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

ljlbox@xxxxxxxxxxxxx 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>

.

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