[Image-SIG] Remove noise from high ISO photos

danielsforummail danielsforummail at terra.com.br
Thu Dec 20 13:34:40 CET 2007

Hi Douglas, thanks for the advices. I will give them a try.
Best regards,
Daniel Ferber

---------- Cabeçalho original -----------

De: "Douglas Bagnall" douglas at paradise.net.nz
Para: "Daniel Felix Ferber" danielsforummail at terra.com.br, image-sig at python.org
Data: Thu, 20 Dec 2007 11:38:03 +1300
Assunto: Re: [Image-SIG] Remove noise from high ISO photos

> Daniel Felix Ferber wrote:
> > When taking a photo with high ISO on a digital camera, there will be
> > many pixels that have a random color that is completely different color
> > from the expected one. But the photo still has a good sharpness
> > ImageMagic uses a stategy that is similar to the median filter: it looks
> > at the neighborhood o a pixel. If the color of the pixel is considerably
> > different from the color of adjacent pixels, then the color of the pixel
> > is changed to a color close to the neighborhood. If not, the pixel is
> > preserved.
> > 
> > It seems that the median filter always changes the color of the pixel,
> > and I am afraid that the image might loose sharpness.
> What you are describing could be achieved thus:
> 1. Take a 3x3 median image. Most pixels will hardly be changed, but the
> noisy ones will be.
> 2. find the difference between the original and the median (using
> ImageChops.difference). Good pixels will be black or nearly so, but the
> noisy ones won't be.
> 3. Use the difference image as a mask for pasting the median over the
> original.  You will need to convert the difference image to grey-scale
> in some way, or split it into bands and do each band of the image
> separately. image.convert('L') is probably not the best way. You might
> want to make it into a threshold image (rather than have it blending
> proportionately to the difference).
> But I'm not sure how much better than a simple median it will be--after
> all, the pixels that this process doesn't change are exactly the pixels
> that the median hardly changes.
> In any case, the median will be much better than SMOOTH and SMOOTH_MORE.
> Douglas
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