Chroma Key

Using a Green Screen

Chroma Key is a technique commonly used to mask out video in live recordings/streaming. Green is chosen as the chroma key most of the time since it is the brightest RGB colour that doesn't typically appear on human bodies (red is an undertone colour for skin and blue is commonly found in clothing).

The chroma key feature in OBS allows you to remove a specific background colour (like green) from your video and replace it with a custom image, video, or transparency. Technically, you can remove any colour you'd like. Try it out on windows with lots of white or black background colours.

Step 1: Set Up Your Green Screen

Make sure the green screen is evenly lit to minimize shadows and colour variations. You can do this with multiple lamps from different angles. Be sure to evenly light the green screen and use soft lighting on the subject to avoid harsh shadows. Place the subject (you) in front of the green screen with no visible gaps between the person and the background.

The goal is to create an evenly coloured green surface! Any dark points or light points add new colours to the array that needs to be keyed away.

Step 2: Add the Chroma Key Filter to an existing video source

In the Sources panel, right-click your video source (webcam, window capture, etc.) and select Filters. Under the Effect Filters section, click the + button and choose Chroma Key. Give the filter the default name (e.g., "Chroma Key") and click OK.

Step 3: Customize the Chroma Key Colour

The green screen we are using in class isn't a true bright green, so you'll need to adjust the key colour. In the Chroma Key Filter settings: Under the Key Color Type, select Custom. Click on the Select Colour button.

In the Colour Picker dialog, use the Eyedropper Tool to sample the actual green colour from a photo of your green screen (you can take a screenshot of the screen in your camera feed if necessary).

Alternatively, manually adjust the RGB values to match the darker green. Adjust the following sliders to fine-tune the keying effect:

Similarity: Determines how much variation in the green is removed. Increase this for a more forgiving key, but avoid making your subject transparent.

Smoothness: Softens the edges of the keyed-out area.

Key Colour Spill Reduction: Reduces green reflections on the subject.

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