Multi-sampling and "2400x4800 dpi" scanners

Many flatbed scanners claim to offer a vertical resolution that is
twice the horizontal resolution, such as 2400x4800 dpi. I understand
this to mean that, while there are only 2400 cells in the CCD, the
stepping motor can move by steps of 1/4800th of an inch.

Additionally, these scanners' CCDs usually do not have a single row of
2400 cells, but two rows of 1200 each, which are positioned at an
half-pixel offset.

Now, if this is true (please confirm), don't we effectively have 4x
multi-sampling when scanning at 1200 dpi?

There are several issues that I don't find clear.

First: when scanning at 1200 dpi, do scanners actually use both CCD
arrays and "mix" the results (I'm not simply saying "average" the
results, since it might be too simplicistic given the half-pixel
offset), or do they only "turn on" one array?

Second: when scanning at 2400 dpi, do scanners give out pixels in the
order "1st pixel of 1st array | 1st pixel of 2nd array | 2nd pixel of
1st array | 2nd pixel of 2nd array", or do they somehow consider the
fact that nearby pixel overlay one another by half their width?
Of course, this also applies vertically, since while the motor moves by
1/2400th of an inch steps, pixels are 1/1200th of an inch "wide".

Third: when scanning at "4800" dpi, what do scanners do about the
horizontal resolution? Interpolation, I suppose. What kind of
interpolation? Does it vary from scanner to scanner?
And, do scanners that claim 2400x4800 resolution *really move the motor
by 1/4800th steps when instructed to scan at 4800 dpi*, or do they just
interpolate (since I know there are also other reasons for having
1/4800th stepping motors)? Does this vary from scanner to scanner?


Now, let's see how all this relates to multi-sampling.

Let's suppose I want to scan at 4800 dpi, with 2x multi-sampling -- for
the moment, let's ignore the fact that it might really be 4x
multi-sampling because of the double CCD array.

The scanner gives me an image. I can turn it into *two* images, one
made of the even lines of the original image, and the other made of the
odd lines (clearly, I must first downsample the original image
horizontally, since it was interpolated to 2x by the scanner).
I can then average the two images. Have I just obtained 2x
multi-sampling?

Apparently not, since I forgot that even and odd lines were sampled at
1/4800th of an inch apart from each other.

But I do know they're separated by a consistent 1/4800th of an inch. So
I could first sub-pixel-align the two images (a no brainer, since I
know they're misaligned by exactly one pixel), and only then do the
merge.

Have I now obtained 2x multi-sampling? Apparently, I have. But now I
wonder: what would have happened if I had just scaled down the original
image to half its size vertically?
Wouldn't that be equivalent to the procedure I described of splitting
it in two, aligning and merging?

Programs usually offer more than one algorithm for scaling down images:
bilinear, bicubic, etc.
Which of these is equivalent to splitting/aligning/merging, if any?


Now you probably also see why I asked all those questions about scanner
behavior above, since to answer my doubts about multi-sampling one must
be aware of how the scanner really behaves, and whatever it does to the
data *before* giving them out to the user.

Perhaps this whole article can be "scaled down" to the question: is
scanning at 4800 dpi and then scaling down to 1200 dpi (with what?
bilinear, bicubic...) equivalent to 4x multi-sampling at 1200 dpi?
(Make substitutions between 4800, 2400 and 1200 above, and you'll get
the other possible scenarios)


by LjL
ljlbox@tiscali.it

On 8 Sep 2005 07:10:03 -0700, ljlbox@tiscalinet.it wrote:

>Additionally, these scanners' CCDs usually do not have a single row of
>2400 cells, but two rows of 1200 each, which are positioned at an
>half-pixel offset.
....

Check the archives (for example on Google). Kennedy
(<rkm@nospam.demon.co.uk>) wrote many extensive and very technical
articles about this in quite some detail, for example:

Subject: Re: filmscanner vs hi-res flatbed
Subject: Re: REPOST: Re: Plustek OptikFilm 7200
etc.

Don.

Don ha scritto:

> On 8 Sep 2005 07:10:03 -0700, ljlbox@tiscalinet.it wrote:
>
> >Additionally, these scanners' CCDs usually do not have a single row of
> >2400 cells, but two rows of 1200 each, which are positioned at an
> >half-pixel offset.
> ...
>
> Check the archives (for example on Google). Kennedy
> (<rkm@nospam.demon.co.uk>) wrote many extensive and very technical
> articles about this in quite some detail, for example:
>
> Subject: Re: filmscanner vs hi-res flatbed
> Subject: Re: REPOST: Re: Plustek OptikFilm 7200
> etc.

Maybe even a bit *too* technical ;-)
I've read these and similar threads before, and I am aware that the
topic of "staggered CCD arrays" (and stepping motors that step less
than one pixel is wide) has been investigated to death.

However, it was mainly about "does a 1200+1200 dpi scanner resolve as
much as a 2400 dpi scanner?", and "does a 1200+1200 dpi resolve
anything more than a 1200 dpi scanner at all?", and "staggered arrays
reduce aliasing but make the image softer".

Instead, my post wanted to investigate the question: is scanning with a
1200+1200 dpi scanner comparable to multi-sampling with a 1200 dpi
scanner?
And if it is, should we process the image taking account of the pixel
offset/overlap, and if so, how?

I've read clues that unsharp masking can be a perfectly valid technique
to compensate for sensor overlap, for example... but it's all a bit too
vague in the threads I've read, covering wider topics than I am
currently focusing on -- such as resolution, aliasing, etc.


by LjL
ljlbox@tiscali.it

On 8 Sep 2005 18:13:58 -0700, ljlbox@tiscalinet.it wrote:

>> >Additionally, these scanners' CCDs usually do not have a single row of
>> >2400 cells, but two rows of 1200 each, which are positioned at an
>> >half-pixel offset.
>> ...
>>
>> Check the archives (for example on Google). Kennedy
>> (<rkm@nospam.demon.co.uk>) wrote many extensive and very technical
>> articles about this in quite some detail, for example:
>>
>> Subject: Re: filmscanner vs hi-res flatbed
>> Subject: Re: REPOST: Re: Plustek OptikFilm 7200
>> etc.
>
>Maybe even a bit *too* technical ;-)

Yes, Kennedy does that! ;o)

But I like it and always file such messages for future use even if
most of it over my head at the time.

>Instead, my post wanted to investigate the question: is scanning with a
>1200+1200 dpi scanner comparable to multi-sampling with a 1200 dpi
>scanner?
>And if it is, should we process the image taking account of the pixel
>offset/overlap, and if so, how?
>I've read clues that unsharp masking can be a perfectly valid technique
>to compensate for sensor overlap, for example... but it's all a bit too
>vague in the threads I've read, covering wider topics than I am
>currently focusing on -- such as resolution, aliasing, etc.

I haven't really looked into all that because I'm too busy with my
film scanner so someone else will have to jump in...

Don.

In article <1126228438.116479.174010@g43g2000cwa.googlegroups.com>,
ljlbox@tiscalinet.it writes
>
>Don ha scritto:
>
>> On 8 Sep 2005 07:10:03 -0700, ljlbox@tiscalinet.it wrote:
>>
>> >Additionally, these scanners' CCDs usually do not have a single row of
>> >2400 cells, but two rows of 1200 each, which are positioned at an
>> >half-pixel offset.
>> ...
>>
>> Check the archives (for example on Google). Kennedy
>> (<rkm@nospam.demon.co.uk>) wrote many extensive and very technical
>> articles about this in quite some detail, for example:
>>
>> Subject: Re: filmscanner vs hi-res flatbed
>> Subject: Re: REPOST: Re: Plustek OptikFilm 7200
>> etc.
>
>Maybe even a bit *too* technical ;-)

Sorry, but sometimes it needs that technical detail to explain the true
implications of the concept.

<snip>
>
>Instead, my post wanted to investigate the question: is scanning with a
>1200+1200 dpi scanner comparable to multi-sampling with a 1200 dpi
>scanner?

That depends on whether the subject contains any information at higher
than 1200ppi and if the lens is capable of resolving it. If it isn't
then it is exactly the same as multisampling - which is why I always
jump on posters who claim that their is no advantage to this scanning
approach: even when there is no resolution advantage there is always the
multisampling advantage.

In simple terms, the double CCD captures twice as much information as a
single line device. If that information does not go into increased
resolution then it appears as increased signal to noise similar to
multisampling.

This is no different from a single line sensor with double the pixel
density when scanning an object which does not have as much resolution
in the original - there is always an advantage to getting more samples
of nominally the same data, but it can be debatable whether that
advantage is worth the time and effort to do so.

>And if it is, should we process the image taking account of the pixel
>offset/overlap, and if so, how?
>
The simplest method of doing this is a pixel average and downsample by a
factor of two. Suffice to say that there isn't an exact method of
separating the resolution from the SNR gain. Half pixel realignment
isn't really a solution in these cases because it involves resampling
losses in itself which are likely to exceed any benefit that they are
intended to gain. Some blurring, up to a quarter of a pixel may be
advantageous.

>I've read clues that unsharp masking can be a perfectly valid technique
>to compensate for sensor overlap, for example... but it's all a bit too
>vague in the threads I've read, covering wider topics than I am
>currently focusing on -- such as resolution, aliasing, etc.
>
Yes, this is essentially the opposite of what you are trying to do - put
more of the additional information of the double scan into increased
resolution rather than improved signal to noise ratio at the lower
resolution. Hence my comment that limited blurring may offer a benefit.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's ****ed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)

Kennedy McEwen ha scritto:

> In article <1126228438.116479.174010@g43g2000cwa.googlegroups.com>,
> ljlbox@tiscalinet.it writes
> >
> >Don ha scritto:
> >
> >> [snip]
> >>
> >> Check the archives (for example on Google). Kennedy
> >> (<rkm@nospam.demon.co.uk>) wrote many extensive and very technical
> >> articles about this in quite some detail, for example:
> >>
> >> Subject: Re: filmscanner vs hi-res flatbed
> >> Subject: Re: REPOST: Re: Plustek OptikFilm 7200
> >> etc.
> >
> >Maybe even a bit *too* technical ;-)
>
> Sorry, but sometimes it needs that technical detail to explain the true
> implications of the concept.

Oh but it wasn't a bad comment on you, I meant too technical *for me*.
When you talk about MFT and so on, I think I can grasp the basic ideas
behind those concepts, but can't really *understand* them to any
extent.

But it's certainly a very good thing that you can discuss the more
technical details on a newsgroup with people who understand them,
that's just what the Internet is good for in research!

> <snip>
> >
> >Instead, my post wanted to investigate the question: is scanning with a
> >1200+1200 dpi scanner comparable to multi-sampling with a 1200 dpi
> >scanner?
>
> That depends on whether the subject contains any information at higher
> than 1200ppi and if the lens is capable of resolving it. If it isn't
> then it is exactly the same as multisampling - which is why I always
> jump on posters who claim that their is no advantage to this scanning
> approach: even when there is no resolution advantage there is always the
> multisampling advantage.

Yes. But I can see two scenarios:
1) when there is no resolution advantage, is it really *exactly* as
multisampling, or does it lose some ground because of the misalignment?
or can the lost ground be re-gained with appropriate post-processing?
2) when there *is* resolution advantage, can the multisampling
advantage exploited *together* with the resolution advantage, or must a
choice be made?

What I suspected is that a choice must be made, and that the choice
typically favors resolutions over multi-sampling (i.e. noise
reduction).

Anyway, you see, I was thinking more about the *vertical* axis of
scanning (i.e. the "4800 dpi" of my scanner), where the resolution gain
appears to be practically null, with pixels overlapping for three
fourths their size.

There is also a post by you where you say that half-stepping on the
vertical axis is next to useless, at least concerning resolution.

But I can clearly see that it *is* useful in terms of noise reduction,
just by taking a scan at 2400x4800 (and then downsampling the 4800) and
one at 2400x2400.

When half-stepping, scanners usually interpolate on the horizontal axis
to get a 1:1 ratio. This I don't like (and in fact I'm trying to modify
my SANE driver accordingly): I'd like to take a purely 2400x4800 scan,
and then downsample *appropriately* on the vertical axis.

My main concern, which you address below, was on the meaning of
"appropriate downsampling" when downsampling an image that is made by
3/4ths overlapping pixels.

> [snip]
>
> This is no different from a single line sensor with double the pixel
> density when scanning an object which does not have as much resolution
> in the original - there is always an advantage to getting more samples
> of nominally the same data, but it can be debatable whether that
> advantage is worth the time and effort to do so.

More than "debatable", I'd call it a personal choice.
My scans at 1200x1200 are awfully noisy; those at 2400x2400 are better,
but I certainly do appreciate the benefit of 2400x4800, at least for
some pictures.

What worries me is the "nominally the same data" part. It's not
nominally the same data in the real world, unless the original is of a
much lower resolution than the sampling rate.
It's *almost* the same data, but shifted -- half a pixel horizontally
(double CCD), and 1/4 of a pixel vertically (half-stepping).

So, I'm under the impression that scanning at 2400x4800 (let's talk
about the half-stepping and ignoring the double CCD) and then
downsampling the vertical axis gives me a less noisy, but blurrier
image than scanning at 2400x2400.

This wouldn't happen with "real" multi-sampling, i.e. samples taken at
exactly the same position. Question is, is there a software fix for
this? I'm taking your answer, below, as a "mostly no"...?

> >And if it is, should we process the image taking account of the pixel
> >offset/overlap, and if so, how?
> >
> The simplest method of doing this is a pixel average and downsample by a
> factor of two.

I.e. an image made by (all pixels from line n + every pixel from line
n+1) / 2 (that is considering only one direction)?
But this is really the same as treating it as a "standard"
multi-sampling, i.e. with no offset, isn't it?

Then what about the various bilinears, biquadratics and bicubics?

> Suffice to say that there isn't an exact method of
> separating the resolution from the SNR gain.

Which is to say that the offset between each pair of scan lines can't
be really accounted for in software?

> Half pixel realignment
> isn't really a solution in these cases because it involves resampling
> losses in itself which are likely to exceed any benefit that they are
> intended to gain. Some blurring, up to a quarter of a pixel may be
> advantageous.

Hm. Blurring, at what stage? Scans taken at 4800 and then resampled to
2400 (Photoshop, bicubic) look already blurrier than scans taken at
2400, as I said.
So, I take it you'd be blurring by 1/4 of a pixel and then
downsampling? But you'd still be downsampling with the method you
described above (average), rather than the standard functions in say
Photoshop, correct?

In any case I don't fully understand why you say that half-pixel
realignment isn't worth doing. I know the explanation would get
technical, but just tell me, shouldn't it be just as worthless when
done on multi-scans (the Don way, I mean, taking multiple scans and
then sub-pixel aligning them)?
The only difference is that, in "our" case, the amount of misalignment
is known. Which should even be an advantage, or shouldn't it?

> >I've read clues that unsharp masking can be a perfectly valid technique
> >to compensate for sensor overlap, for example... but it's all a bit too
> >vague in the threads I've read, covering wider topics than I am
> >currently focusing on -- such as resolution, aliasing, etc.
> >
> Yes, this is essentially the opposite of what you are trying to do - put
> more of the additional information of the double scan into increased
> resolution rather than improved signal to noise ratio at the lower
> resolution. Hence my comment that limited blurring may offer a benefit.

I see. But do you agree with me, in any case, that on the vertical
axis, the 4800 dpi of "resolution" are worthless as *resolution* and
much more useful as a substitute for multi-sampling (i.e. for improving
SNR)?

But anyway, what do you have to say about the unsharp masking -- which
I certainly consider doing on 2400x2400 scans?
My impression is that the standard, consumer-oriented Internet sources
say "just apply as much unsharp masking as you see fit".

But shouldn't there be an exact amount and radius of unsharp masking
that can be computed from the scanner's characteristics, seeing from
the things you said in the various threads (which I only very partially
understood, though)?

> [snip]

by LjL
ljlbox@tiscali.it

Kennedy McEwen ha scritto:

> [snip]

I forgot one more thing I wanted to ask.

Assume I settle on a solution I like for downsampling my vertical 4800
dpi to 2400.
As I wrote in the other post, I'm trying to path my scanner driver to
have it output 2400 dpi on the *horizontal* axis instead of
interpolated 4800, but I'm afraid I might not make it.

(SANE doesn't even natively support 4800x4800dpi with interpolated x
axis, I have to patch it for that; and then, doing 2400x4800dpi
*without* interpolated x axis looks very hard, because the driver isn't
really written with different x/y resolutions in mind.)

So, assuming I only get 4800x4800dpi with interpolated x axis, how do I
downsample that axis?
Bicubic resize in Photoshop gives blurrier data, *on the x axis* (also
on the y axis, but I've treated that in the other post), than a simple,
uninterpolated 2400x2400 scan does.

Must I know the exact interpolation algorithm used by my scanner, in
order to recover the original data? Or doesn't even that suffice, and
some data gets lost irremediably with the interpolation?

I suppose an interpolation that works as
pixel1 -- (pixel1 + pixel2)/2 -- pixel2 -- (pixel2 + pixel3)/2 --
pixel3 -- etc

should be easily "reversed". But I currently have no clue about the
interpolation used by my scanner, and don't know whether some
interpolation methods are "irreversible".

Is there even perhaps a safe bet about my scanner's algorithm, that is
do most or all scanners use a specific algorithm?


by LjL
ljlbox@tiscali.it

ljlbox@tiscalinet.it wrote:

> Many flatbed scanners claim to offer a vertical resolution that is
> twice the horizontal resolution, such as 2400x4800 dpi. I understand
> this to mean that, while there are only 2400 cells in the CCD, the
> stepping motor can move by steps of 1/4800th of an inch.
>
> Additionally, these scanners' CCDs usually do not have a single row of
> 2400 cells, but two rows of 1200 each, which are positioned at an
> half-pixel offset.
>
> Now, if this is true (please confirm), don't we effectively have 4x
> multi-sampling when scanning at 1200 dpi?

Actually, many linear CCDs are 8400 or 10200 cells (pixel sites), though
divided by three to give each colour Red, Green, and Blue. Kodak have some
nice White Papers on these.

So in theory an 8400 element linear CCD should be able to resolve 2800
dpi, and a 10200 element CCD should be able to do 3400 dpi. The reality is
that each pixel site is not that efficient, and only resolves a fraction
of the total possible. Often that can be 0.3 to 0.8 of the cell site for
commercial imagers. That would give us an actual best of 2720 dpi for the
10200 element CCD, and 2240 dpi for the 8400 element CCD.

You should be aware that there are linear CCDs in scanners that are less
than 8400 elements, and expect those to perform worse. The stepper motors
and scanner optics will affect resolution. The size of the cell site for a
linear CCD will affect resolution and colour. The scanner optics could
have the most affect on resolution, and often are the limiting factor in
low end and mid range gear.

>
>
> There are several issues that I don't find clear.
>
> First: when scanning at 1200 dpi, do scanners actually use both CCD
> arrays and "mix" the results (I'm not simply saying "average" the
> results, since it might be too simplicistic given the half-pixel
> offset), or do they only "turn on" one array?
>
> Second: when scanning at 2400 dpi, do scanners give out pixels in the
> order "1st pixel of 1st array | 1st pixel of 2nd array | 2nd pixel of
> 1st array | 2nd pixel of 2nd array", or do they somehow consider the
> fact that nearby pixel overlay one another by half their width?
> Of course, this also applies vertically, since while the motor moves by
> 1/2400th of an inch steps, pixels are 1/1200th of an inch "wide".
>
> Third: when scanning at "4800" dpi, what do scanners do about the
> horizontal resolution? Interpolation, I suppose. What kind of
> interpolation? Does it vary from scanner to scanner?

Interpolation can happen at an up or down value. It is controlled by fixed
sets of algorithms determined by the scanner manufacturers. Obviously,
this would vary between companies. In short, there is not one answer to
your questions, since different scanners will arrive at final files by
using different methods.

>
> And, do scanners that claim 2400x4800 resolution *really move the motor
> by 1/4800th steps when instructed to scan at 4800 dpi*, or do they just
> interpolate (since I know there are also other reasons for having
> 1/4800th stepping motors)? Does this vary from scanner to scanner?

Usually interpolated. Don't think this is all bad. While more resolution
and details might not be visible, overscanning can give smoother colour
transitions, since there are more final pixels in the resulting file. Of
course this only works if your printing output can use that extra
information.

>
>
> Now, let's see how all this relates to multi-sampling.. . . . . . . . .
> . . . . . .

Multi-sampling is usually just done to decrease noise or sometimes to help
colour accuracy. The effectiveness of this will vary for each type of
scanner, each scanner manufacturer, and the software in use.

>
>
> . . . . . . . . . . .
>
> Now you probably also see why I asked all those questions about scanner
> behavior above, since to answer my doubts about multi-sampling one must
> be aware of how the scanner really behaves, and whatever it does to the
> data *before* giving them out to the user.
>
> Perhaps this whole article can be "scaled down" to the question: is
> scanning at 4800 dpi and then scaling down to 1200 dpi (with what?
> bilinear, bicubic...) equivalent to 4x multi-sampling at 1200 dpi?
> (Make substitutions between 4800, 2400 and 1200 above, and you'll get
> the other possible scenarios)

Scanning at some multiple of the claimed resolution might improve your
scans, if that is what you are after with all this investigation. If you
really want to get technical, check out the Dalsa and Kodak web sites,
then find the White Papers for their linear CCDs. You will get far more
technical information that way, though maybe more than is practical.

Ciao!

Gordon Moat
A G Studio
<http://www.allgstudio.com/technology.html>

Gordon Moat ha scritto:

> ljlbox@tiscalinet.it wrote:
>
> > [snip]
>
> [snip]
>
> Scanning at some multiple of the claimed resolution might improve your
> scans, if that is what you are after with all this investigation. If you
> really want to get technical, check out the Dalsa and Kodak web sites,
> then find the White Papers for their linear CCDs. You will get far more
> technical information that way, though maybe more than is practical.

I don't want to get *too* technical.

In short, my scanner's got 2400 dpi horizontal. Sure, there are
complications: it's a "staggered" CCD, for one, and then all you've
written that I snipped (although I believe my scanner has three --
actually six -- linear CCDs, one for each color, not one -- actually
two -- very big linear CCD).

But let's just pretend for a moment that it's 2400 dpi optical, period.

What I want to do is scan at 4800 dpi in the *vertical* direction, i.e.
run the motor at "half-stepping". My scanner can do that.

The problem is twofold:

1) (the less important one) My scanner's software insists on
interpolating horizontally in order to fake 4800 dpi on both the x and
y axis, and I don't know how to "revert" this interpolation to get the
original data back (just downsampling with Photoshop appears to lose
something). But as you said, the interpolation algorithm varies between
scanners, so I'll have to find out what mine does, I suppose -- or,
hopefully, just manage to hack the open-source driver I'm using to
support 2400x4800 with no interpolation.

2) (the more important one) I, of course, don't want a 2:1 ratio image.
I just want 2400x2400, and use the "extra" 2400 I've got on the y axis
as one would use multi-sampling on a scanner supporting it. Yes, to get
better image quality and less noise, as you said.
But the question is, how to do it *well*?
I feel that I shouldn't just pretend I'm really multi-sampling (i.e.
taking two readouts for each scanline), because I am not. I ought to
somehow take into account the fact that each scanline is shifted by
"half a pixel" from the previous one.
Should I ignore this, and go on processing as if I were "really"
multi-sampling? Or should I downsample the image using bilinear,
bicubic, or something else more appropriate -- something that can take
the half-pixel offset into account?


I realize that simply downsampling the picture to 2400x2400 in
Photoshop or something gives decent results. But I'd just like to know
if there's something I'm missing.

In my mind, the "right" thing to do would be to consider the scan as
two separate scans (one made from the even scanlines, one made from the
odd scanlines); then merge the two image at an half-pixel offset. But
Kennedy said this is not such a great idea.
And in any case, even if Kennedy were wrong, I suppose there must be
some simpler transformation that gives the same result as the alignment
thing above... after all, it seems stupid to actually perform the
alignment and then the merging, when we know the misalignment is
exactly x=0, y=0.5.


All the other questions I posed in the original message were mostly
about how all this relates (if anyhow) with the fact the CCD is
"staggered" (which in turn means that each sensors already overlaps
each other sensors by half their size -- or *about* half their size,
since as you pointed out, things get actually a bit more complicated).


by LjL
ljlbox@tiscali.it

Kennedy McEwen ha scritto:

> [snip]

Hey, I've come across anrticle by you (in "EPSON Scan wouldn't make
large files (>1000 MB)", 2004), where you say

--- CUT ---

You are right about resolution, even the theoretical resolution gain is
marginal and almost certainly well below the sample to sample
production
variation. But I don't think there has ever been any question about
the
noise reduction aspect - if you resample the image back to 3200ppi
using
nearest neighbour resizing it is mathematically exactly the same as 2x
multiscanning. That yields exactly 1.414x noise reduction - and all in
a single pass with a scanner which formally doesn't provide
multisampling at all. With no significant resolution gain, the noise
reduction is just there in the image without resampling.

--- CUT ---

Is nearest neighbour resizing (though I've got no idea what it is! but
thankfully there's the Internet for that) what I am looking for?

I mean, "mathematically exactly the same as 2x multiscanning" is really
close to what I had in mind. Confirm?

But I think I can see some bad news, too, as in

--- CUT ---

Not always. Some, indeed most flatbeds these days, exploit what is
known as "half stepping" of the stepper motor drive. These half steps
are less precise than the full step and less robust because only half
the holding force is produced by the motor coils [...]

--- CUT ---

So does that mean that I might possibly losing more than I gain by
half-stepping? Although I suppose that, at most, I would end up with a
scan whose geometry doesn't perfectly match that of the original... or?


by LjL
ljlbox@tiscali.it