This article explains how a small, Utah-based company used asynchronous Javascript and XML (AJAX) to add advanced features to the user interfaces in its Linux-based cameras.
Tutorial Details:
Several years before I started working on network cameras, I was already sure that web interfaces are ideal for embedded devices. No software to install- nice for the users-and it's easier to write some HTML, instead of programming all these user interface elements in executable programs, and then making sure they support major OSes and all their versions. Originally, I was impressed by this idea when I got a Toshiba ISDN router-it still works in my basement, although only for phone lines now, since the Internet was switched to ADSL, which was not available back then.
When I started the network cameras project, I did not have any doubt as to the nature of the interface software: the GNU/Linux software for the Axis ETRAX100LX processor even included a web server that was able to serve html pages and images, and interface with CGI programs and scripts that ran in the camera.
That was nice-as soon as the hardware was ready, it did not take me long to modify the software, add the JPEG library, and have basic camera functionality that supported passing acquisition parameters from a simple GET method HTML form and receiving images back. Problems started later, however, when I needed to add some otherwise basic functionality that was not easily available with this technology.
Asynchronous JavaScript
I wanted to receive not just images, but usable data, such as status
information. I did not expect a problem with such a simple task, but it proved difficult. For example, the camera can be used for capturing single events that are triggered externally, by sensors, for example. The first approach I tried was to request an image from the camera. The camera was armed by this request, but would not send an image until after the trigger event had happened. After an interval, the browser would abandon waiting, and there was no easy way to reset the waiting-not a really nice solution.
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