Dalsa Genie HM640, Руководство пользователя

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Genie HM-HC Series-GigE Vision Camera Operational Reference
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Flat Field (Image Shading) Correction
Image Shading correction, also known as Flat Field Correction is the process of eliminating small gain differences
between pixels in a sensor, eliminate sensor hotspots by automatically doing pixel replacement, and also to
compensate for light distortion caused by a lens. That sensor when exposed to a uniformly lit field will have no gray
level differences between pixels when calibrated flat field correction is applied to the image. The Genie camera
stores flat field correction data until power off or in its user setting state for recall at any time (see "Power-up
Configuration" on page 34). Flat field correction data is composed of gain and offset coefficients.
The CamExpert Flat Field Correction tool (or Sapera Flat Field Demo) allows calibrating, saving, and loading the
flat field calibration data for any Genie camera being controlled. CamExpert will automatically apply frame rate and
exposure limits to optimize calibration (see the calibration notes below).
Information on the Flat Field Data File
A Sapera application (such as CamExpert) creates a new SapBuffer object of the same width as the image buffer but
with twice the number of lines. This provides the room to store both offset and gain Flat Field data. The Flat Field
offset data is contained in the top half of the new buffer, while the gain buffer is in the bottom half.
A Sapera application saves the new buffer using SapBuffer::Save with the "-format tiff" option, which allows
saving both 8-bit and 10-bit offset/gain data without loss of significant bits.
The Flat Field correction formula is:
correctedPixelValue = (originalPixelValue - offset) * (gain/128)
Note: If the offset data = 0xff, then that is a special flag, indicating that the pixel is replaced with an adjacent pixel,
without any calculation. This is the factory calibration technique for elimination of dead or hot pixels. A pixel on
the left edge (beginning of the line) would be replaced with the pixel to its right, while a pixel on the right edge (end
of the line) is replaced with the pixel to its left. Any pixel within a line is replaced with the average of the its
neighboring pixels (on the same line). For color sensors, the same algorithm is used except the replacement pixel is
of the same color.
Important Factors about Flat Field Processing
Important: During calibration, no other Genie features should be accessed or modified. The calibration process
will disable functions such as image crop or flip setting. These features need to be re-enabled by the application or
user only after the flat field calibration completes.
Important: Before calibration, the Genie should be powered on long enough to achieve its nominal temperature
(a minimum of 30 minutes). A metallic camera mount or a low ambient temperature may increase the time
required for the Genie to reach a stable internal temperature.
Calibration via CamExpert: The CamExpert calibration function will automatically reduce the frame rate and
the exposure is limited to 40ms maximum. These settings are for optimal performance from the sensor.
Calibration via a User Application: The user application must not exceed 15fps and limit exposure to 40ms
(same conditions set by CamExpert). These requirements are for both free run mode (internal trigger) or when
using an external trigger to capture the calibration frames.