User guide

KeylightThe Foundry
ADVANCED KEYING
16
Screen Colour
Advanced Keying
The following section describes the parameters you need to fine tune keys
and get the most out of Keylight. Basic parameters covered in the previous
chapter may also be covered here in more detail.
Screen Colour
The screen colour represents the colour of the pure blue (or green) screen.
The first thing you should do when pulling a key is pick the Screen Colour.
This colour has a primary component, blue or green, and that has a
saturation. Once the screen colour has been picked, Keylight analyses all
the pixels in the image and compares the saturation of the primary
component in each of these pixels with the corresponding saturation of the
screen colour, setting the alpha and modifying the colour accordingly.
Background Pixel If the saturation of the pixel in the image is as strong, or greater than the
screen colour, then it’ll be a pixel from the blue screen background, and that
pixel will be set to completely transparent and black. See Figure 18.
Figure 18. Blue screen pixel set alpha to zero.
Edge Pixel If the saturation of the pixel is less than the screen colour, then it’ll be the
edge of the foreground object, and we subtract some of the screen colour
from the pixel (despilling) and set the image to semi-opaque. See
Figure 19.
Figure 19. Edge pixel gives partial alpha.