[Tig] Key/Fill (was dual link on NVIDIA SDI card)

Ted Langdell 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
>> matte.
> 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.
> Bob

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  
you see.

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.

</memory lane>


Ted Langdell
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