User`s manual

STOC USER’S MANUAL ©2005 VIDERE DESIGN
produce an image that is overall neutral (called white balance). The STOC-
C provides manual color balance by allowing variable gain on the red and
blue pixels, relative to the green pixels. Manual balance is useful in many
machine vision applications, because automatic white balance continuously
changes the relative amount of color in the image.
The manual gain on red and blue pixels is adjusted using the Red and Blue
controls on the Video Parameters dialog. For a particular lighting source,
try adjusting the gains until a white area in the scene looks white, without
any color bias.
7.3 Gamma Correction
To display properly for human viewing, most video images are formatted to
have a nonlinear relationship between the intensity of light at a pixel and
the value of the video signal. The nonlinear function compensates for loss
of definition in low light areas. Typically the function is x
γ
, where γ is 0.45,
and the signal is called “gamma corrected.”
The STOC has on-chip gamma correction that can be turned on or off.
When on, it compresses the 10-bit output of each pixel into 8 bits. The
compression occurs mostly in the higher light levels. With on-chip gamma
correction, the STOC can handle larger dynamic ranges of light, such as
occur outdoors with bright sunlight and deep shadow.
Without on-chip gamma correction, the display looks very dark in low-light
areas. You can add gamma correction to the displayed image by choosing
an appropriate gamma value in the slider under the right display window
(
Figure 7-2).
7.4 Video Digitization Parameters
The CMOS imagers have electronic exposure and gain controls to
compensate for varying lighting conditions. The exposure can vary from a
maximum of a full frame time to a minimum of one line time. Gain is an
additional amplification of the video signal, for low-light situations. It is
settable from 0 to 12 dB (1x to 4x).
Both imagers are treated in exactly the same manner. It is not possible to
set a different exposure or gain on each imager.
Digitization control can operate in either manual or automatic mode. Refer
to
Figure 7-1 for the controls in the video capture program. Both manual
and automatic modes are available for the STOC(-C) devices.
In manual mode, the user program sets the exposure and gain. The
exposure and gain are based on a 0 to 100 scale. Here are some tips for
setting exposure and gain.
• In general, keep the gain as low as possible, since it introduces
additional noise into the system. Use it only if the exposure is set
to maximum, or if the exposure must be kept low to minimize
Sam
p
lin
g
mode
Frame size
Gamma
correction
Figure 7-2 Frame size and sampling controls in the main capture window.
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