Black and white moving character in a color video This question is a consequence of :


How to make a film in black and white and add color to some objects?
How to show a character in black and white mode with colorful background?


This is just for my personal culture. Using gimp + path + transform, I can do it in one 2D image, but I was trying to understand how you guys would make a character in black and white in a colored scene, if the movie is already done. 

From the previous answers, I understand that the key word is this case is rotoscoping.
But, I don't understand the principle !

Is it some kind of dynamic path with the time ?
What software would you use to do it ?
Have you got a link to explain that to a guy without video processing culture like me ?

Thanks for reading,
You'd achieve the effect by masking one part of the image and applying a desaturation effect to it. The hard part is masking off one part of the image.

In software such as After Effects and Nuke you can mask off an area of the image with bezier splines, and the shape of the masks can be animated. Tracing and animating these masks is what is called rotoscoping (it was invented by animator Max Fleischer in 1915, where he hand traced frames of film projected onto paper). 

After Effects does come with an automatic rotoscoping tool called "Roto Brush" that works a bit like the magic wand in Photoshop, except that it changes shape over time. This can take a lot of the work out of rotoscoping, but it is far from infallible.