anti-aliasing and scanners, what's the deal?

"Paul Rubin" <http://phr.cx@NOSPAM.invalid> wrote:
> For example, say I have an 8x10" view camera transparency with lots of
> fine detail. I scan it in a flatbed scanner which uses a linear CCD
> sensor at 300 dpi.
>
> Do these scanners have any AA filters? If not, don't I get aliasing?

Dunno about 300 ppi, but there is a phenomenon known as "grain aliasing" in
which the grain in negative films is exacerbated due to aliasing. This was a
nasty problem with 2700 ppi scanners and consumer color negative films, and
is less of a problem with 4000 ppi scanners. It's even less of a problem
than it looks at 4000 ppi (high-res scans of negative films look pretty
ugly*), since 9x is about the limit for quality enlargements (please, note
the word "quality" in there) and you can be fairly aggressive with noise
reduction at 4000 ppi and then downsample to 2700 ppi for printing at 300
ppi.

The folks at Minolta had the right idea of scanning at 5400 ppi, since
that's a full factor of two overscan of my desired target final resolution.
A pity it doesn't scan MF. Sigh.

*: See the following page for examples.
http://www.terrapinphoto.com/jmdavis/

David J. Littleboy
Tokyo, Japan
 >> Stay informed about: anti-aliasing and scanners, what's the deal? 

"Bart van der Wolf" <bvdwolf DeleteThis @no.spam> writes:
> The native scanner sampling density is much higher than that, in the
> 3200-6400 ppi range. Don't confuse that with the output resolution,
> because that is reached by resampling the scanner data.

Are you serious? I'm talking about an ordinary cheap desktop
page-sized flatbed scanner, not a film scanner or some high end Epson
Perfection scanner. They are usually specified at 300 or 600 dpi and
I thought they got higher density by software interpolation.

> > Do these scanners have any AA filters? If not, don't I get aliasing?
>
> No AA filter, therefore any detail smaller than half of the sampling
> density will be subject to aliasing,

OK, thanks, this (and similar posts from other folks) clears up the
mystery.

> e.g. film grain aggregates / dye-clouds. The detail in film itself
> is of relatively low modulation due to the interaction between the
> camera lens and the film emulsion, and image detail is somewhat
> irregular due to graininess, so the image aliasing is not a real
> issue with most films.

I wonder what happens when you scan pages with text on them (small
print). These do have sharp edges and therefore high frequencies, but
maybe not over large areas.
 >> Stay informed about: anti-aliasing and scanners, what's the deal? 

"Paul Rubin" <http://phr.cx@NOSPAM.invalid> wrote in message
news:7xfyej5pp1.fsf_-_@ruckus.brouhaha.com...
> For example, say I have an 8x10" view camera transparency with
> lots of fine detail. I scan it in a flatbed scanner which uses a
> linear
> CCD sensor at 300 dpi.

The native scanner sampling density is much higher than that, in the
3200-6400 ppi range. Don't confuse that with the output resolution,
because that is reached by resampling the scanner data.

> Do these scanners have any AA filters? If not, don't I get
> aliasing?

No AA filter, therefore any detail smaller than half of the sampling
density will be subject to aliasing, e.g. film grain aggregates /
dye-clouds. The detail in film itself is of relatively low modulation
due to the interaction between the camera lens and the film emulsion,
and image detail is somewhat irregular due to graininess, so the image
aliasing is not a real issue with most films.

--
Bart
 >> Stay informed about: anti-aliasing and scanners, what's the deal? 

"Paul Rubin" <http://phr.cx@NOSPAM.invalid> wrote in message
news:7xpsdmno5w.fsf@ruckus.brouhaha.com...
> "Bart van der Wolf" <bvdwolf.RemoveThis@no.spam> writes:
>> The native scanner sampling density is much higher than that, in
>> the 3200-6400 ppi range. Don't confuse that with the output
>> resolution, because that is reached by resampling the scanner
>> data.
>
> Are you serious? I'm talking about an ordinary cheap desktop
> page-sized flatbed scanner, not a film scanner or some high end
> Epson Perfection scanner.

I'm talking about flatbed film scanners (with a light lid for
transparent film), not a simple document scanner. Reasonable document
scanners, e.g. the Canon Lide types, typically have 600-1200 ppi
sampling densities and are good enough for documents and half tone
images as found in process printed matter.

> They are usually specified at 300 or 600 dpi and I thought they
> got higher density by software interpolation.

Yes, anything higher than the native sampling density is interpolated.

SNIP
> I wonder what happens when you scan pages with text on them (small
> print). These do have sharp edges and therefore high frequencies,
> but maybe not over large areas.

A lot depends on the paper they are printed on. Very high quality
type-setters on glossy paper can achieve 1200 dpi or more, but there
is some spreading of ink (dot gain). Yet any detail smaller than half
the sampling density wil to some degree cause aliasing artifacts which
are by definition larger (more visible) than the actual structures
causing it.

--
Bart
 >> Stay informed about: anti-aliasing and scanners, what's the deal?