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[filmscanners] Re: Scanning with too much resolution? (was:PS sharpening...)

On 8/16/02 4:15 PM, "Paul D. DeRocco" <pderocco@ix.netcom.com> wrote:

> What's happening is this. If you have an image that has lots of
> high-frequency (spatial frequency, that is) irregular detail, such as grass
> or leaves, and you downsample "correctly", the detail is averaged out. But
> if you shoot or scan the image originally at a lower resolution, your camera
> or scanner may not produce the reduced resolution correctly, but may merely
> take point samples that are further apart.

I think, perhaps, you meant

"... may not be able to reproduce the original details correctly, ..."

or, at least, that wording makes more sense to me.

> (The correct method would be to
> blur the image to roughly the size of the pixels.) This results in the
> high-frequency energy being aliased down to lower frequencies, so you end up
> with the same general sort of irregular detail that you would have had in
> the higher-res image. It isn't a true image at all, but in some cases, it
> may be more pleasing to the eye, because people expect grass or trees to
> have some sort of random detail in it.

Yes.  But what I found most surprising about the example images in Margulis'
book, was that the image scanned at the lower sampling rate appeared to be
sharper, and not just in the areas of texture, like the grass.  Try viewing
these images with Acrobat set to display at 200% zoom.  Look at the white
straps and the hems around the edge of the horse blanket.  In the image
scanned at low res, the lines are rough from pixellation, but they have good
definition.  In the the high res, downsampled scan, the lines are smoother,
but less sharp.

Julian Vrieslander <julianv@mindspring.com>

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