From DXOMark: More pixels offsets noise3

Started May 30, 2009 | Discussions thread
Steen Bay Veteran Member • Posts: 7,418
Re: Bob..

bobn2 wrote:

Steen Bay wrote:

Just one question. You said in the other thread that a gaussian blur
before downsampling wouldn't affect detail/sharpness (and Emil said
that it would reduce noise in the downsampled image), but you also
said in a reply to zxaar that "you cannot know which pixels are
'noisy' that is the nature of noise". So, how can gaussian blur
remove noise, without removing detail at the same time?

Emil explained this nicely in his post on the subject. Remember, you
are reducing detail/sharpness by downsampling. If you do that with
high frequency noise still there, that noise aliases into the
downsampled image, so before you downsample you blur which averages
the detail and noise above the maximum spatial frequency to be
present in the output image. The blur is a crude low pass filter, it
eliminates noise and detail, but only that which will not appear in
the output image, due to the lower sampling rate. Since you've taken
away the high frequency noise, it can't be aliased into the lower
resolution image. Not doing the blur was the mistake PIX was making,
and his downsamples showed that aliased high frequency noise.


Thanks. Reading Emil's reply again, I can see that he actually did explain it nicely (and you too, of course), but still I find it a bit counter-intuitive, that a gaussian blur affects the visible noise in the downsampled image, but not the visible detail. Anyway, I'll take your word for it, that it's the best method when downsampling, also to prevent aliasing. (And it would of course be pretty easy to check, downsampling the 50D resolution testchart shot, with and without the gaussian blur)

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