Allied Vision Technologies F-131B, Technical Manual, Page: 70 / 183

Description of the data path
8.9 Shading correction
Shading correction is used to compensate for non-homogeneities caused by lighting or optical
for
nloading it to the camera, the camera allows
correction data to be generated au
Upon generation of the shading image in he came
onvolatile storage purposes.
pro
characteristics within specified ranges. To correct a frame, a multiplier from 1...2 is calculated
each pixel in 1/256 steps – this allows for shading to be compensated by up to 50 %.
Besides generating shading data off-line and dow
tomatically in the camera itself.
ra, it can be uploaded to the host computer for
n
The following pictures describe the process of automatic generation of correction data. The line
files were created using MVTEC’s “ActivVision Tools”.
Figure 37: Shading correction: Source image with non-uniform illumination
On the left you see the source image with non-uniform illumination. The graph on the right
clearly shows the brightness level falling off to the right.
By defocusing the lens, high-frequency image data are removed from the source image, therefore
its not included in the shading image.
8.9.1 Automatic generation of correction data
Requirements
Shading correction compensates for non-homogeneities by giving all pixels the same gray value
as the brightest pixel. This means that only the background must be visible and the brightest
pixel has a gray value of less than 255 when automatic generation of shading data is started.
It may be necessary to use a neutral white reference, e.g. a piece of paper, instead of the real
image.
lgorithm
ra pulls in the number of frames set in the
GRAB_COUNT register. Recommended values are 4, 8 or 16. An arithmetic mean value is
calculated from them (to reduce noise).
A
After the start of automatic generation, the came
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