National Instruments IMAQ Vision for LabWindows TM /CVI - Manual (Page 27)

Chapter 2 Getting Measurement-Ready Images
© National Instruments Corporation 2-11 IMAQ Vision for LabWindows/CVI User Manual
Convolution Filter
The imaqConvolve() function allows you to use a predefined set of
lowpass and highpass filters. Each filter is defined by a kernel of
coefficients. Use
imaqGetKernel() to retrieve predefined kernels. If the
predefined kernels do not meet your needs, define your own custom filter
using a 2D array of floating point numbers.
Nth Order Filter
The imaqNthOrderFilter() function allows you to define a lowpass or
highpass filter depending on the value of N that you choose. One specific
Nth order filter, the median filter, removes speckle noise, which appears as
small black and white dots. Use
imaqMedianFilter() to apply a median
filter. For more information about Nth order filters, refer to Chapter 5,
Image Processing, of the IMAQ Vision Concepts Manual.
Grayscale Morphology
Perform grayscale morphology when you want to filter grayscale
features of an image. Grayscale morphology helps you remove or
enhance isolated features, such as bright pixels on a dark background.
Use these transformations on a grayscale image to enhance non-distinct
features before thresholding the image in preparation for particle analysis.
Grayscale morphological transformations compare a pixel to those pixels
surrounding it. The transformation keeps the smallest pixel values when
performing an erosion or keeps the largest pixel values when performing
a dilation.
Refer to Chapter 5, Image Processing, of the IMAQ Vision Concepts
Manual for more information about grayscale morphology
transformations.
Use imaqGrayMorphology() to perform one of the following seven
transformations:
• Erosion—Reduces the brightness of pixels that are surrounded by
neighbors with a lower intensity.
• Dilation—Increases the brightness of pixels surrounded by neighbors
with a higher intensity. A dilation produces the opposite effect of an
erosion.
• Opening—Removes bright pixels isolated in dark regions and smooths
boundaries.