As a first step, I wanted to implement the country selection and highlighting. I tried reading through the source code, but eventually got lost in the morass of details. Fortunately, I stumbled upon a blog post written by Michael Chang about creating this visualization; read it at http://mflux.tumblr.com/post/28367579774/armstradeviz. There are many great ideas discussed here, in particular about encoding information in images. Summarizing briefly: to determine the country that is clicked, there is a grayscale image in the background, with each country colored a unique shade of gray. Determine the gray value (0-255) of the pixel that was clicked and the corresponding country can be identified by a table lookup. (This technique is also mentioned at well-formed-data.net.) The highlighting feature is similarly cleverly implemented: a second image, stored as a canvas element with dimensions 256 pixels by 1 pixel, contains the colors that will correspond to each country. The country corresponding to the value X in the previously mentioned table lookup will be rendered with the color of the pixel on the canvas at position (X,0). A shader is used to make this substitution and color each country appropriately. When a mouse click is detected, the canvas image is recolored as necessary; if the country corresponding to value X is clicked, the canvas pixel at (X,0) is changed to the highlight color, all other pixels are changed to the default color (black).
For aesthetic reasons, I also decided to blend in a satellite image of Earth. Doing this with shaders is straightforward: one includes an additional texture and then mixes the color (vec4) values in the desired ratio in the fragment shader. However, the grayscale image that Chang uses is slightly offset and somehow differently projected from standard Earth images, which typically place the longitude of 180 degrees aligned with the left border of the image. So I did some image editing, using the satellite image as the base layer and Chang's grayscale image as a secondary translucent layer, then I attempted to cut and paste and move and redraw parts of the grayscale image so that they lined up. The results are far from pixel perfect, but adequate for my purposes. (If anyone reading this knows of a source for a more exact/professional such image, could you drop me a line? Either stemkoski@gmail or @ProfStemkoski on Twitter.) Then I used an edge detection filter to produce an image consisting of outlined countries, and also blended that into the final result in the fragment shader code. The result:
The code discussed above is available online at http://stemkoski.github.io/Three.js/Country-Select-Highlight.html.
More posts to follow on this project...