Ensemble Designs 7420 HD/SD Logo Inserter - Manual (Page 22)

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Page 22
Avenue Logo HD Application
Keying with the 7420 — A Mini Tutorial
The 7420 uses what is called a “linear” system because it responds linearly to the Alpha channel. This
means that the keyer will do the following three things:
1. Suppress ALL of the background signal where the alpha channel is full white;
2. Pass ALL of the background where the alpha channel is full black; and
3. Attenuate the background signal where the alpha is grey — and do it in proportion to the exact
grayscale value of the Alpha.
One of the visual features that this technique provides is to have the Alpha signal cause a shadow to
appear to be cast on the background signal by the foreground signal. It is called an “additive” system
because the foreground (key fill) is added to the background signal in order to make the final image.
Below is a sample logo where the RGB portion and Alpha portion are shown separately.
The 7420 keys this graphic over a background in two steps.
1. The 7420 cuts a hole in the background using the linear characteristics of the Alpha portion.
2. The portions of the Alpha that are gray will attenuate (or darken) the background video. But it will
still be possible to see details of the background.
Numerically this is done by multiplying the Alpha against the background. We can think of a full white
Alpha as being equal to 1.0; a black Alpha as being equal to 0.0; and intermediate gray values as being
a fraction, such as 0.45. Then we multiply the background with the alpha signal like this:
New Background = Background x (1 - Alpha)
Loading a Project from SD
Card to 7420 Module
Insert the Secure Digital Card with your Project
into the 7420 module. Refer to the 7420 HD/
SD Logo Inserter User Guide for additional
information.
The Output Window
Use the Process button to process Assets “offline”
and then later load the Assets onto the SD card
manually.
The Process and Copy To Selected Drive button
processes the Assets in the Project and then
copies them to the selected SD card all in one step.