STH-DCSG
U
SER
’
S
M
ANUAL
©
2005
V
IDERE
D
ESIGN
18
7 User
Controls
The CMOS imagers are fully controllable via the 1394 interface. User
programs may input color images (STH-DCSG-C only), set video
digitization parameters (exposure, gain, red and blue balance), and
subsampling modes. All of these parameters can be set 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 controls are accessed through a dialog
invoked with the
Video…
menu item. Figure 7-1 shows the dialog.
7.1 Color
Color information from the stereo digital head (STH-DCSG-C only) is
input as raw colorized pixels, and converted by the interface library into
two monochrome and one or two RGB color channels. The primary color
channel corresponds to the left image, which is the reference image for
stereo. The right image color channel is also available. The color images
can be de-warped, just like the monochrome images, 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.
The cameras have on-imager binning from 640x480 to 320x240
(monochrome only). Full-frame images at 320x240 use the binning mode.
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 STH-
DCSG-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.
7.2 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
γ
, where
γ
is 0.45,
and the signal is called “gamma corrected.”
Figure 7-1 Video Parameters dialog.