[Tig] Key/Fill (was dual link on NVIDIA SDI card)
ted at tedlangdell.com
Wed May 19 17:20:42 BST 2010
On May 19, 2010, at 7:16 AM, Bob Friesenhahn replied to:
>> "fill/key" terminology comes from live keying applications, where the
>> picture comes out of the A link (4:2:2) and the B link is used for a
> Does anyone have a reference for how these terms ('fill' & 'key')
> were orignally derived? It seems likely that there is a long
> history, similar to other industry designations which stop making
> sense but continue to be used.
Here's the "long" background.
It's a takeoff on motion picture "matting" techniques where high
contrast images on film are used as a matte to hold back exposure from
some areas, while allowing images to pass through on others.
The early electronic technique was often referred to as electronic
matting, and the first circuits used were often called "matting
On early analog video switchers, a high contrast signal like white
lettering on a black background was used to "cut a hole" in a
background picture. You'll easily see the difference between "Keyed"
titles and "Supered" (Superimposed) white titles in old TV shows.
The high-con signal is called a key. What goes into the hole cut by
the key is the fill. (IE Fill the Key Hole.)
In electronic terms, the high con signal controlled an electronic
switch. The Key signal controlled whether the foreground (Fill) or
background picture was on the screen as the scanning line swept across.
The same thing is done with Alpha channels on computer-based images
today. The alpha allows the background to show through.
The cards mentioned allow the alpha channel to generate a key signal
sent out of the KEY output and the picture to fill the hole is output
via the FILL output.
Those outputs go to the Key and Fill inputs on a switcher, which
selects the background, Key and Fill images and composites the picture
The first switchers needed a simple Black/White transition to work
well. Now, the sensitivity of the keyer circuits ("Linear Keyers" in
particular) is much better, allowing gradients, smoke and hair to be
more easily "keyed" over a background picture.
Outboard boxes such as the Ultimatte are often used for blue and green
screen work, and they provide the key and fill signals which can be
either run through a switcher, or recorded to tape or files for use in
Before computer compositing became the norm... high-end compositing
was done with large switchers with lots of keyers so you could make a
lot of layers.
For moving images, a pair of VTR's would be used—one for the Key and
another for the Fill, synced by timecode.
The concept was to try to use lots of VTR pairs to keep the images as
close to first generation as possible. Otherwise you needed multiple
passes and incurred generation loss when using smaller numbers of
VTR's and keyers.
One example of the "lots of VTR's" technique that comes to mind is a
Coke spot (posted at Charlex if I recall) that had a whole bunch of
things flying around.
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