Chapter2 GettingMeasurement-Ready Images
IMAQVision for LabWindows/CVI User Manual 2-12 ni.com
An image can haveextraneous noise, such as periodic stripes, introduced
during the digitization process. In the frequency domain, the periodic
pattern is reduced to a limited set of high spatial frequencies. Also, the
imaging setup may produce non-uniform lighting of the field of view,
which produces an image with a light drift superimposed on the
information you want to analyze. In the frequency domain, the light drift
appearsas a lim ited set of lowfrequencies around the average intensity of
the image (DC component).
Youcanusealgorithmsworkinginthefrequencydomaintoisolateand
removethese unwanted frequencies from your image. Follow these steps to
obtain an image in which the unwanted pattern has disappeared butthe
overallfeatures remain:
1. Use imaqFFT()to convert an image from the spatial domain to the
frequencydomain. This function computes the Fast Fourier Transform
(FFT) of the image and results in a complex image representing the
frequencyinformation of your i mage.
2. Improveyour image in the frequency domain with a lowpass or
highpass frequency filter.Specify which type of filter to use with
imaqAttenuate()or imaqTruncate(). Lowpass filters smooth
noise, details, textures,and sharp edges in an image. Highpass filters
emphasize details,textures, and sharp edges in images, but they also
emphasize noise.
•Lowpassattenuation—The amount of attenuation is directly
proportional to the frequency information. At low frequencies,
there is little attenuation. As the frequencies increase, the
attenuation increases.This o peration preservesall of the zero
frequencyinformation. Zero frequency information corresponds
to the DC component of the image or the average intensity of
the image in the spatial domain.
•Highpass attenuation—The amount of attenuationis inversely
proportional to the frequency information. At high frequencies,
there is little attenuation. As the frequencies decrease, the
attenuationincreases. The zero frequency component is removed
entirely.
•Lowpasstruncation—Specify a frequency. The frequency
componentsabove the ideal cutoff frequency are removed,and the
frequencies belowit remain unaltered.
•Highpass truncation—Specify a frequency.The frequency
components abovethe ideal cutoff frequency remain unaltered,
and thefrequencies belowit are removed.