The Keyer Nodes
Compositing wouldn't be compositing if foregrounds could not be removed from their original backgrounds to be placed on top of brand new backgrounds. This requires that the background be selected in some way. This selection may be done manually with the Roto or RotoPaint node or it may be done with keying tools. These will make the selection based on the color information of the background.
Channel Based Keyers
The following keyers pull mattes that correspond to channel information: the luminance and the red, green and blue channels. As they are simple channel operations, the functionality of these keyers can be replicated with such nodes as the ColorLookup, the Grade and suchlike.
The Keyer is the simplest keying node in Nuke's arsenal. One very useful application of this keyer is to produce a luminance mask based on the brightness values of the sky (the sky is usually the brightest thing in a scene).
The Difference keyer compares two inputs and calculates the difference between them. In order to use this, a cleanplate will be needed. A clean plate is simply the background of a shot, without the foreground. Is compared to the shot with the action, and the difference between the two is returned as the mask. The results of this Keyer are usually a bit rough, but sometimes still usable.
High End Keyers
The preceding keyers only consider the separate channels of the image: red, green, blue and luminance. The following keyers are more than just simple channel operations. They treat the color information as a 3D volume and the mask as a separate volume within it.
This has the reputation of being good at fine details like hair and also semi-transparent items like thin cloth. It is also slightly newer. One advantage of this keyer is that it has inputs for junk masks and holdout masks.
This prduces a slightly harder matte than Keylight. It is common practice to use both Primatte and Keylight in one matte pull. This is the only high end keyer that this course will support. More info on the Primatte page.
Ultimatte is the only Keyer to have ever won an Oscar. It is very sophisticated and even has separate controls for cast shadows.
IBK stands for Image Based Keyer. It consists f two nodes: the IBKGizmo' and IBKColor. IBKColor first makes a clean plate using the shot footage as a source. Then using IBKGizmo a key is pulled. This analyses the difference between the clean plate and the original image. Essentially, it is a sophisticated Difference Keyer. Unlike the preceding advanced Meyers, it does not reference a single color. Rather a range of colors, variable across the background. The advantage of this approach is that it is very tolerant of uneven lighting. This is particular useful for shoots that include the floor area.
- Green Screen
- Tips on green screen use:
- Keying Tips
- Various keying tips:
- Combining Alphas
- It is often required that the output from two or more keyers need combining. Here's how.