First things first, improving the filter. I had become rather rusty at working on filters for The GIMP but eventually everything came flooding back to me.
The first thing that helps when writing a Python filter in The GIMP is to run The GIMP from the command line in a terminal window. That way you get to see all the error messages the plug-in produces and are not working "blind". You can also see the output of any print statements you add to help you debug.
The second thing I remembered was that you should use a symbolic link to the filter in The GIMP's plug-in folder, so you can work somewhere more convenient than a hidden folder that's several levels down.
Sugary Spectrum - click to enlarge
Once I'd got myself working sensibly I could have a look at improving the filter. The first thing I did was to speed the filter using this technique described in Akkana's blog. It cuts down on writing to the actual image, which is slow. Instead you copy the image to a byte array, work with that and then copy all the bytes back to the image when you have finished. Using Akkana's technique had the added bonus of allowing the filter to be adapted easily work with either RGB or RGBA images.
However, the resulting changes didn't seem to generate the desired increase in speed until I realised I had stupidly queried the image class' size and width repeatedly instead of storing the values in variables. Once I did that the filter literally flew.
Nitrofurano (Paulo Silva) has been lovely and very encouraging as I've been hacking his lovely code to bits. He's also as enthusiastic about free software as I am. I think it's fantastic that people who have never met before can work on each other's software, share ideas and get to know each other - the GPL really does work as advertised.
Sinclair TV - thanks to Nitrofurano
The reason the code was written originally was to be part of a very interesting project Paulo is working on to create "retro" vision web-cams. You can find out more about it here.
You can download the updated python-fu ZX Spectrum filter for The GIMP here.