Posts with the “script” tag

Calculating the geodesic distance between two points

I was recently tasked with recreating an existing supplier search for a client; I was provided with a database of suppliers, most of which had been geocoded, and not much else. This scenario is fairly standard when dealing with mapping applications: a user enters in a postcode and the system will return a list of the closest suppliers to that location. The postcode part of this equation is well travelled - the Post Office in the UK will not relinquish the mapping from a postcode to a latitude, longitude tuple without a large outlay of cash (and numerous non-disclosure agreements), the easiest option is to use an external service for this. I opted for PostcodeAnywhere as I had used them before with great success. The latter part of this challenge - the return of the closest database entries - was something that I wanted to try myself as I didn't known when I would get such an opportunity again.

if something is worth doing, then it's worth overdoing

To say there are many different ways of calculating the distance between two points would be an understatement. One which I had used before involved northing and easting co-ordinates from a known point within the UK (usually the centroid or London). Using this meant a smattering of trigonometry would be enough to return a decent list of matches; this always struck me as crude, despite it's usefulness, using an antiquated and subjective co-ordinate system seemed the wrong way to approach the problem. Latitude and longitude are globally recognised and provide a precise way of defining points on the globe - reading up on how they are calculated was the step one. Step two was finding an algorithm that calculated the distance between two arbitrary points. The first one I found was the Haversine formula: simple, easy to follow and easy to implement. Knowing that this formula was based upon the assumption that the Earth was perfectly spherical grated slightly with me - I reasoned there must be a more accurate algorithm. I found this precision in Vinencty's algorithm, it was then I decided to enact a contrived but deliciously fun maxim: if something is worth doing, then it's worth overdoing.

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Javascriptery: Tabbed forms

Forms are perhaps the bane of web development for me; you can't get them to look good, you can't find a foolproof way to make them act well and lets not even start of trying to get them into a pacified state, free from the dangers of user input (surprise ending: form input will never be completely trustworthy). A lot of sites would appear to have aesthetically pleasing forms, however this is a careful ruse by them as they sidestep the problem of forms by having only one or two of them, and then they usually only have a few fields. The monstrosities I am required to deal with almost daily are things of grotesque beauty, veritable Rube Goldberg machines of complexity.

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An "automatic" screenshot taker is something that I've always wanted, but the commercial offerings leave much to be desired and the only other option seems to be the "manual" approach. I am of course talking about screenshots from video files rather than screenshots of your desktop, that sort of thing is well covered.

One of the problems with making your own is that the options are fairly limited on just how you go about opening video files and pulling out the candy frame goodness. For Windows users, the option is to use DirectShow which I can only describe as The Crystal Maze for it's Byzantine ways of operating are beyond mortal ken. The other option is to use a pre-built library such as ffmpeg or similar. This was out as well as not only was it a whole new way of working for me (Windows development files were few and far between) it was a whole new set of a programming challenges which made the learning curve more of a learning cliff.

"during testing I had a number of problems with this"

So I turned forlornly to existing media-players in the slim hope that one of them would have the abilities required for scripting a makeshift screenshotter. Media Player Classic has limited command line support, VLC is more geared towards client/server setup and I couldn't even figure out whether that route would lead to any semblance of success, BSPlayer... The list goes on as to the number of players which don't supply a full body of command line options.

The silver lining, the angel of hope was MPlayer. If you're prepared to wade through a bit of fudge to get there, MPlayer provides everything you need to script a screenshotter:

  • jump to any part of the file from the command line
  • output into different (static) formats such as PNG and JPEG
  • can output file information (length, dimensions etc.)

With these three functions MPlayer is almost all you need. Almost.

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