[filmscanners] RE: Suggestions for scanning 4x5 transparencies

["Paul D. DeRocco" <pderocco@ix.netcom.com>]

> Laurie Solomon wrote:
>
> While I will make no claims to completely comprehending the technical
> arguments and specifics of the engineering nor do I wish to argue
> those things, I do think that your example is a bad one in that it is
> an example of exactly the oppositie of what I said.  I said that the
> multiples of 720 had to be even multiples which would be along the
> lines of the following
> 90,120,150,180,210,240,270,300,330,360,390,420,450,480,510,540,570
> ,600,630,6 60,690....750,780,810,...,1440,..., and 2880 - not uneven
> or odd multiples which 700 dpi would be.  I would even go as far as
> to suggest that the reason that Epson talks of those resolutions like
> 720, 1440, 2880, and 5760 un terms of dpi rather than ppi may be
> precisely because it is in printing terms which does account for
> dithering and stands as such as being equivalent in that regard to
> halftones whose resolutions take into account the line screen
> multiple of the halftone dots in stating halftone resolutions. At any
> rate, it is because you have selected an uneven multiple of the 720
> figure that you wound up with the extra line of black and an extra
> line of white (or double width line on one of the lines of each).  In
> that regard, I believe you example illustrates the reasoning behind
> my statement and supports my contention that it is more frequently
> than not factors such as imputing an uneven multiple of the 720 dpi
> figure that produces the aliasing and artifacts more than the
> closeness of the match between the two numbers.

The numbers you mention all seem to be multiples of 30, but most are not
multiples of 720, so I'm not sure what you're driving at. But what
eliminates all moire is to make the resolution of the image an integral
_submultiple_ of 720. (That way, the spatial harmonics of the image that
exceed the 360lpi Nyquist alias back on top of other harmonics, instead of
to harmonically unrelated frequencies--if anyone's interested.) It isn't
sufficient to make them submultiples of the dot resolution (e.g., 1440 or
2880), since, as I said, the aliasing occurs when the driver resamples to
720ppi, not when it dithers to 1440 or 2880dpi. For instance, if you print
alternating 1-pixel black and white lines at a resolution of exactly
1440ppi, the dumb resampling will simply choose every other pixel, resulting
in either all white or all black, not gray.

--

Ciao,               Paul D. DeRocco
Paul                mailto:pderocco@ix.netcom.com

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