Mouse controls (was: Re: Survex 1.1.3 test version uploaded)

[John Halleck]

On Tue, 13 Sep 2005, Andy Waddington wrote:

> Date: Tue, 13 Sep 2005 18:00:06 +0100
> From: Andy Waddington <surveys at pennine.demon.co.uk>
> To: survex at survex.com
> Subject: Re: Mouse controls (was: Re: Survex 1.1.3 test version uploaded)
> 
> On Tuesday 2005-09-13 14:44, David Loeffler wrote:
> 
> > That's mine - it's the one that calculates the length, depth, N-S and
> > E-W ranges, and horizontal extent of a cave given a .3d file.
> 
> Given that one doesn't often draw a cave survey with N straight up the page,
> but rotates it to make the best use of the paper, it would be useful for it
> to print the top-bottom and left-right extent of the cave given, as input,
> some arbitrary bearing up the page.
> 
> *Much* harder, would be to tell me the best orientation to draw the cave
> given paper of some given aspect ratio, or to tell me the bearing that
> minimises the width. This is not only harder, but also rather less useful,
> since you can probably get this by rotating the survey in one of the
> viewers, to get a bearing to feed into the program with the extension
> suggested above.

  It can, however, be done mechanically quite easily.
  For the Geeks among you, here is the algorithm...
  Anyone else is free to skip it.

  Assume you have the "convex hull" of the data points.
    (Lots of algorithms around to do this efficiently.)
  For a "typical" several thousand point survey the
  convex hull has on the order of 20 points.
  
  There is a theorm (due I think To Dr. Frank Ford Little)
  that says that:
    1) The minimum width enclosing rectangle lines up
       with one edge of the convex hull.
    2) The minimum area rectangle lines up with one
       edge of the convex hull.

  So, the program can just try the 20 or so orientations,
  rotating the JUST the points 20 or so points on the hull
  and computing the width and aspect ratio for each, and
  select the best.


> 
> Andy
> 
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>