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!

You'll save yourself a lot of (render) time if you create any type of mask for your window (roto, id material pass or whatever) and use the compositor or any other image application to desaturate that area only. — brockmann, Nov 08 '17 at 22:10
@brockmann - Thanks very much; this was an excellent pointer. I think I've got something that's working now. — Colin Conwell, Nov 09 '17 at 21:04

score 2 · Accepted Answer · answered Nov 10 '17 at 03:52

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.

Combining Black and White with Color Rendering

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