle draws (lays down a visible track)
when its pen is down.

forward | fd <distance>
Moves the turtle forwards in the direction it is facing.
Example: forward(100) advances the turtle by 100 steps.

back | bk <distance>
Moves the turtle backwards along its line of motion.
back <distance> == forward -<distance>
Example: back(100) backs up the turtle by 100 steps.

right | rt <angle>
Turns the turtle clockwise by <angle> degrees.
Example: right(90) turns the turtle clockwise by a right angle.

left | lt <angle>
Turns the turtle counterclockwise by <angle> degrees.
left <angle> == right -<angle>
Example: left(45) turns the turtle counterclockwise by 45 degrees.

--------------------

These commands are not needed to reproduce any of the sample designs, but they
are found in all implementations of turtle graphics that I know of.

home
Places the turtle at the origin, facing north, with its pen up. The
turtle does not draw when it goes home.

clear
Homes the turtle and empties out it's track. Sending a turtle a clear
message essentially reinitializes it.

xy
Reports the turtle's location.
Example: Suppose the turtle is 10 turtle steps north and 15 turtle steps
west of the origin, then xy will return [-15.0, 10.0].

set_xy | xy= <point>
Places the turtle at <point>. The turtle does not draw when this command
is executed, not even if its pen is down. Returns <point>.
Example: Suppose the turtle is at [10.0, 20.0], then self.xy = [50, 80]
moves the turtle to [50.0, 80.0], but no line will drawn between the [10,
20] and [50, 80].

Reports the direction in which the turtle is facing. Heading is measured
in degrees, clockwise from north.
Example: Suppose the turtle is at the origin facing the point [100, 200],
then heading will return 26.565 (approximately).

Sets the turtle's heading to <angle>. <angle> should be given in degrees,
measured clockwise from north. Returns <angle>.
Example: After self.heading = 135 (or set_h(135) which is easier to
write), the turtle will be facing southeast.

pen_up? | pu?
Reports true if the turtle's pen is up and false otherwise.

pen_down? | pd?
Reports true if the turtle's pen is down and false otherwise.

Optional Commands
-----------------

These commands are only found in some implementations of turtle graphics. When
they are implemented, they make the turtle capable of doing global (coordinate)
geometry in addition to local (coordinate-free) geometry.

I used one of these commands, go, to draw the mandala design (see
designs/mandala.tiff and samples/mandala.rb). If you choose not to implement the
optional commands, you might try writing a turtle program for drawing the
mandala design without using go. But, believe me, it is much easier to implement
go than to write such a program.

go <point>
Moves the turtle to <point>.
Example: Suppose the turtle is home (at the origin facing north). After
go([100, 200]), the turtle will be located at [100.0, 200.0] but will
still be facing north. If its pen was down, it will have drawn a line
from [0, 0] to [100, 200].

toward | face <point>
Turns the turtle to face <point>.
Example: Suppose the turtle is at the origin. After toward([100, 200]),
its heading will be 26.565 (approximately).

distance | dist <point>
Reports the distance between the turtle and <point>.
Example: Suppose the turtle is at the origin, then distance([400, 300])
will return 500.0 (approximately).

Interfacing to the Turtle Graphics Viewer
=========================================

Implementing turtle graphics without being able to view what the turtle draws
isn't much fun, so I'm providing a simple turtle graphics viewer. To interface
with the viewer, turtle instances must respond to the message track by returning
an array which the viewer can use to generate a line drawing.

The viewer expects the array returned by track to take the following form:

track ::= [segment, segment, ...] # drawing data
segment ::= [point, point, ...] # points to be joined by line segments
point ::= [x, y] # pair of floats

Example: [[[0.0, 0.0], [200.0, 200.0]], [[200.0, 0.0], [0.0, 200.0]]]

This represents an X located in the upper-right quadrant of the viewer; i.e.,
two line segments, one running from the center of the viewer up to its
upper-right corner and the other running from the center of the top edge down to
the center of the right edge.

[Editor's Note: I added a script to dump your turtle graphics output to PPM
image files, for those that don't have TK up and running. It works identically
to Morton's turtle_viewer.rb, save that it writes output to a PPM image file in
the current directory. For example, to output the included tree image, use
`ruby turtle_ppm_writer.rb samples/tree.rb`. --JEG2]

Unit Tests
==========

I'm including the unit tests which I developed to test turtle commands. For the
purposes of the quiz, you can ignore tests/turtle_view_test.rb. But I hope you
will find the other test suite, tests/turtle_test.rb, helpful. It tests every
one of the turtle commands described above as well as argument checking by the
commands. Don't hesitate to modify any of the unit tests to meet the needs of

References
==========

[1] Abelson, H. & A. diSessa, "Turtle Geometry", MIT Press, 1981.
[2] Harvey, B., "Computer Science Logo Style", Chapter 10.
http://www.cs.berkeley.edu/~bh/pdf/v1ch10.pdf
[3] Wikipedia, http://en.wikipedia.org/wiki/LOGO_programming_language

Harold Hausman
Guest
Posts: n/a

 12-01-2006
On 12/1/06, Ruby Quiz <(E-Mail Removed)> wrote:
>
> by Morton Goldberg
>
>
> Turtle Graphics
> ===============
>

3 cheers, great quiz!

Ironically, I very recently implemented a turtle graphics system using
GTK and Ruby.

Here are some images that came from it to whet your appetite:
http://www.danceliquid.com/images/LS/

It used cairo for the anti-aliased line drawing.

Whee!
-Harold

James Edward Gray II
Guest
Posts: n/a

 12-01-2006
On Dec 1, 2006, at 9:42 AM, Harold Hausman wrote:

> It used cairo for the anti-aliased line drawing.

I tried to use the new pure Ruby PNG library to get anti-aliased line
drawing in the quiz files, but that sucker is a bit broken.

James Edward Gray II

Kurt Hindenburg
Guest
Posts: n/a

 12-01-2006
On 12/1/06, Ruby Quiz <(E-Mail Removed)> wrote:
> [Editor's Note: I added a script to dump your turtle graphics output to PPM
> image files, for those that don't have TK up and running. It works identically
> to Morton's turtle_viewer.rb, save that it writes output to a PPM image file in
> the current directory. For example, to output the included tree image, use
> `ruby turtle_ppm_writer.rb samples/tree.rb`. --JEG2]
>

Hello,
Does the above work for everyone?

(~/turtle_graphics) > ruby turtle_ppm_writer.rb samples/tree.rb
turtle_ppm_writer.rb:47:in `run_code': undefined method `each' for
nil:NilClass (NoMethodError)
from turtle_ppm_writer.rb:27:in `initialize'
from turtle_ppm_writer.rb:75:in `new'
from turtle_ppm_writer.rb:75

ruby 1.8.5 (2006-08-25) [powerpc-darwin8]

Kurt

James Edward Gray II
Guest
Posts: n/a

 12-01-2006
On Dec 1, 2006, at 10:50 AM, Kurt Hindenburg wrote:

> On 12/1/06, Ruby Quiz <(E-Mail Removed)> wrote:
>> [Editor's Note: I added a script to dump your turtle graphics
>> output to PPM
>> image files, for those that don't have TK up and running. It
>> works identically
>> to Morton's turtle_viewer.rb, save that it writes output to a PPM
>> image file in
>> the current directory. For example, to output the included tree
>> image, use
>> `ruby turtle_ppm_writer.rb samples/tree.rb`. --JEG2]
>>

> Hello,
> Does the above work for everyone?
>
> (~/turtle_graphics) > ruby turtle_ppm_writer.rb samples/tree.rb
> turtle_ppm_writer.rb:47:in `run_code': undefined method `each' for
> nil:NilClass (NoMethodError)
> from turtle_ppm_writer.rb:27:in `initialize'
> from turtle_ppm_writer.rb:75:in `new'
> from turtle_ppm_writer.rb:75
>
> ruby 1.8.5 (2006-08-25) [powerpc-darwin8]

Making it work is the quiz. You need to finish the turtle.rb file so
a track() is returned to draw.

James Edward Gray II

Harold Hausman
Guest
Posts: n/a

 12-01-2006
On 12/2/06, James Edward Gray II <(E-Mail Removed)> wrote:
> On Dec 1, 2006, at 9:42 AM, Harold Hausman wrote:
>
> > It used cairo for the anti-aliased line drawing.

>
> I tried to use the new pure Ruby PNG library to get anti-aliased line
> drawing in the quiz files, but that sucker is a bit broken.
>
> James Edward Gray II
>
>

Ha! I did the exact same thing the day they announced that PNG lib.
There is some kind of bug in their implementation of that tricksy line
drawing algorithm they're using. I spent about 20 minutes trying to
unravel it and then quit.

I found it kind of ironic that they chose some super speedy line
drawing routine meant to save cycles when writing directly to video
ram in mode 13h from the early ninety's. Sitting right next to a pure
ruby implementation of png, compression and all, the contrast was
stark.

(:,
-Harold

Daniel Martin
Guest
Posts: n/a

 12-01-2006
"Harold Hausman" <(E-Mail Removed)> writes:

> Ha! I did the exact same thing the day they announced that PNG lib.
> There is some kind of bug in their implementation of that tricksy line
> drawing algorithm they're using. I spent about 20 minutes trying to
> unravel it and then quit.
>
> I found it kind of ironic that they chose some super speedy line
> drawing routine meant to save cycles when writing directly to video
> ram in mode 13h from the early ninety's. Sitting right next to a pure
> ruby implementation of png, compression and all, the contrast was
> stark.

To be fair, they don't do the compression in pure ruby - they call
Zlib, so the really computationally intensive bit isn't in ruby.

Also, the fast line-drawing algorithm used for writing to that old
320x200 VGA mode wasn't what they're using here. Here, they're doing
*anti-aliased* lines, which no one would ever think of trying when you
only have 256 colors available and the pixels are going to be visibly
squares no matter what you do.

That old line drawing algorithm I could just pull out of a book on my
shelf; what they're doing here is enough different that it's very
tough to disentangle.

--
s=%q( Daniel Martin -- http://www.velocityreviews.com/forums/(E-Mail Removed)
puts "s=%q(#{s})",s.map{|i|i}[1] )
puts "s=%q(#{s})",s.map{|i|i}[1]

James Edward Gray II
Guest
Posts: n/a

 12-01-2006
On Dec 1, 2006, at 3:44 PM, Daniel Martin wrote:

> That old line drawing algorithm I could just pull out of a book on my
> shelf; what they're doing here is enough different that it's very
> tough to disentangle.

Unfortunately, that kept me from trying very hard to fix it. I also
worried they weren't too interested in the library since the example

I also had issues where I would draw lines in bounds, but it would
access pixels outside the image area in anti-aliasing which caused
exceptions to be tossed.

James Edward Gray II

Edwin Fine
Guest
Posts: n/a

 12-02-2006
/usr/local/lib/ruby/site_ruby/1.8/tk.rb:1187: warning: instance variable
@encoding not initialized

Is it something I am doing wrong, or some other problem? I don't know

--
Posted via http://www.ruby-forum.com/.

Harold Hausman
Guest
Posts: n/a

 12-02-2006
On 12/2/06, Daniel Martin <(E-Mail Removed)> wrote:
> To be fair, they don't do the compression in pure ruby - they call
> Zlib, so the really computationally intensive bit isn't in ruby.
>

Thanks for the correction here.

> Also, the fast line-drawing algorithm used for writing to that old
> 320x200 VGA mode wasn't what they're using here. Here, they're doing
> *anti-aliased* lines, which no one would ever think of trying when you
> only have 256 colors available and the pixels are going to be visibly
> squares no matter what you do.
>

People were most definitely drawing anti-aliased lines in mode 13h:
http://freespace.virgin.net/hugo.eli...s/x_wuline.htm

Aesthetically sensible when, as you say, each pixel is a visible rectangle? no.

Still pretty fun though. hehe.

> That old line drawing algorithm I could just pull out of a book on my
> shelf; what they're doing here is enough different that it's very
> tough to disentangle.
>

It's a bit of a shame too, this lib would otherwise be *perfect* for
doing cross-platform turtle graphics in Ruby.

Regards,
-Harold