Proxima ASA STH-MD1/-C - Manual (Page 11)

STH-MD1 U SER ’ S M ANUAL
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2001 V IDERE D ESIGN
11
6 User Controls
The CMOS imagers are fully controllable via the 1394 interface. User
programs may input color images (STH-MD1-C only), set video
digitization parameters (exposure, gain, red and blue balance),
subsampling modes, and region-of-interest (subwindowing). All of these
parameters can be set with the included capture application, or with the
SRI Small Vision System. They are also accessible to user programs
through the capture API (Section 8).
User controls for frame size and sampling modes are on the main capture
window dialog. Video digitization and Subwindowing controls are
accessed through a dialog invoked with the Video… menu item. Figure 6-1
shows the dialog.
6.1 Color
Color information from the MEGA-D digital head (STH-MD1-C only) is
input as raw colorized pixels, and converted by the interface library into
two monochrome and one RGB color channel. The color channel
corresponds to the left image, which is the reference image for stereo. The
color image can be de-warped, just like the monochrome image, to take
into account lens distortion (see the Small Vision System User’s Manual).
Color information from the camera is input only if the Color button is
pressed on the main window (Figure 2-1).
Because the typical color camera uses a colorizing filter on top of its pixels,
the color information is sampled at a lower resolution than a similar non-
colorized camera samples monochrome information. In general, a color
camera has about ¼ the spatial resolution of a similar monochrome
camera. To compensate for the reduced resolution, use binning (Section
6.4) to increase the fidelity of the image. For example, if you need a
320x240 frame size, use 640x480 and binning x2.
The relative amounts of the three colors, red/green/blue, affects the
appearance of the color image. Many color CCD imagers have attached
processors that automatically balance the offsets among these colors, to
produce an image that is overall neutral (called white balance). The
MEGA-D 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.
6.2 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
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, where
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is
0.45, and the signal is called “gamma corrected.” Digital cameras, such as
the STH-MD1/C, do not necessarily have gamma correction. This is not a
Figure 6-1 Video Parameters dialog.