Firstly we learned how to key an actor filmed in front of a green screen using Keylight plugin, and about the different techniques you can use to refine the key.
After erasing the 'green' part of the image using the Screen Colour ink dropper, we used the Screen Pre-Blur attribute to smooth the edges slightly. We then changed the view to 'Screen Matte', which showed us something like below:
It's very important to ensure when keying that the actor is entirely opaque whilst everything around him is transparent. The upper image shows tones of grey, which indicates different levels of transparency. You can increase and decrease the Clip White and Clip Black attributes to achieve something like the bottom image, which makes for a clean key.
Colour spill from the green screen can be altered using the Replace Colour tool, and you can use different blend modes such as Hard Light and Soft Light depending on what the footage needs.
Additional refinements can be added to further tighten up the key, such as the Simple Choker under the Matte effects menu. A very slight amount should be enough to get the effect we want.
Finally, using the pen tool to create a mask around the actor omits unnecessary background elements. The key is then ready to be composited.