Nuke: Green screen
Keying is a compositing technique used in visual effects and post-production to separate a subject from its background. This process involves creating a matte or mask that isolates the subject, allowing compositors to replace the background with a new image or scene.
There are many different types of keying, and they can be used together to achieve
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.
It separates color information (hue) from the grayscale (value/lightness), allowing for more straightforward adjustments to color intensity and brightness.
R = HUE: Hue literally means colour, measured in degrees from 0 to 360
G = Saturation: Saturation pertains the amount of white light mixed with a hue. It measures the intensity or purity of the color, ranging from 0% (gray) to 100% (full intensity)
B = Luminance/Value (Brightness). Luminance is a measure to describe the perceived brightness of a colour, from 0% (black, no light) to 100% (full brightness, maximum light).
Colorspace node
Colorspace node can be used to convert RGB channels from Linear color space to HSV color space to help analyze color of the plate.
HueCorrect Node
HueCorrect node can be used to mute, suppress or desaturate colors
Mute: Shift color to another color, tone down color, keep luminence
Suppress: Remove color entirely, with luminence
Desaturated: Reduce color
Keyer (Luminance Key) node
The Keyer (luminance key) node analyzes the luminance values of the footage, allowing you to select a range of brightness to create a matte or mask based on the brightness levels within an image.
Key Features:
- Flexibility: Allows for keying based on luminance, is especially useful in monochromatic scenes or when dealing with unevenly lit backgrounds.
- Detail Preservation: Capable of preserving fine details in the keyed element, such as hair or edges, by carefully adjusting the luminance range and softness of the key.
- Spill Suppression: While primarily focused on luminance, additional nodes may be used in conjunction with the Keyer to manage spill or color cast from the background, ensuring a clean and natural integration with the new background.
IBK Gizmo/Colour
In Nuke, IBKGizmo and IBKColour are keying nodes designed to work together for extracting high-quality mattes from footage, especially useful in complex keying scenarios where traditional chroma key methods may struggle.
IBK stands for Image Based Keyer. It operates with a subtractive or difference methodology
IBKGizmo
- Is the core node used for generating mattes, handling difficult keying challenges.
- Example: fine hair details, uneven background tones, severely motion blurred edges etc…
IBKColour
- Works in tandem with IBKGizmo to address color spill issues.
- After a matte is generated using IBKGizmo, IBKColour helps to neutralize or remove color spill from the background, ensuring that the foreground elements integrate seamlessly with a new background.
ChromaKeyer Node
- Uses an eyedropper to select the background color you wish to key out
- Works well with evenly lit screens of saturated color.
- Takes advantage of GPU devices for efficient processing.
Keylight Node
- Provide high-quality keys with detailed edge control and effective spill suppression.
- For challenging keying scenarios, consider using EdgeBlur or Roto to address specific issues or enhance the key.
Primatte Node
- 3D keyer that uses a special algorithm in 3D color space
- Offers an Auto-Compute feature for step-by-step alpha data extraction.
Green Despill
Blue Despill
Clamp node: used for clamping/control max/min value of color
Despill madness gizmo
EdgeExtend node: Premult by default, automatically detects the edges within an image and extends them outward, filling in empty or problematic areas.
addmix vs merge over:
Tips for Effective Keying in Nuke:
– Clean Plates: Whenever possible, use clean plates to help with the keying process, especially for difference keying.
– Preprocessing: Adjusting the input footage for contrast or color balance can significantly improve keying results.
– Combination of Tools: Often, the best results come from combining several keying tools, leveraging the strengths of each to address different aspects of the keying challenge.
Reflection:
I was quite confused about the concept of HSV color space and working with luminance at first, but after going through example nodes and reading about it, it makes sense how useful it is in ensuring high-quality, detail-rich mattes for complex visual effects sequences.
Luminance keying is particularly useful for isolating elements from either a very bright (high luminance) or very dark (low luminance) background when traditional chroma keying (based on color) is not feasible.