Geometry Boost List: [ggl] sample: poisonous cloud

Subject: [ggl] sample: poisonous cloud
From: Barend Gehrels (Barend.Gehrels)
Date: 2009-04-27 15:59:27

Hi,

Maybe it's nice to give an example of what can can be done with ggl, and 
especially what's currently being added and why.

I created a small sample web-page here: 
http://www.xs4all.nl/~barend/geodan/ve_mal.htm, showing a poisonous 
cloud (in Dutch: gasmal). Those clouds are normally modelled, calculated 
and displayed in 2D but for a project we decided to add a 3D version for 
visualization. So if you go to the referred page and download the 
Microsoft Virtual Earth plugin, you'll see a 3D representation of a 
dome-shaped ellipse-like geometry.

How is this created?
- a polygon is read from a shapefile
- its longest axis is determined using the new algorithm "diameter" 
which calculates the diameter (furthest point pair and its distance) 
from a geometry (in this case a polygon). The diameter.hpp is not yet 
added to svn, will be soon. It uses brute force now (n^2) but it is 
possible to use O(n log n).
- the half of the polygon is selected (using the new 
"circular_iterator") and copied to a linestring
- another version of this "half" linestring is generated: where all 
vertices are exactly in between the vertices of the original (this is 
called the algorithm "intermediate" but maybe a more appropriate name 
can be invented)
- it is then rotated along its longest axis using a 4D matrix 
(generation of this matrix will be added as a transformation), in steps 
of 18 degrees, 10 times until it is totally rotated 180 degrees. 
Rotation is done alternating the original  and the "intermediate". All 
those rotated 3D linestrings are collected in a std::vector
- then, using two of these rotated 3D linestrings at a time (so a normal 
one and an "intermediate" one) 3D triangles are generated (--> a 3D mesh)
- those triangles are transformed from Dutch Coordinate System (epsg 
28992) to LatLong using "transform" with the new 
"project_inverse_transformer". In transformation height is copied
- detail: within this generic transformation a corresponding type must 
be generated with another point-type. So for polygon<P> --> 
polygon<LatLong>. This is the new meta-function replace_point_type
     * note that this one can be enhanced (if possible at all) by adding 
constness, and by using the original point type. The last one is 
probably hard or requires specializations per point-type...
- all triangles are streamed to "VEShapes", these are Javascript Virtual 
Earth shapes. Virtual Earth is Microsoft's Google Earth answer, a nice 
environment where 3D can be displayed in your webbrowser
- it's also possible to generate KML (of course)
- I'll create a new sample of these procedure
- part of this procedure (rotation, intermediate and 3D mesh generation) 
can be added in another algorithm, e.g. "create_mesh"
- there were already question raised: what's the volume of that cloud? NYI.

See also http://www.xs4all.nl/~barend/geodan/ve_mal_ggl.htm where more 
steps are displayed:
- the ground shape in darker green
- the 3D triangles in lighter green
- the rotated linestrings in black
- the pair of points indicating the longest axis as pushpins

Note that VE even shows the weather (also clouds). Note also that during 
weather-display the semi-transparante dome is sometimes not painted 
correct...

Regards, Barend