National Instruments IMAQ Vision for Measurement Studio - Manual (Page 63)

Chapter 5 Machine Vision
© National Instruments Corporation 5-17 IMAQ Vision for LabWindows/CVI User Manual
However, there may be applications in which the assumptions used in the
algorithm are not optimal. Knowing your particular application and the
images you want to process is useful in selecting the pattern matching
parameters. The following are parameters the influence the IMAQ Vision
pattern matching algorithm.
Match Mode
You can set the match mode to control how the pattern matching algorithm
treats the template at different orientations. If you expect the orientation of
valid matches to vary less than ±5
° from the template, set the
mode element
of the options parameter to IMAQ_MATCH_SHIFT_INVARIANT . Otherwise,
set the mode element to IMAQ_MATCH_ROTATION_INVARIANT .
Shift-invariant matching is faster than rotation-invariant matching.
Minimum Contrast
You can set the minimum contrast to potentially increase the pattern
matching algorithm’s speed. Contrast is the difference between the smallest
and largest pixel values in a region. The pattern matching algorithm ignores
all image regions where contrast values fall beneath a set minimum contrast
value. If the search image has high contrast but contains some low contrast
regions, you can set a high minimum contrast value. By using a high
minimum contrast value, you exclude all areas in the image with low
contrast, significantly reducing the region in which the pattern matching
algorithm must search. If the search image has low contrast throughout, set
a low minimum contrast parameter to ensure that the pattern matching
algorithm looks for the template in all regions of the image. Use the
minContrast element of the imaqMatchPattern() options parameter
to set the minimum contrast.
Rotation Angle Ranges
For matching objects that may rotate in the image, the pattern matching
algorithm, by default, allows any orientation between 0
° to 360°. If the
pattern rotation in your application is restricted to a certain range (for
example, from –15
° to 15°), you can provide this information to the pattern
matching algorithm by setting the angleRanges element of the
imaqMatchPattern() options parameter. This information improves
your search time because the pattern matching algorithm looks for the
pattern at fewer angles.