It's important to only isolate three separate elements per ID pass, or the technique becomes redundant. For example, if I had made both the windows and door red in the image for Part One of this blog, then any colour correction I applied to the red channel in NUKE would affect them both. You've already seen a broad example, but there's no reason you can’t use ID passes to focus on more specific areas of your render. The image above shows an example of an ID pass refined to specialise in the windows only. This kind of pass would be useful if the texture I added to the window transoms (red bits) had rendered far too dark, but everything else rendered correctly, as I would be able to adjust them without damaging the rest of my render.
So you've batch rendered an ID pass, and what to do next you ask? Let’s head to NUKE, where the magic happens. I've got my beauty render of my house, and my ID pass as per image in Part One. Let’s imagine I want to colour correct the windows, so I need to retrieve the information stored in the red channel of the ID pass. Easy! Place a shuffle node under the ID pass. In the shuffle node, you'll see columns for the rgba channels. If you place all the crosses in the red 'r' boxes, then you are shuffling out (extracting) just the red information from the ID pass. In my case, this means I've pulled out just the red squares of the windows.
That 'shuffle' can then be used as a mask, much like you would use an alpha as a mask. Meaning, I can place a colour correct node under my beauty, then feed the shuffle node into the mask of the colour correction. Now, if I select the colour correct node and play with any of the setting, it will only affect the information stored in the mask - in this case, the red windows. So the colour correct only affects the windows! Dramatically useful.
Any comments or questions, sound off below.