One of my favourite movies – especially at Christmas – is It’s a Wonderful Life; a 1946 American movie directed by Frank Capra and starring James Stewart, a man who is financially ruined and is about to commit siucide on Christmas Eve until he is visited by a guardian angel.

Today I saw a new colorized version of the movie being advertised on DVD (or maybe Blu-Ray, whatever that is). It turns out that it is not the first colorized version of this movie. The first was introduced in 1986 and improved upon in 1989. The process of colorization is a difficult and time-consuming process; essentially an artist adds colour to each frame of the movie (obviously this is usually done digitally on a computer). There are two interesting properties of the world and of colour vision that makes the process a little easier than it might otherwise be.

Firstly, colour varies slowly over the scene so that it is not necessary to individually colour each pixel; the artist can define an area an add a single colour to that area with a single mouse click. The importance of luminance for detecting edges and defining form allows some error in this process with the knowledge that it probably wouldn’t be noticeable to most observers. However, in addition to spatial smoothness, there is also temporal smoothness. So, most adjacent frames are very similar. This allows the artist to colorize every, say, 10th frame, and use interpolation to do the rest.

The results of colorization are sometimes disappointing, especially for flesh tones. However, the process is improving all the time and I look forward to seeing the latest results.