On Oct 19, 11:38 am, David Fanning <n...@dfanning.com> wrote:
> kBob writes:
>> To answer my own question ...
>
>> Yes, IDLanROI is NOT the IDL routine to perform this check.
>
>> After pondering on this for a couple of weeks, I figure what I needed
>> to do.
>
>> IDLgrROI is the prefer IDL routine to use.
>
>> It is a tessellation thing, especially when dealing with State and
>> Country Shapefiles that may contain gaps. You may get away with very
>> simple polygons using IDLanROI when dealing with a handful of
>> vertices, but IDLgrROI and IDLgrROIGroup are the routines you should
>> be using along with incorporating IDLgrTessellator in the code.
>
> Can you elaborate a bit on why this "is a tessellation thing"?
> I don't really understand what that means. And, supposing
> I did understand what it means, what would it have to do
> with choosing IDLgrROI over IDLanROI. Isn't IDLgrROI a
> subclass of IDLanROI?
>
> Cheers,
>
> David
>
> --
> David Fanning, Ph.D.
> Fanning Software Consulting, Inc.
> Coyote's Guide to IDL Programming:http://www.dfanning.com/
> Sepore ma de ni thui. ("Perhaps thou speakest truth.")
In Object Graphics, polygons that are overlapping, disjointed, or self-
intersecting are not handled very well by routines like IDLgrPolygon,
thus IDLgrTessellator is used to convert polygons that are more
suitable for IDLgrPolygon. From the IDL documentation, it converts the
concave polygon into a convext polygon. Tessellation is the process of
tiling a plane so there are no overlaps or gaps.
When I tried to use the US shapefile in IDLanROI (and IDLgrROI) there
was a line running across the United States from the Northeast to
Hawaii. Using IDLanROI, some States above this line were considered
exterior polygons. See my original post. :( After tessellating, the
line from the Northeast to Hawaii is gone, so all the States that fall
in the United States polygon are considered interior polygons. :)
Your right, IDLgrROI inherits IDLanROI, thus allowing IDLgrROI to use
IDLanROI classes, like ->ContainsPoints().
IDLgrTesselator is an IDLgr object class, thus I recommend the use of
the IDLgr classes IDLgrROI and IDLgrROIGroup. I haven’t tried, but I
may at a later date to see if just the IDLanROI and IDLanROIGroup will
give the desired results.
Check out the example below that I modified from IDL Help, it tries
to draw an intersecting polygon in Object Graphics, with and without
tessellation.
PRO TessAux, DoTess = DoTess
x = [0,1,0,1]
y = [0,0,1,1]
colors = [[0,255,0],[0,255,0],[0,64,0],[0,64,0]]
IF ( KEYWORD_SET( DoTess ) ) THEN BEGIN
oTess = OBJ_NEW('IDLgrTessellator')
colors = [[0,255,0],[0,255,0],[0,64,0],[0,64,0]]
oTess -> AddPolygon, x, y, AUXDATA=colors
result = oTess -> Tessellate(v, c, AUXDATA=aux)
oPoly = OBJ_NEW('IDLgrPolygon', v, POLYGONS=c, VERT_COLORS=aux,
SHADING=1)
ENDIF ELSE BEGIN
oPoly = OBJ_NEW('IDLgrPolygon', x, y )
ENDELSE
XOBJVIEW, oPoly, /BLOCK
IF ( OBJ_VALID( oTess ) ) THEN OBJ_DESTROY, oTess
IF ( OBJ_VALID( oPoly ) ) THEN OBJ_DESTROY, oPoly
END
;---------------------------------------------------------
Kelly Dean
Milliken, CO
;+
;
;
;-------------------------------------------------------
PRO TessAux, DoTess = DoTess
x = [0,1,0,1]
y = [0,0,1,1]
IF ( KEYWORD_SET( DoTess ) ) THEN BEGIN
oTess = OBJ_NEW('IDLgrTessellator')
colors = [[0,255,0],[0,255,0],[0,64,0],[0,64,0]]
oTess -> AddPolygon, x, y, AUXDATA=colors
result = oTess -> Tessellate(v, c, AUXDATA=aux)
oPoly = OBJ_NEW('IDLgrPolygon', v, POLYGONS=c, VERT_COLORS=aux,
SHADING=1)
ENDIF ELSE BEGIN
oPoly = OBJ_NEW('IDLgrPolygon', x, y )
ENDELSE
XOBJVIEW, oPoly, /BLOCK
IF ( OBJ_VALID( oTess ) ) THEN OBJ_DESTROY, oTess
IF ( OBJ_VALID( oPoly ) ) THEN OBJ_DESTROY, oPoly
END
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