Anti-aliasing, resolution , and downsampling

  • Thread starter Thread starter Neil Baylis
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Neil Baylis

Do film scanners contain low pass filters like digital cameras do, to
prevent aliasing of high frequency image components?

If they do, I'm assuming that it would only be effective at the
highest resolution, which raises the question of what happens when you
scan at a lower resolution?

One way to do it would be to acquire the image at the highest
resolution, and then blur it (digitally) before downsampling. Is this
in fact what they do?

Finally, I'm wondering if editing apps like Photoshop apply a low pass
filter before they downsample.. seems like it would be necessary. The
evidence of my own eyes tells me that Photoshop is not doing this, or
is not doing it enough, because downsampling does seem to introduce
aliasing artifacts.

Thanks,

Neil
 
Do film scanners contain low pass filters like digital cameras do, to
prevent aliasing of high frequency image components?

If they do, I'm assuming that it would only be effective at the
highest resolution, which raises the question of what happens when you
scan at a lower resolution?

One way to do it would be to acquire the image at the highest
resolution, and then blur it (digitally) before downsampling. Is this
in fact what they do?

Finally, I'm wondering if editing apps like Photoshop apply a low pass
filter before they downsample.. seems like it would be necessary. The
evidence of my own eyes tells me that Photoshop is not doing this, or
is not doing it enough, because downsampling does seem to introduce
aliasing artifacts.
Click to expand...



Anti-aliasing needs to be implemented optically,
at the time of the scan. The AA filter characteristic
needs to be matched to the mechanical resolution
of the sensing element.

I suspect both the scanner driver and Photoshop
are using good intelligence when they downsample --
ie., not simply "throwing away" data but averaging
the original data. You can prove this easily by
looking at a featureless area of a "real" film scan,
both before and after the downsampling, with the
histogram tool. The downsampled version will
a smaller standard deviation.


rafe b.
http://www.terrapinphoto.com
 
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