Hello, everyone! I've been programming in IDL for a little under a
year now, and I am currently building a program part of which involves
graphically selecting a 4-point polygon on a sphere (each side of
which is a geodesic).
For that, I record the user's clicks and moves on the screen and store
the x- and y- coordinates of the points they selected in a 2x4 array.
The array is then processed by a function that transforms it into x-
and y- coordinates of a spherical polygon. To do so, it first converts
the coordinates to spherical using cv_coord. Then it uses the library
function map_2points to find the longitude-latitude path arrays
between the 4 point pairs. Then it combines all the longitudes and
latitudes into one array, and cv_coord's them back into cartesian
coordinates. The points are then used as data for an IDLgrPolyline
object.

The function works, but the resulting polygon looks extremely choppy,
making it practically impossible to do any precision work (which is
necessary). So my question is - how do I draw a geodesic curve without
using three precision-degrading processes (cv_coord, map_2points, and
cv_coord again) in a row?

Thank you,
~Aram Panasenco

P.S. I can post the function that renders the polygon online if
necessary.

In article 
<018e5283-b8e4-446a-9ba1-35625dcf678c@g11g2000yqe.googlegroups.com>,
 Aram Panasenco <panasencoaram@gmail.com> wrote:

> Hello, everyone! I've been programming in IDL for a little under a
> year now, and I am currently building a program part of which involves
> graphically selecting a 4-point polygon on a sphere (each side of
> which is a geodesic).
> For that, I record the user's clicks and moves on the screen and store
> the x- and y- coordinates of the points they selected in a 2x4 array.
> The array is then processed by a function that transforms it into x-
> and y- coordinates of a spherical polygon. To do so, it first converts
> the coordinates to spherical using cv_coord. Then it uses the library
> function map_2points to find the longitude-latitude path arrays
> between the 4 point pairs. Then it combines all the longitudes and
> latitudes into one array, and cv_coord's them back into cartesian
> coordinates. The points are then used as data for an IDLgrPolyline
> object.
> 
> The function works, but the resulting polygon looks extremely choppy,
> making it practically impossible to do any precision work (which is
> necessary). So my question is - how do I draw a geodesic curve without
> using three precision-degrading processes (cv_coord, map_2points, and
> cv_coord again) in a row?
> 
> Thank you,
> ~Aram Panasenco
> 
> P.S. I can post the function that renders the polygon online if
> necessary.

How far apart are your points?  Precision should not be a problem
unless they are very close together.  We use the same basic 
approach all the time to draw great circles on maps (using
widget events and CONVERT_COORD).

How many points are you using to create each side of the polygon?

Ken Bowman

On Apr 4, 7:03=A0am, "Kenneth P. Bowman" <k-bow...@null.edu> wrote:
> In article
> <018e5283-b8e4-446a-9ba1-35625dcf6...@g11g2000yqe.googlegroups.com>,
> =A0Aram Panasenco <panasencoa...@gmail.com> wrote:
>
>
>
>
>
> > Hello, everyone! I've been programming in IDL for a little under a
> > year now, and I am currently building a program part of which involves
> > graphically selecting a 4-point polygon on a sphere (each side of
> > which is a geodesic).
> > For that, I record the user's clicks and moves on the screen and store
> > the x- and y- coordinates of the points they selected in a 2x4 array.
> > The array is then processed by a function that transforms it into x-
> > and y- coordinates of a spherical polygon. To do so, it first converts
> > the coordinates to spherical using cv_coord. Then it uses the library
> > function map_2points to find the longitude-latitude path arrays
> > between the 4 point pairs. Then it combines all the longitudes and
> > latitudes into one array, and cv_coord's them back into cartesian
> > coordinates. The points are then used as data for an IDLgrPolyline
> > object.
>
> > The function works, but the resulting polygon looks extremely choppy,
> > making it practically impossible to do any precision work (which is
> > necessary). So my question is - how do I draw a geodesic curve without
> > using three precision-degrading processes (cv_coord, map_2points, and
> > cv_coord again) in a row?
>
> > Thank you,
> > ~Aram Panasenco
>
> > P.S. I can post the function that renders the polygon online if
> > necessary.
>
> How far apart are your points? =A0Precision should not be a problem
> unless they are very close together. =A0We use the same basic
> approach all the time to draw great circles on maps (using
> widget events and CONVERT_COORD).
>
> How many points are you using to create each side of the polygon?
>
> Ken Bowman

Ah, that's the problem! I used points 2 degrees apart (dpath=3D2). Now I
changed dpath to 10, and it renders perfectly. I think I tried that
approach briefly before and didn't like it because it made my
polygon's corners sloppy, but I can easily insert the corner points
manually.

Thank you very much!

~Aram Panasenco

In article 
<f8831fc9-6365-488e-bc28-b01ab05024f9@k13g2000yqe.googlegroups.com>,
 Aram Panasenco <panasencoaram@gmail.com> wrote:

> > How many points are you using to create each side of the polygon?
> >
> > Ken Bowman
> 
> Ah, that's the problem! I used points 2 degrees apart (dpath=2). Now I
> changed dpath to 10, and it renders perfectly. I think I tried that
> approach briefly before and didn't like it because it made my
> polygon's corners sloppy, but I can easily insert the corner points
> manually.
> 
> Thank you very much!
> 
> ~Aram Panasenco

I do like problems that are easy to solve.  :-)

Ken