Re: Nikon D800 announced -- 36mp, w/ & w/o AA filter
Jan Brittenson wrote:
This is really only true if you sitting staring at it in PS. It\'s not really true if you\'re tasked with creating a filter to remove it. The latter is a global change in that it looks at the entire image, although it could be selectively applied using a mask of course. But it needs to do a complex DFT of the image since moiré shows up as spikes in the (2D) image frequency; the filter locates (possibly by correlation or heuristically) the spikes and attenuates them, leaving the phase (and hence edge response) unaltered. This is something you can\'t easily accomplish with the end-user tools provided in PS. (With matlab or mathematica it\'s pretty easy. And that\'s how you\'d prototype it of course, then go off and create the production filter.)
Been there, done that, written the commercial video plugins. No, it doesn\'t work in that easygoing way...
*The correlations is always unknown, since the algorithm doesn\'t know if the moire is caused by blue aliasing or red aliasing, or a combination.
*The DFT (FFT) of the image only shows spikes if the moire is present on a perfectly flat surface covered in a perfectly repetitive (and uniform) pattern. Take a wavy veil over a bride - Where\'s the perfect repetition that would give a spike?
Try an FFT on the image in that was linked earlier, on photo.net.
Ok, this is an already destroyed image - he would have a much better chance starting from the individual channels in the raw file. But anyway.
Feb 18, 2012 at 06:25 PM
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