User guide

KeylightThe Foundry
BASIC KEYING
14
Screen Strength
background in the final composite. White pixels show areas that will be pure
foreground. Mid-grey pixels will be a blend of foreground and background
pixels in the final composite. You need grey pixels around the edge of the
foreground to get a good key at the foreground edge. However, if there are
grey pixels where there should be pure background, you should try to
remove these with the Screen Strength, Clip Min or Outside Matte. If you
have grey pixels where there should be pure foreground this tells you that
parts of the background will show through here and you will need to firm up
the foreground with Clip Max or an Inside Matte.
Pixels that are a blend between the foreground and background are shown
in just one shade of grey. This is done to highlight potential problems with
the key. These grey pixels may represent a foreground/background blend of
50/50 or 99/1. No distinction is made as to this ratio.
You may occasionally see other colours in the Status and these are covered
on page 23 in the Advanced Keying Chapter.
Screen Strength
The Screen Strength controls how much of the screen colour is removed to
make the screen matte. Increasing this value will key more.
Figure 12 shows
the Status after picking the Screen Colour. You can clearly see that parts of
the background are grey where they should be black. When composited you
may see faint pixels from the foreground where you should be seeing pure
background. Increasing the Screen Strength will fix this, as shown in
Figure 13, but increasing it too much will destroy your good work.
Like many keying parameters, it’s a balance - not too much, not too little.
Increasing the Screen Strength too much will lead to the background
showing through the foreground and edge detail will be destroyed.
Figure 15 on page 15 shows this quite well. Note the steering wheel is black
when it should be white. If you look at the composite you will see the
Figure 12. Status View after picking the
Screen Colour.
Figure 13. Status showing the increase in
Screen Strength.