[MWForum]large coordinates for "setpos" command

[Zehava Wizman]

Hi Wendy!
thanks for your answer, now I understand the strange behaviour of my turtle 
when its position was the same in negative and positive coordinate  on some 
values.
                                                       thanks, Zehava


>From: "Wendy Petti" <wpetti@mathcats.com>
>Reply-To: mwforum@lists.mathcats.com
>To: <mwforum@lists.mathcats.com>
>Subject: RE: [MWForum]large coordinates  for "setpos" command
>Date: Mon, 13 Sep 2004 16:59:10 -0400
>
>Hi Zehava,
>
>Daniel offered you a procedure for calculating exactly where a turtle will
>end up (given a certain projectsize) if you supply it with coordinates that
>are larger than the limits of the project page.  But maybe we can also help
>you with a simpler solution to your question, if what you're actually 
>trying
>to figure out is the outer coordinates of the project page.  In the Command
>Center, type:
>
>show projectsize
>
>After you hit Enter/Return, you'll see two numbers below this command:  the
>width and height of the project, in pixels.  Let's suppose your project is
>480 x 420 (that's a small project but it will do for this example).  Since
>home position is [0 0] in the middle of the page, we can divide each
>dimension in half assume that there will be 240 pixels to the left and 240
>pixels to the right to make 480 pixels in width overall, and that there 
>will
>be 210 pixels above and 210 pixels below the home position.
>
>However, the [0 0] position actually occupies a pixel in width and height.
>On a project with an even number of pixels in width and height, the 0
>position for the x or y coordinates cannot actually be the exact center of
>the page.  (At least this is what I'm assuming based on some experimenting
>I've been doing just now.)
>
>So setpos [-240 0] puts the turtle at the far left edge of the project 
>page,
>but setpos [240 0] does *not* put the turtle at the far right edge of the
>project page; instead it appears at the left edge again.  setpos [239 0]
>does put the turtle at the far right edge of the project page.  If you turn
>the turtle into a line segment just one pixel in width, you will see it 
>hugs
>the right edge of the project page exactly.
>
>But when we explore the top and bottom edges of this 480 x 420 page with a
>turtle shaped as a horizontal line segment, setpos [0 210] puts the
>line-segment turtle at the top of the page and setpos [0 -210] also puts 
>the
>turtle at the top of the page.  We need
>
>So (for instance) if you wish to draw a border only one pixel thick exactly
>around the outer dimensions of a project page, using the 480 x 420 example
>you would need these commands:
>
>pu setpos [-240 210]
>pd setpos [239 210]
>setpos [239 -209]
>setpos [-240 -209]
>setpos [-240 210]
>
>Wendy
>
>
>
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