DIGITAL COMPOSITING: Keying and Green screen.

Chroma keying, often referred to as ‘green screening’ is stripping a selected colour away from footage, leaving it transparent. This allows the remainder to be composited in front of other footage.
Keying can be done with any hue, not just green, but since we often use it to key out people, green is most common as it is the colour least likely to be present in human skin.

In most compositing packages, Keying is done via an included plugin called Keylight. The user manual for which is listed below.

Keylight User Manual: keylight2-0v2_fcp
BASIC KEYING IN AFTER EFFECTS

With footage taken in front of a green screen, choose Effects > Keylight and drag it onto your footage.

keylight.jpg
In the effects Controls window you will be given a variety of options to tweak your key. The most important are as follows:

VIEW: View determines how we see the effects of what keylight is doing. There are a number of different settings:
Final Result – The Default view will show you what the key will look like. It can be hard to get accurate visual feedback from this though, so the other options are handy for checking the quality of the key.

Screen Matte: Shows the footage in a greyscale matte, useful to help determine how clean your Key is. White is visible, Black is transparent, and grey is in between – usually you want to minimize grey areas.

Status: An exaggerrated view of the information presented in Screen Matte view, helping to identify problem areas.

SCREEN COLOUR: Use the colour picker to select the colour on screen you actually want to remove. Since there may be a variation caused by the lighting on your green screen, choose a hue in the midtones to get a fairly uniform key.

DE-SPILL BIAS: Spill is when our object has some of the background colour reflected on it. This can create either unusual hues, or transparency in our keyed footage. De-spilling is replacing this colour (usually greenish) with nearby hues, before the key is applied.

SCREEN GAIN – Increase this to romove greys and increase blacks in your matte. This can start to eat away at your edges though, so one or two points is usually enough before moving on to other methods.
SCREEN BALANCE – Opposite of screen noise – Increase this to boost the whites in your image. This will help to eliminate transparency where you dont want it, but can introduce noise in your footage, so use sparingly.
SCREEN PRE-BLUR – This can eliminate some harsh edges but can obviously blur the footage. Also useful for grainy footage.

SCREEN MATTE
Inside this little section are a few more options worth playing with:
Clip Black: to start to remove harsh line from outside edge of keyed footage.
Clip White: to enhance white areas and eliminate any partial transparency you may be getting
Screen Shrink/Grow –  Use to extend or shrink the edge of the keyed footage.

Keying is more art than science. Every piece of footage will require different values in the options above. Some footage will be more suitable than others. Once you have got a decent key, Use the PEN TOOL to cut out parts of the footage outside the boundaries of the green screen.

LUMA MATTE METHOD
If you are getting noise in your keyed footage that you cant remove any other way you can use this method –

1: Run keylight on your footage, but instead of final result, set it to screen matte. adjust settings til its good.
2: Have a duplicate of your original footage underneath with its TrkMat set to luma matte. This will take the matte data from the layer above to effectively mask it- hiding the green area.
3: We may now need to colour correct the original footage. Apply a hue/sat effect to remove any green spill from skin. Shift hue slightly to red, and remove green sat. pull the little markers in the hue bar to the left to increase the hue range being affected to the new hue.

SIMPLE LIGHT WRAP

A light wrap helps to make keyed footage more convincing by simulating a backlight on the figures, whose colour and intensity matches the background environment.

1: Add your background image and your foreground footage to a new composition
1: Add keylight to the foreground footage and adjust as necessary, then Precompose your footage. We do this so it has an alpha transparency.
2: Duplicate your background layer and bring it to the top of your timeline, then precompose it.*
3: On this top background layer, add a Fast Blur and a Set Matte effect. Bump up the fast blur then in Set Matte, set the ‘Take Matte From Layer’ to our keyed precomp.
4: Add a Channel Blur effect, increasing the Alpha Blurriness setting.
5: Duplicate the Set Matte effect. Move one ‘Set Matte’ below the Channel blur effect, and keep one above. Invert the one above with the ‘Invert Matte’ check box.
6: The light wrap is complete. You can adjust its strength with the layer opacity, and also setting it’s mode to screen.

*We only need to precompose the background if the source image is a different size to our composition. If it is the same resolution exactly – dont worry about it!

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One thought on “DIGITAL COMPOSITING: Keying and Green screen.

  1. Pingback: DIGITAL COMPOSITING – Green Screening (Keying) in After Effects. | Michael Arbuthnot

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