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Zooming Rectangles

Date: 05/11/2005 at 17:58:43
From: Cassi
Subject: Zooming rectangle

What is the math behind zooming rectangles?  You see them all the time
in computer programs, and even on calculators.  For example, say you
have a graph.  You can create a zooming rectangle which will "zoom in"
on whatever is in the rectangle.  But all it's essentially doing is
increasing the relative size and changing the position of the objects
inside the rectangle.

I'm trying to write a program that mimics a zooming rectangle (i.e.,
it zooms in on a line using math instead of some built-in function),
but it's not working the way I think it should.  I'm having problems
because objects close to the center don't enlarge/move much, while
objects close to the edge do.


Date: 05/12/2005 at 11:20:07
From: Doctor Ian
Subject: Re: Zooming rectangle

Hi Cassi,

Whenever you're drawing anything on a computer screen, there are two
coordinate systems involved.  One is the set of coordinate axes where
you're representing the thing to be drawn--a line, or a curve, or an
ellipse, or whatever.  The other is the set of coordinate axes on the
display device.  

To draw something, you have to construct a mapping function that takes
any point in the first coordinate system, and maps it into the second
coordinate system.  Note that for _most_ points in the first system,
the corresponding point in the second system will be outside the
drawing area, i.e., it won't show up.  

To construct the function, you need to know which point (a,b) in the
first system corresponds to the lower-left corner (0,0) in the second
system; and which point (c,d) in the first system corresponds to the
upper-right corner (x_max,y_max) of the second system.  

Now, let's say you have a point (x,y) that is somewhere in the
rectangle defined by (a,b) and (c,d).  The scale factor is whatever
takes (c-a,d-b) to (x_max,y_max), i.e., 

        x_max            y_max
  k_x = -----      k_y = -----
        c - a            d - b

Now you can take any point (x,y) in the first system, and the place
where it should show up is

  (x,y) -> ( (x-a) * k_x,  (y-b)*k_y )

Convince yourself that if (x,y) is at (a,b), it will map to (0,0); and
if (x,y) is at (c,d) it will map to (x_max,y_max).  

Does this help? 

- Doctor Ian, The Math Forum
  http://mathforum.org/dr.math/ 


Date: 05/12/2005 at 15:14:22
From: Cassi
Subject: Thank you (Zooming rectangle)

Thank you SO MUCH!!!  Your answer was very clear and I finally 
understood what was going on.  I was able to shorten my code from 52 
lines down to 6.  I had been working on this problem for far too long.  
I got a response extremely fast.
Associated Topics:
High School Euclidean/Plane Geometry

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