National Instruments IMAQTM - Manual (Page 75)

Chapter 5 Performing Machine Vision Tasks
© National Instruments Corporation 5-23 IMAQ Vision for Visual Basic User Manual
The time required to locate a pattern in an image depends on both the
template size and the search area. By reducing the search area or increasing
the template size, you can reduce the required search time. Increasing the
size of the template can improve search time, but doing so reduces match
accuracy if the larger template includes an excess of background
information.
Setting Matching Parameters and Tolerances
Every color pattern matching algorithm makes assumptions about the
images and color pattern matching parameters used in machine vision
applications. These assumptions work for a high percentage of the
applications.
In some applications, the assumptions used in the algorithm are not
optimal. In such cases, you must modify the color pattern matching
parameters. To efficiently select the best pattern matching parameters for
the application, you must have a clear understanding of the application and
the images you want to process.
The following sections discuss parameters of the IMAQ Vision color
pattern matching algorithm, and how they influence the algorithm.
Color Sensitivity
Use the color sensitivity to control the granularity of the color information
in the template image. If the background and objects in the image contain
colors that are very close to colors in the template image, use a higher color
sensitivity setting. A higher sensitivity setting distinguishes colors with
very close hue values. Three color sensitivity settings are available in
IMAQ Vision: low, medium, and high. Use the low setting, which is the
default, if the colors in the template are very different from the colors in the
background or other objects that you are not interested in. Increase the
color sensitivity settings as the color differences decrease. Use
CWIMAQMatchColorPatternOptions.ColorSensitivity to set the
color sensitivity. For information about color sensitivity, refer to
Chapter 14, Color Inspection, of the IMAQ Vision Concepts Manual.
Search Strategy
Use the search strategy to optimize the speed of the color pattern matching
algorithm. The search strategy controls the step size, sub-sampling factor,
and the percentage of color information used from the template.