ALLIED Vision Technologies Marlin F-201 - Manual (Page 85)

Description of the data path
MARLIN Technical Manual V2.4.0
85
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 back-
ground 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.
Algorithm
After the start of automatic generation, the camera 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).
After this, a search is made for the brightest pixel in the mean value frame.
A factor is then calculated for each pixel to be multiplied by, giving it the
gray value of the brightest pixel.
All of these multipliers are saved in a shading reference image. The time
required for this process depends on the number of frames to be calculated.
Correction alone can compensate for shading by up to 50 % and relies on 10
bit pixel data to avoid the generation of missing codes.
How to proceed:
Figure 43: Automatic generation of a shading image
Set GrabCount to # of
desired frames
Set BuildImage Flag to true
Poll SHGD_Control
Register until Busy and
BuildImage Flag are
resetted