the how to
In this tutorial I cover how to setup and use the UV projection mode in the ScanlineRender node to make cleanup a breeze in Nuke. In my experience, I've found myself using this method to paint out dolly tracks or other objects on a flat surface. It's also came in handy when doing cleanup on pattern surfaces where painting from the camera's perspective might be challenging. The only thing you need to get this method working is a tracked camera for your shot. The better the camera track, the easier this type of cleanup will be.
To get started, you'll need to place a card in 3D space over the area which you want to do your cleanup on. Think of this card as your cleanup canvas, anything that falls inside this card's area you'll be able to work on, anything that falls outside the card's area will not be able to be used. This method can be repeated using multiple cards if necessary. I used a PointCloudGenerator node to help me confirm that my card was where I wanted it in 3D space. Next, take a Project3D node and hook the inputs in to the tracked camera and the BG plate. Take your card and plug that into the Project3D. Next, drop in a ScanlineRender and hook that into the card. Set the "projection mode" knob on the ScanlineRender to "UV". This will have the ScanlineRender output the UVs of whatever geometry is plugged in, which is where we're going to be doing the cleanup. This ScanlineRender does not require a camera to be plugged in because UV maps can exist independently of the camera. Think of it like modeling geometry when in a 3D package. You can model the geo with UVs without needing a camera in your scene. With the viewer hooked up to the ScanlineRender set to UV, start working through the cleanup.
After you've finished the cleanup, it's time to reverse the UV stabilize that we had set up above. Copy the card that was created earlier and hook it up at the bottom of your cleanup. Next, place a ScanlineRender node and connect the camera input to the tracked cam, and the obj/scn input to the copied card. This ScanlineRender needs to have its "projection mode" knob set to the default "render cam". Finally, merge your completed cleanup over the BG plate. All done!
The image below shows the basic setup for using this stabilization technique.
Thanks for reading and happy comping!