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

Gordon Moat ha scritto:

> ljlbox@xxxxxxxxxxxxx wrote:
>
> [snip]
>
> > I don't want to get *too* technical.
>
> Though you want to hack the driver. ;-)

Programming is my field, optics isn't. Not that I'm terribly good at
that either (in fact I haven't hacked the driver succesfully so far),
but off hand I can't think of anything I'm terribly good at.

>> [snip]
>
> If it is moving the optics, and not the CCD, then it has a three or four row
> CCD with RGB filtering over it. If it is moving the CCDs, then it could be
> several. Of course, you could crack it open and find out. ;-)

Well, I dunno, but it looks like it's moving everything through the
glass.
In any case, I'm not really interested in the CCD layout, except for
its - ehm - staggeredness, in that the 2400dpi are obtained with two
shifted 1200dpi CCDs.

> > But let's just pretend for a moment that it's 2400 dpi optical, period.
>
> You would be lucky for it to be much better than half that, but for sake of
> discussion . . . . . . .

Well, if you can suggest a simple method for measuring real resolution,
I would be happy to try and find out. Well, I wouldn't necessarily be
*happy*, on a second thought.
But anyway, yes, let's just pretend for the sake of discussion.

> > 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.
>
> Make that a three fold problem . . . how and what do you plan to use to view
> that image? In PhotoShop, you would view 2400 by 4800 as a rectangle; if all
> the information was 2400 by 2400 viewing, then you have a square; if you want
> a square image and have 2:1 ratio of pixels then your square image will be
> viewed like a stretched rectangle. This is similar to a problem that comes up
> in video editing for still images; video uses non-square pixels, so the
> square pixel still images need to be altered to fit a non-square video
> display.

But keeping the image at a 2:1 ratio is not what I plan to do.
What I plan to do is to take it back to a 1:1 ratio by downsampling on
the y axis: this way I get a 2400x2400 dpi picture, which is less noisy
than the same picture taken directly at 2400x2400 dpi from the scanner.

My main doubt was, what is the "best" or the "right" algorithm to do
the downsampling? I usually just use (bi)cubic resampling when I want
to resize something; but in this case, I've got some specific
information about the original data -- i.e. that each line is shifted
by half a pixel from the previous line (a quarter of a pixel actually,
since the CCD is staggered, but I think this shouldn't be important as
long as I want to keep the final image at 2400x2400).

I just thought that this knowledge might have allowed me to choose an
appropriate downsampling algorithm, instead of just using whatever
Photoshop offers.

> > 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*?
>
> Or how to actually still view it as a square image.

By downsampling.
After all, it's the same with scanners that can do "real"
multi-sampling, only that
1) lines do not exhibit a sub-pixel shift, since the CCD is kept in the
same positing for each sample
2) the scanner firmware, or the driver, hides it all to the user and
just gives out a ready-to-use 1:1 ratio image

> [snip]
>
> I have not heard of anyone outside of Canon still using a staggered idea. I
> think Microtek may have tried it, or possibly UMAX.

Well, Epson certainly does it in (most of) their flatbeds.

> In order to really do
> something different with that, much like the video example above, it seems
> you would need to get the electronic signal directly off the CCD prior to any
> in-scanner processing of the capture signal. Basically that means hacking
> into the scanner. I don't see how that would be practical; even if you came
> up with something, you still have a low cost scanner with limited optical
> (true) resolution and colour abilities.

But I don't think the data that come out of my scanner are very much
adulterated (at least if I disable color correction, set gamma=1.0 and
such stuff).

Well, when I scan at "4800 dpi" from Windows, they're certainly
adulterated in the sense that interpolation is used on the x axis, in
order to get a 1:1 ratio, apparent 4800x4800 dpi image.

But I know (really, I'm not just assuming) that this interpolation is
done in the *driver*, not in the scanner; so assuming that I can hack
the Linux driver to support 4800 dpi vertically, horizontal
interpolation becomes a non-issue: after all, it's easier to *not*
interpolate than to interpolate!


by LjL
ljlbox@xxxxxxxxxx

.

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