Chroma Keying

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The Effect module has an extremely powerful feature built in: the chroma keyer. Chroma keying is the process where a uni-coloured background is replaced with either a static graphic or video. Although the Auto Key command makes it very easy to use the chroma keyer, professionals will want access to the Advanced Settings dialog.

The following controls are available when opening the dialog.

Auto Key

Automatically computes the chroma key, use this when Source 1 shows the green/blue screen.

Pick Key

Click this button to pick a colour from the video window. Notice when this mode is active that the mouse cursor will change to a cross when positioned over the video window and that the chroma keyer is temporarily disabled to show the background.

Set Key

Opens dialog to manually select the RGB colour.


Sets all chroma key parameters (blur, hue, saturation, value, spill & edge shrink) to their default value.

Chroma Blur

This filter is used to cope with video sources that have a low colour resolution (UV colour sampling), which is quite common on most consumer video hardware (e.g. video grabbers, camcorders and webcams). Often these devices also have a lot of noise in their video signal. This filter helps softening the low resolution colour information, as well as removes noise. For professional cameras both horizontal and vertical should be set to 0 for optimal resolution.


Hue Distance specifies the range of hues (colours) starting from the key colour's hue that we want to key out. All the colours in this range will be keyed out 100 %. Hue Feather is used to soften this so that values further away from the key colour will be keyed out less.


This is quite an important parameter that also could be called "Exclude Grey Shades". This is because we don't want to key out anything that is completely desaturated, as the background (the green screen) should be very saturated indeed! So Distance is the distance from the fully desaturated (greys) to saturated (colours). Feather is used to soften the effect.


Another important parameter is value, which could also be called "Exclude Dark Shades". Again we should not need to key out very dark pixels as the background should always be very bright. Distance is the distance from completely black. Again Feather is used to soften the effect.


Spill suppression effectively desaturates the key colour from the image. This is really useful when you have some of the background screen light reflecting onto the back of the subject. This usually creates a halo of the key colour around the subject, especially around the hair. The severity of this will depend on the setup of the lighting.

Edge Shrink

This effect shrinks the alpha mask which can improve the quality of the edge of a mask. It also has a nice softening effect which can improve a noise mask edge.

Note 1: The chroma keyer uses the video card's GPU (Graphics Processing Unit) to take most of the workload of  the CPU. Most video cards manufactured after 2002, and all video cards manufactured after 2005 will support the chroma keyer. Be assured that if your video card does not support chroma keying, it will be completely unsuitable for real time video processing and should be replaced.

Note 2: Distance is also known as threshold, and feather is also known as falloff.