- In Nuke, keying is the process of isolating specific elements, typically from a video or image, based on their color or luminance values. This is often done to remove backgrounds, separate objects, or create matte elements for compositing.
HSV
HSV Color Scale: The HSV (which stands for Hue Saturation Value)
scale provides a numerical readout of your image that corresponds to the color
names contained therein. Hue is measured in degrees from 0 to 360
R = HUE: Hue literally means colour
G = Saturation: Saturation pertains the amount of white light mixed with a hue.
B = Luminance: Luminance is a measure to describe the perceived brightness of a colour
Hue corrections
Luminance Key
Can be used to create mask with the luminance value of an image.
This can be used to key various channels like RGB/
Luma key can also be used is various ways like experimenting with different color spaces.
Here is an example with keying red channel in various color spaces.
Hue / Luma separator
We can separate Hue from an image by dividing the luminance with the colour of the image in the merge division mode.
Color Difference Key
We can separate individual colors by negating colors from two channels using merge minus function.
Adding Channels
We can create separate channels for certain needs for example we can create a Roto and add it as a new channel and we can use grade or blur node’s mask as the new channel which was created. These Channels are separate from alpha so it is only visible on its assigned name.
ChromaKeyer
ChromaKeyer is basic keying node useful for quick color keying and it is not accurate around hairs or other tiny moving objects.
Image Based Keyer – IBK
IBK stands for Image Based Keyer It operate with a subtractive or difference methodology. It is one of the best keyers in NUKE for getting detail out of fine hair and severely motion blurred edges.
IBK Color: This node we can use it to make up the clean plate for the background of the subject.
IBK Gizmo: With this node we can make use of the produced clean plate of the IBK Color and aid it to key the subject especially for hairs.
KeyLight
Key light It is a really great keyer over all and does color de-spill.
Primatte
Primatte is what is called a 3D keyer. It uses a special algorithm that puts the colors into 3D color space and creates a 3d geometric shape to select colors from that space.
Ultimatte
The advantage is you can get phenomenal keys that can get really fine detail and pull shadows and transparency from the same image.
Green De-spill
It’s the process of removing green scatter around the subject which occurs mainly due to the green screen.
We first key the green using key-light and after the comes the difference function which will extract the green only from the image and then we desaturate the green and we add back to the image.
we can use the same principle for the blue screen.
De-spill map
We can create a De-spill map with the help of minus function with the color function. Here we use Green – Red thus getting a alpha as a result which can be used for de-spill.
Edge extend
Sometimes we will get black halo/edges in an keyed footage, to eliminate this we use edge extend, It works by extruding the color values around the edge of the alpha and we can use this value to reduce the black halo which will be appearing.
Fixing uneven green screen
Having an irregular green screen will develop problems down the pipeline especially during keying. so to counter that we need to even out the greenscreen in comp.