image frequency:
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In radio reception using heterodyning in the tuning process, an undesired input frequency that is capable of producing the same intermediate frequency (IF) that the desired input frequency produces. Note: The term image arises from the mirror-like symmetry of signal and image frequencies about the beating-oscillator frequency.
Image in spatial domain is made up of pixels. the image can have low or high frequency content. most of the natural images have a low frequency content, and this is why DCT performs energy compaction at low frequencies, giving rise to jpeg compressed images.
In the image it contain both high freqency and low frequency components.If there is a high variation in the grey level between the two adjacent pixels then that region is called high frequency region.High frequencies usually occur in the edges of the image.If there is smooth variations b/w the adjacent pixels then this is called as low frequency region.
The term (spatial) frequency is used in Neat Image to denote image elements (both important details and noise) of certain size.High frequency corresponds to image elements of smallest size. Medium frequency corresponds to elements of medium size. Low frequency corresponds to image elements of (relatively) large size.