descreening

Definition: Process in image processing of avoiding or removing moiré effects in images due to scanning of photomechanically reproduced photographs or the capture of subjects with fine, regular pattern e.g. weave of jacket. * May take place during image capture or in post-processing.
Why is it never straightforward?
After going through all the palaver of improving CCD resolution and so on, we discover that we have to reduce some of that quality we fought for. Why? Because certain subjects like woven jackets, the fine grid in some metalwork will cause moiré effects with the CCD array. The only way to avoid it is to reduce the high frequency pattern i.e. to lower the resolution of the image. Kodak use a brilliantly designed anti-aliasing or low-pass filter in their professional digital cameras (an extraordinarily clever use of birefringence), but others will just have to put up with the problem. The majority of DSLRs use a low-pass filter, which does soften image detail but in a way which can legitimately be retrieved by unsharp masking.

Related Terms: USM

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