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


Gordon Moat ha scritto:

> ljlbox@xxxxxxxxxxxxx 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@xxxxxxxxxx

.

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