Volume 10, Issue 2
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Scanning a Dithered Image
A problem occurs when the dithering mask is not made up of randomly placed black-and-white pixels but has some regularity. The dithered image then contains many highly regular spatial frequencies, which may give rise to interference patterns when the image is sampled with a regular scanner, which likely has a different sampling frequency. The result is a range of extra harmonics (i.e., frequencies that are the result of the interference). They come as the products of the frequencies involved. An example of the period of the frequencies that emerge follows.
This occurs because the product signal is actually the addition of the sum and the difference of the base frequencies.
The lower frequencies are rarely a problem. The higher frequencies can be removed by proper low-pass filtering.
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