HSVKey Plugin

The HSVKey plugin is a difference keyer that uses the different in hue between the image and the reference background image.

Difference keying off hue alone is useful when the background is moving but relatively uniform or when it changes shade or brightness significantly when compared to the reference background image. It's used when there is not enough intensity difference between foreground and background to lumakey but the background isn't clean enough to use a normal chromakeyer.

Hue alone is usually not enough to pull a good matte. However, it's a good starting point when mixed with other mattes (using a Max or Min operator) or as a holdout or garbage matte.

Using HSVKey

If you have a reference background, lucky you. Drop it into the rightmost input of the HSVKey. Otherwise, you'll have to get artistic, approximating one by hand, probably chopping up different frames of the input sequence to help you.

Drop the input onto the left hand side. (Ensure the input already has an alpha channel by using a SetAlpha or setting autoAlpha to 1 in the filein node). The from HSVKey is in the alpha channel of the input, so you'll need to be viewing the alpha channel to see what's going on (push A).

Adjust huemin until the background appears completely black. Then adjust huemax until the foreground appears white, but only just. You probably won't get very soft edges, but have a play.

If there are grey areas in the image, these will not really have a hue. You can knock them out of the matte by increasing satMin, or leave them in. Either way, you'll probably have to do some kind of touching up afterwards.

Parameters

Parameter Purpose
hueMincolour differences less than huemin are represented by alpha=0
hueMaxcolour differences greater than huemax are represented by alpha=1
satMinIf the saturation falls below this threshold, alpha is set to 0.

If you need a special kind of keyer, get in touch: pedro@peterhillman.org.uk