Let me try this again, this time directly linking to the files instead of
cutting and pasting...
> ----------
>
> > I always thought that down sampling consisted of some kind of averaging
> > (of
> > samples).
> > I thought bicubic and bilinear and such terms could as well be related
> to
> > down sampling as they could to upsampling.
> > Now I wonder: how does downsampling work?
> > Does it exist of sampling only one of the pixels in the previous larger
> > image for each pixel in the new image?
> > It more or less explains why I have some grainy images that retain a lot
> > of
> > their graininess when downsampled. But why is downsampling often called
> > better than downsizing?
> >
> > Thank you all in advance,
> >
> > Jerry
> >
> > > -----Original Message-----
> > > From: Shough, Dean [SMTP:[EMAIL PROTECTED]]
> > > Sent: Wednesday, December 06, 2000 4:38 PM
> > > To: '[EMAIL PROTECTED]'
> > > Subject: RE: filmscanners: RE: Film Scanners and what they see.
> > >
> > [cut]
> > > I expect you are right except perhaps for the Epson 1200 and 1600
> series
> > > scanners. I am not sure if they use a custom CCD with smaller pixels
> or
> > > if
> > > they are micro-stepping with an ordinary 600 and 800 dpi array. But,
> > now
> > > that I think about it, if you use a scanner at 1/2 or 1/4 of its full
> > > resolution, then the pixel size remains the same but the Nyquist limit
> > is
> > > much lower. Sounds like a recipe for alaising and another good reason
> > to
> > > always scan at higher resolution and average down (not down sample).
> > >
> > [cut]
> >
>
> I created some test files so everyone could see what I am talking about.
> This first image shows a section of a 512 by 512 pixel zone plate.
>
<<Zone256crop.jpeg>>
> The original file is symmetric about the bulls eye. The fringes seen in
> the
> image increase in frequency as one goes from the center to the edge of the
> image. The edge of the zone plate is exactly at the Nyquist frequency.
> Anything in the image outside the inscribed circle is alaised (In the
> original image, of which only a small section is shown above.
>
> If I reduce the size of the image by a factor of 4 in both directions
> using
> Photoshop's bicubic interpolation, I get:
>
> <<Zone128bicubic.jpeg>>
>
> This is very similar to what a 128 by 128 pixel scanner would see when
> looking at the above image. Because of the averaging effect, the bulls
> eye
> pattern fades away about 1/4 o the way out from the center. Some slight
> modulation is still apparent out at the edge o the image - this is alaised
> information whose magnitude has been greatly reduced due to the low pass
> filtering of the pixels finite size.
>
> If I reduce the size by using Photoshop's nearest neighbor algorithm (the
> same as down sampling) I get:
>
> <<Zone128nearestNeighbor.jpeg>>
>
> Outside the central 1/4 of the image, the strongly alaised image no longer
> bears any resemblance to the correct image. Further more, any filtering I
> apply to this image will not produce correct image - not even close. You
> could say that this alaised image is sharper than the non-alaised image,
> but
> I don't think anyone interested in obtaining an accurate image would like
> it.
>
> In a real image, the same type of effects will take place, although not
> nearly as dramatic. There are two reasons for this: 1) No real scanner
> is
> going to be able radically alias the image to the extent shown here.
> Unless
> of course you tell the scanner to do a low resolution scan and the scanner
> uses down sampling. 2) No real image will have such strong and regular
> features above the Nyquist limit.
>
> I hope that I made the images small enough to not trip the email filters
> yet
> large enough to demonstrate the point.
>
> ----
> Dean Shough
> [EMAIL PROTECTED]
>
>
Zone256crop.jpeg
Zone128bicubic.jpeg
Zone128nearestNeighbor.jpeg
Zone256crop.jpeg
Zone128bicubic.jpeg
Zone128nearestNeighbor.jpeg