On the PC-side, a simple PHP-program to control the whiteboard electronics runs as a daemon. It listens on a certain PHP-port; clients can connect to that and send a pair of coordinates and a text to it. The program then uses the excellent and public-domain Hershey fontset to turn the text into lines, and then proceeds to draw these lines on the whiteboard.

A second program on the PC reads out the webcam. To do that, I choose to use webcam-server, an old but fine-working piece of software which spits out the webcam-image using a built-in http-server.There's a problem with the webcam, though: it isn't exactly looking straight at the whiteboard:

(The webcam is the green piece of PCB, for unknown reasons, somewhere in the past I decided to dismantle it.) Because my camera looks at the whiteboard at an angle and I wanted to get an image of just the whiteboard, I've hacked perspective correction into it. I'll release the patch with the rest of the software.

The final thing to tie everything together is a php-page running on my webserver. This page serves up the webcam-picture as an image map. Click on that picture and an text input box appears. Enter the text and it will be plotted on the whiteboard at the location you clicked.

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