Re: [Opengrads-devel] Shapefile extension: first crack

[Arlindo da S. <da...@al...>]

Efren,

  I am back from vacation. I read quickly the ESRI white paper that
describes the shapefile format. What an awkward format! SVG appears a
much more elegant approach to vector graphics. In any event, here are
my thoughts on how to produce shapefiles from within GrADS:

0) I would produce shapefiles associated with 2D lat-lon "naked
plots", that is, those without maps, frames or fonts. Anything else
may not make much sense. (Since GrADS draws its own characters, fonts
could encoded as polylines but they would be deformed if a map
transformation is later applied.)

1) GrADS vector graphics includes both POLYLINES and POLYGON type of
shapes. Since one can only have one type of shape per shapefile, a
given image (say contour lines on top of shaded contours) would have 2
separate shapefiles associated with it: one with POLYGONS, another
with POLYLINE objects, for a total of 6 files if you factor in the
.shp, .shx and .dbx subfiles. This is ugly.

2) There is an annoying requirement on the enconding of polygons: one
must order the vertices so that the "inside" is to the right of the
path, something that should have been handled by the renderer.   I
believe cairo provides functions to determine whether a point is
inside or outside a closed path, so I may be able to use it.

3) Each polygon object would have a single part so as to avoid dealing
with similar vertex ordering issues.

4) Now, each POLYLINE or POLYGON object will have a color associated
with it. I am hoping one could save the (r,g,b) values in the
associated .dbx file, but I am not sure what are the conventions for
encoding this.

5) In the case of shaded contours, I am not sure how to encode the
color bar (mapping of (r,g,b) into field values).

   With all these caveats, would GrADS produced shapefiles still be of
any interest? I believe Brian has looked into this as well, I'm cc'ing
him as he may not be subscribed to this list.

Brian: for reference, here is the thread:

http://sourceforge.net/mailarchive/forum.php?thread_name=77fcd6b20711251803j746a965j689063db5d6e4da8%40mail.gmail.com&forum_name=opengrads-devel

   Arlindo

-- 
Arlindo da Silva
da...@al...