Tim
I think you (mistakenly?) sent this to me instead of Ken.
Cheers
Henrik Dahl
Ken,
This is related to "Aliasing" (in photo speak "moire"). We need to
sample the picture with more than 2 pixels per line of resolution in any
direction to avoid this. (this is the so called theoretical Nyquist sampling
criterion). In practise you need a bit more than the theoretical 2 pixels ,
especially since the pixel spacing is not even the same on the diagonal as it
is in vertical or horizontal scan directions. Aliasing, from pixel sampling
adds low frequency noise that does no exsist in the original. The fact that
the pixels have spaces between them also increases noise slightly because the
sample does then not average over the theoretical whole pixel area.Your 9
pixel estimate for a dot translates to about 2 pixels per "line"
(line=1dark,1light transition) in any direction. Right on the limit. Even
cheap scanners can improve aliasing a little by scanning and averaging
overlapping pixel positions in the direction of the scan.
Regards,
Tim Hughes
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