Point Grey Research Grasshopper Technical Reference
Camera Operations and Features
Revised 26-Nov-10
Copyright (c) 2010 Point Grey Research Inc.
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exposing images even when ISO is off in order to keep things such as the auto exposure
algorithm (if enabled) running. This is done to ensure that when a user starts requesting images
(ISO turned on), the first image they receive will be properly exposed.
4.5.3. Gain
The Grasshopper supports automatic, manual and one-push gain modes. The A/D converter
provides a PxGA gain stage (white balance / preamp) and VGA gain stage (GAIN register 0x820).
The main VGA gain stage is available to the user, and is variable from 0 to 24dB in steps of
0.046db.
Formulas for converting the fixed point (relative) gain values reported by GAIN register 0x820 to
floating point (absolute) values are not provided. Users wishing to work with real-world values
should refer to the Absolute Value CSR Registers section of the PGR IEEE-1394 Digital Camera
Register Reference.
L
Increasing gain also increases image noise, which can affect image
quality. To increase image intensity, try adjusting the lens aperature
(iris) and shutter time first.
L
The GRAS-20S4M/C models are limited to 16dB of gain when running
in 30fps mode. It is possible to manually override this limit by setting
the ABS_VALUE_GAIN register.
4.5.4. Auto Exposure
Auto exposure (AE) allows the camera to automatically control shutter and/or gain in order to
achieve a specific average image intensity, and is controlled using the AUTO_EXPOSURE
register 0x804. There are three AE states:
State Description
Off Control of the exposure is achieved via setting shutter and/or gain.
On
Manual AE
The camera automatically modifies shutter and/or gain to try and match
the average image intensity to one-quarter of the specified AE value.
On
Auto AE
The camera modifies the AE value in order to produce an image that is
visually pleasing.
If only one of shutter and gain is in auto mode, the auto exposure controller attempts to control
the image intensity using that one parameter. If both of these parameters are in auto mode, the
auto exposure controller uses a shutter-before-gain heuristic to try and maximize the signal-to-
noise ratio by favoring a longer shutter time over a larger gain value.
The auto exposure algorithm is only applied to the active region of interest, and not the entire
array of active pixels.
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