In mid-2010 I wanted to know more about texture mapping in Google Sketchup. As simple practical example, I decided to create a sphere on which a texture map of a globe can be placed. Several of those maps are available online in the form of globe gores, which had been printed in previous centuries on papers which were physically applied onto real globes.
The first step therefore was an import plugin for Sketchup. I developed an input format (.glb) to specify longitude/latitude and texture coordinates for vertices on the sphere. The sphere is split into lots of small flat patches over which the image texture is draped patch by patch. If two patches share a corner that lies inside the texture image, you only have to declare it once, and the texture will appear to run smoothly around the point. The mesh, i.e. the definition of which vertices form which triangle or quad, is also specified in the file, so I could now just import a fully textured globe into Sketchup. An added benefit is that the areas around the poles look better than that of a sphere with the usual texture mapping, where an equirectangular map is wrapped around.
The limitation of this approach of course is the existence of a coordinate grid in the globe gores. Its resolution also usually defines the tesselation quality of the sphere, you may compare the faceted look of the Mercator globes compared to the much smoother appearence of Coronelli's 5-degree-grid globes. If you have real globe gores, you can overlay a polyconic mesh.
It is not necessary, though, to have globe gores designed to be applied onto spheres. You can put other maps onto the sphere, again, it helps to have a coordinate grid! For instance, there are Renaissance "world maps" illustrating Ptolemy's Geographia using some double-cone projection, which will display approximately as quart-spheres.
Getting the texture coordinates just by reading the pixel coordinates in an image editing application and typing into the .glb file turned out way too cumbersome, so after searching the Web and some further programming I am using a simple HTML/javascript file which provides mouseclick coordinates, which I paste into the .glb file. Entering the coordinates for a globe now takes less than two hours or so, depending on grid resolution.
Having a globe in Sketchup is fine, but what to do next? Sketchup Pro comes with an OBJ exporter, and there are also 3rd-party exporters for Sketchup Free, however, the OBJ format is simple enough to directly create it. The Sketchup plugin uses the Ruby programming language, so creating a set of standalone Ruby scripts to create and play with virtual globes seemed natural. The script, glb2obj, now creates OBJ directly.
Another idea was to re-project the texture patches onto a single map of the world. There is a Ruby extension using ImageMagick, which I am using to create equirectangular maps which could in principle be used as standard texture maps for spheres - however, patching direcly provides better results. The full 2D maps can however be compared more easily. Of course I am aware of "professional" solutions using typically proprietary and expensive GIS software, which may however provide an even better job if the scanned paper was heavily distorted and you need some arbitrary rubber-banding in between the grid points.
The final question was, how to present the globes? OBJ can be displayed in Meshlab, but many people won't even touch such a program. So, an online solution seemed best. There are some commercial packages for this, but finally I found what I needed: an OBJ object viewer for the upcoming standard for 3D graphics on the Web, WebGL! I had to make some slight modifications, but now you can enjoy my globes whereever you are. The only thing (and here, sorry, you may have to install new things!) is that you need a WebGL-capable system, consisting of, typically, post-2007 hardware, true 3D hardware support (NVidia and ATI/AMD should all work, and Intel HD2000/HD3000 as well. Intel GMA950 or the Atom/GMA3150 netbook platform are not usable, sorry!), recent drivers, and a good web browser. On Windows, recent versions of Firefox, Google Chrome or Opera are capable, on a Mac, Safari does the trick, on Linux at least Firefox should be available. Heck, it even works on my Android phone!
(c) 2011 Georg Zotti - Last changed on 2011-12-31