Craig wrote:
>Good point. I haven't heard the interference you
>refer to called aliasing (although apparently it has
>been done)and I've not heard a really good technical
>explanation of this effect.
Pete (photoscientia) has a good explanation on his web site. I first saw
this kind of aliasing years ago when I started using flatbed scanners.
I found that scanning an offset printed photograph in a magazine at more
than 100dpi was a waste of time because at the higher resolutions I tried
(up to 300dpi) the quality of the image got *worse* due to interference
between the scanner resolution and the offset dot pattern. This interference
also exists when scanning film, but as I mentioned - the grain or dye pattern
is random so the interference isn't an obvious moire pattern.
> However, it seems to be caused by diffraction of the
> scanner's light source by the dye clouds in the film.
> This has the effect of increasing the area sampled by
> the scanner and reducing sharpness (or alternatively
> increasing the apparent size of small artifacts
> i.e. grain). A higher sampling frequency would not
> necessarily diminish this effect as it is a function
> of the light beam and not the sampling device itself.
Interesting theory, but I don't think it's the main problem as I understand
it. See my offset printing example above - a *reflective* medium which
demonstrates the same problem. The scanner CCD is seeing the edges of dot
patterns, and as a result not getting accurate results. If the scanning
resolution is high enough, the scanner should see the offset dots themselves
not the edges - or in the case of film, the dye clouds not their edges.
> So as it's not quite the same as either aliasing effect I
>described previously, this may be a third definition.
I don't think it's any other definition of aliasing. It's a noise effect
caused by the sampling frequency not being high enough. Just the same as
the distortions in sound if it's recorded with too low a sample rate. However,
as I hopefully sorted out with Byron, aliasing is NOT the cause of scanner
image softness. It's the cause of increased apparent grain and as Tony
mentioned, colour distortions.
Regards,
Rob
Rob Geraghty [EMAIL PROTECTED]
http://wordweb.com
