Re: Multi-sampling and "2400x4800 dpi" scanners

Kennedy McEwen ha scritto:

> In article <1126228438.116479.174010@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
> ljlbox@xxxxxxxxxxxxx writes
> >
> >Don ha scritto:
> >
> >> [snip]
> >>
> >> Check the archives (for example on Google). Kennedy
> >> (<rkm@xxxxxxxxxxxxxxxxxx>) 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@xxxxxxxxxx

.

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