Combining Black and White with Color Rendering

Question

I'm looking to create the effect of sliding a window across my scene and to have everything in that window rendered black & white. (Everything outside the window will still render via color channels).

I've considered multiple layers, multiple cameras and using a glassy material, and have tried the last of these directly. (It didn't have the desired effect).

I imagine the solution could be one of a few things: maybe rendering a black and white camera at the same time as rendering a color channel camera and later splicing the scenes together in editor?; or maybe slowly switching from one scene to another (as if a panel effect)?

My thanks in advance for any tips!

Answer 1

My solution was as follows:

Riffing off one of many excellent tutorials by OneMinuteVideoTutorials on YouTube, I used a plane with a transparent material that I linked to a Material ID in node editor.

The first step is assigning the transparent material a Material ID (Material > Settings > Pass Index), and then enabling Material Index passes (Render Layer > Passes > Material Index).

The effect can then be achieved using the following combination of nodes:

Notice that the effect is contingent on the saturation in the Hue Saturation Value node being set to 0.