Wednesday, October 10, 2012

WebGL - dynamic global temperature maps




This is an advance on HTML 5 that I've been using for some previous dynamic temperature plots. Some Web browsers now support WebGL, a Javascript version of openGL for 3D plotting. I've been playing with it for monthly temperature maps - here's September.

What you'll notice is a globe that responds to mouse dragging just like Google Earth - in fact I believe GE, at least initially, used a version of WebGL. The main advantage for presentation is better shading - Gouraud shading rather than the rather kludgy HTML5 canvas version.

A downside is that support for WebGL is quite patchy, and implementation depends also on your graphics card. I've found Chrome is fine; Firefox produces a fragment of picture and then gives an error message, and of course IE is way behind. Some other browsers have the capability but disable it by default. I understand the reason is that it creates a vulnerability to DOS attacks which send dynamic but very slow pictures.

I don't think WebGL will replace my HTML 5 versions for a while. I don't have as much control, so I can't, for example, allow you to click on the picture to bring up local station info.

The other downside is that the files are fairly large, and take a few seconds to download. So I've put them below the jump. There is the WebGL version and a snapshot of the HTML5 version. The color scheme is the same - I haven't figured out yet how to put a bar on the WebGL version. It's a direct plot from the TempLS September station anomalies - exact for each station, and shaded elsewhere.

I've added a technical update describing the methods I used here.




So here is the globe. Give it a spin! The left button rotates, the middle button/wheel enlarges, and the right button changes field of view (which can also enlarge).



Here is a snapshot from the HTML 5 version. You can see that the shading is more ragged.




Technical update


I should say more about how this is done. I use my R program which does the interactive monthly presentations, in conjunction with an excellent R package, rgl, in which Duncan Murdoch has a big role. This enables me to show in an R GUI the spinnable globe as you see it (browser permitting).

Then there is an rgl routine writeWebGF, which generates an HTML program with Javascript. I could use that directly, but it is bulky (about 2.5 Mb, mostly data). The JS is beautifully written, but the data layout is extravagant. I've been able to cut it down to about 500 Kb, with some added Javascript. I show that via an iframe, because I need to run the program when loading is complete. Normally that would be a flag in the body element, but Blogger controls that, hence the iframe.

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