TMC22091/TMC22191
PRODUCT SPECIFICATION
A 4-Layer Example
Layering with the TMC22191
For this layering example, a BACKGROUND image is
generated. This image comprises shaded matte levels varying
from black at the top of the screen to white at the bottom.
This could just as well be a color image which will be seen
wherever no other image appears through the layering
process.
Layering is a video production process where various images
or patterns are superimposed (keyed) over each other to form
a layered composite of the input images. Four layers with the
following priority are provided by the TMC22191:
1. The DOWNSTREAM KEY layer keys over all other
layers.
The MIDGROUND image comprises a happy face superim-
posed over a white rectangle. Only the happy face and the
white rectangle are of interest for this image and therefore,
the portion of the image outside that area will be replaced by
the BACKGROUND image when MIDGROUND is keyed
over BACKGROUND. A key signal is generated on a pixel-
by-pixel basis. It indicates which image is active. The key
signal for keying MIDGROUND over BACKGROUND is
shown to the right of the MIDGROUND image. This repre-
sents a single bit signal mapped over the image. When the
signal is black (logic LOW), the MIDGROUND image is
active, when it is white (logic HIGH), the BACKGROUND
is active.
2. The FOREGROUND layer keys over MIDGROUND
and BACKGROUND, but not over DOWNSTREAM
KEY.
3. The MIDGROUND layer keys over BACKGROUND,
but not over FOREGROUND or DOWNSTREAM
KEY.
4. The BACKGROUND layer never keys over any other
layer.
It is important not to confuse layers with sources. The
TMC22191 can be programmed to assign any of its input
sources (RGB, YC C , CVBS bus, Overlay bits) to any of
R B
the four layers.
The results of layering MIDGROUND over BACK-
GROUND images are shown in the 2-layer composite image
Figure 19.
The ability to combine various video sources into a 4-layer
composite image is a very powerful tool in the production of
live video. The TMC22191 performs layering operations
entirely in the digital domain, enabling precise digital
control.
BACKGROUND
KEY
2-LAYER COMPOSITE
MIDGROUND
27036A
Figure 19. 2-Layer Image Construction
A FOREGROUND image comprises a shaded matte rectan-
FOREGROUND image is active, when it is white (logic
HIGH), the composite image is active.
gle with "HI KIDS !" alpha characters in its center. This is to
be superimposed over the previous 2-layer composite image.
The key signal needed for superimposing FOREGROUND
over other images is shown to the right of the FORE-
GROUND image. This represents a single bit signal mapped
over the image. When the signal is black (logic LOW), the
A new 3-layer composite image, FOREGROUND over
MIDGROUND over BACKGROUND, is shown in Figure
20.
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