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PROJECTS:
Flash keying
Sometimes it is necessary to insert virtual graphics behind a presenter,
but in front of other real parts of the scene. Chroma-key cannot be used
in such situations, as there is no coloured background. FlashKey is a
method we developed to solve this problem.
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The person or object that needs to be keyed in front
of the graphic is illuminated with pulsating blue light, so the camera
sees the light as being on and off in alternate fields. We have used
bright LEDs triggered by a specially-designed trigger unit, locked
to a video reference signal. |
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The image from the camera is processed to generate a key signal
in areas of flashing blue, using real-time software on a standard
PC. |
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A ‘clean’ camera signal is also generated,
by removing the pulsating blue light using a filter with carefully-chosen
temporal
and colour characteristics. At simplest, pulsating light
at 25Hz can be used, but as this is very distracting for presenters,
we pulse the light at 75Hz, and use a 1/150th second shutter on the
camera so that it only sees one in three pulses. |
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| Two successive fields
from the camera. |
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| Key signal generated
by detecting pulsating blue. |
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| Virtual
graphic inserted into the camera image after removal of the pulsating
blue,
using generated key to force the presenter’s hand into the
foreground (left). His hand can also pass behind the object as
it is then out of the illuminated area (right). |
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movie clip: |
Weather
map and a handheld object
Flashkey
(RealMedia 5.1 Mb,
widescreen
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| patents: |
| European Patent
Application EP1499117 |
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iview