How to get data only inside polygon created by points which is part of whole data from file? Post: 302411426

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Top Forums UNIX for Dummies Questions & Answers How to get data only inside polygon created by points which is part of whole data from file? Post 302411426 by Corona688 on Thursday 8th of April 2010 12:45:38 PM

04-08-2010

Registered User

I don't have a simple program for you. It's not even a simple problem. Furthermore just considering this a polygon may not be strictly correct since we're concerned with points on a sphere not a plane; if you just consider latitude as "Y" and longitude as "X" your polygon would need curved lines. You'd have to map it into mercator projection or something to get a correct representation. But if a naive solution will do, you could do it in a similar way to how they draw polygons. Please excuse my awful ASCII art:

Code:

        /\      /--\
......./**\..../****\....
      /    \  /      \
      |     \/       /
      |             /
      \            /
       \          /
        \---------

So you loop through your list of edges left to right, stopping at the segment following it, starting again at the next segment, stopping at the next, etc, etc until you run out of edges, checking if your data point exists between any of the spans you'd be "drawing" in. Note that the order of segments left to right can change depending on the latitude of the data point in question, you have to keep re-sorting the list!

To speed this up you could first check the data points against a bounding box of [min, min]-[max,max] which your polygon fits inside. If it's outside that box you can skip it without the bother of re-sorting your list of edges and trawling them one by one.

Last edited by Corona688; 04-08-2010 at 01:52 PM..

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glpolygonmode

GLPOLYGONMODE(3G)						  [FIXME: manual]						 GLPOLYGONMODE(3G)

NAME

       glPolygonMode - select a polygon rasterization mode

C SPECIFICATION

       void glPolygonMode(GLenum face, GLenum mode);

PARAMETERS

       face
	   Specifies the polygons that mode applies to. Must be GL_FRONT_AND_BACK for front- and back-facing polygons.

       mode
	   Specifies how polygons will be rasterized. Accepted values are GL_POINT, GL_LINE, and GL_FILL. The initial value is GL_FILL for both
	   front- and back-facing polygons.

DESCRIPTION

       glPolygonMode controls the interpretation of polygons for rasterization.  face describes which polygons mode applies to: both front and
       back-facing polygons (GL_FRONT_AND_BACK). The polygon mode affects only the final rasterization of polygons. In particular, a polygon's
       vertices are lit and the polygon is clipped and possibly culled before these modes are applied.

       Three modes are defined and can be specified in mode:

       GL_POINT
	   Polygon vertices that are marked as the start of a boundary edge are drawn as points. Point attributes such as GL_POINT_SIZE and
	   GL_POINT_SMOOTH control the rasterization of the points. Polygon rasterization attributes other than GL_POLYGON_MODE have no effect.

       GL_LINE
	   Boundary edges of the polygon are drawn as line segments. Line attributes such as GL_LINE_WIDTH and GL_LINE_SMOOTH control the
	   rasterization of the lines. Polygon rasterization attributes other than GL_POLYGON_MODE have no effect.

       GL_FILL
	   The interior of the polygon is filled. Polygon attributes such as GL_POLYGON_SMOOTH control the rasterization of the polygon.

EXAMPLES

       To draw a surface with outlined polygons, call

	   glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);

NOTES

       Vertices are marked as boundary or nonboundary with an edge flag. Edge flags are generated internally by the GL when it decomposes triangle
       stips and fans.

ERRORS

       GL_INVALID_ENUM is generated if either face or mode is not an accepted value.

ASSOCIATED GETS

       glGet() with argument GL_POLYGON_MODE

SEE ALSO

       glLineWidth(), glPointSize()

COPYRIGHT

       Copyright (C) 1991-2006 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see
       http://oss.sgi.com/projects/FreeB/.

[FIXME: source] 						    05/30/2012							 GLPOLYGONMODE(3G)