National Instruments IMAQ Vision for LabWindows TM /CVI - Manual (Page 44)

Chapter 4 Performing Particle Analysis
IMAQ Vision for LabWindows/CVI User Manual 4-2 ni.com
If all the objects in your grayscale image are either brighter or darker than
your background, you can use
imaqAutoThreshold() to automatically
determine the optimal threshold range and threshold your image.
Automatic thresholding techniques offer more flexibility than simple
thresholds based on fixed ranges. Because automatic thresholding
techniques determine the threshold level according to the image histogram,
the operation is more independent of changes in the overall brightness and
contrast of the image than a fixed threshold. These techniques are more
resistant to changes in lighting, which makes them well suited for
automated inspection tasks.
If your grayscale image contains objects that have multiple discontinuous
grayscale values, use
imaqMultithreshold() to specify multiple
threshold ranges.
If you need to threshold a color image, use imaqColorThreshold() .
You must specify threshold ranges for each of the color planes using either
the RGB or HSL color model. The binary image resulting from a color
threshold is an 8-bit binary image.
Improve the Binary Image
After you threshold your image, you may want to improve the resulting
binary image with binary morphology. You can use primary binary
morphology or advanced binary morphology to remove unwanted
particles, separate connected particles, or improve the shape of particles.
Primary morphology functions work on the image as a whole by processing
pixels individually. Advanced morphology operations are built upon the
primary morphological operators and work on particles as opposed to
pixels. Refer to Chapter 9, Binary Morphology, of the IMAQ Vision
Concepts Manual for lists of which morphology functions are primary
and which are advanced.
The advanced morphology functions require that you specify the type of
connectivity to use. Use connectivity-4 when you want IMAQ Vision to
consider pixels to be part of the same particle only when the pixels touch
along an adjacent edge. Use
connectivity-8 when you want IMAQ Vision
to consider pixels to be part of the same particle even if the pixels touch
only at a corner. Refer to Chapter 9, Binary Morphology, of the
IMAQ Vision Concepts Manual for more information about connectivity.
Note Use the same type of connectivity throughout your application.