[filmscanners] RE: Suggestions for scanning 4x5 transparencies

["Laurie Solomon" <laurie@advancenet.net>]

> No, it's been proven by people on one of the Epson lists (perhaps it
> was the old Leben list) that the driver resamples to exactly 720ppi.

I belong to both those lists (or I did belong to the Leben list until it
went defunct) but I do not remember seeing any such proof.  I do remember
some discussion about this on both lists and that is were I got my
information from.  Some one may have said what you claim and I will take
your word on that; but that some one proved it may be overstating things a
little and certainly with respect to at what stage in the printing process
this driver resampling takes place.  If I remember correctly, the discussion
was somewhat vague and ambiguous as to when the resampling took place in the
process and if all resampling had to go through the 720 dpi conversion first
before being resampled to 1440 or 2880 dpi or if it was a direct input
resolution to the end resample resolution if that end resolution was to be
1440 dpi or 2880 dpi.

> For instance, if you create an image with alternating one-pixel black
> and white lines, and then set the resolution to 700ppi (pixels per
> inch), you've created an image whose spatial frequency is 350lpi
> (lines per inch).

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.

Like you, I find that most of this sort of discussion seems to be academic
in the case of my experiences since I find that I have experienced and seen
few defects like aliasing and artifacts as long as the file I input into the
Epson printer has a resolution between 200 and 360 ppi/dpi resolution.  When
I experience morie patterns it is usually due to the fact that I scanned a
previously halftoned image.

filmscanners_owner@halftone.co.uk <> wrote:
>> Laurie Solomon wrote:
>>
>>> The 720dpi figure (360 for large format printers) is the exact
>>> resolution that the Epson driver converts to _before_ dithering.
>>
>> That was not my understanding; but I am willing to stand corrected.
>> My understanding was that the input resolution should be between 200
>> and 300 dpi for the desk top photo inkjets where the printer would
>> convert it to 720 or some higher multiple of that as a result of the
>> dithering process and that the 720 dpi or higher figures should not
>> be used to determine input resolutions since one does not have a
>> means of factoring in the dithering factor unless one had Epson's
>> proprietary algorithms.
>
> No, it's been proven by people on one of the Epson lists (perhaps it
> was the old Leben list) that the driver resamples to exactly 720ppi.
> This means that printing an image with strong high spatial frequency
> content at something close to but not equal to 720ppi will produce
> the same sort of aliasing that you get when resampling to 720ppi in
> Photoshop with "nearest neighbor" mode selected--which is why
> resampling manually in bicubic mode can be used to solve the problem.
>
> For instance, if you create an image with alternating one-pixel black
> and white lines, and then set the resolution to 700ppi (pixels per
> inch), you've created an image whose spatial frequency is 350lpi
> (lines per inch). If you give that to the 720ppi driver, you'll get
> 350 black lines (and 350 white gaps) for every 720 pixels, or 35
> lines for every 72 pixels. This means that there will necessarily be
> one double-width black line (and one double-width white gap) every 72
> pixels. Since 72 pixels is a tenth of an inch, this represents a
> 10lpi alias component, which corresponds to the difference between
> the Nyquist frequency of 360lpi (half the 720ppi resampling
> frequency) and the 350lpi image.
>
> The dithering is something separate that happens after the resampling
> to 720ppi. That is, the resampling to 720ppi has nothing to do with
> the dithering, and is unaffected by the precise dithering algorithm.
> It seems to me that it ought to be possible to invent a dithering
> algorithm that accepts input at any resolution, and has the necessary
> resampling filtering built right in. Epson may have not done this
> because they don't know how, or perhaps because there are patents
> that prevent them. I don't know.
>
>>> And since the conversion to 720dpi is an unfiltered
>>> conversion, there are situations where it causes visible aliasing,
>>> and you get better results by manually resampling.
>>
>> Here again, that is not my understanding; but I am willing to be
>> corrected. Alas, in such situations, I am not sure how one determines
>> if the artifacts and alaising are a product of what you suggest or
>> some other factor in the process.  I do know that many suggest using
>> an input resolution that is a even multiple of 360 dpi and that an
>> uneven multiple of that resolution will cause some of the same
>> problems as you suggest.
>
> The only really objectional form of aliasing is moire, and is
> generally pretty recongizeable. To avoid aliasing in this case,
> resample up to the next higher integral submultiple of 720 (that is,
> 720, 360, 240, 180, 144, 120), or down to 720 if you're starting with
> a resolution above 720.
>
> Aliasing can also occur on incoherent high-frequency information, like
> leaves on a tree. However, in this case, since what is being
> resampled is somewhat random, the aliasing will also be random
> (albeit coarser). Since all randomness looks more or less the same,
> the resulting aliasing can actually be pleasing. For instance, a
> scenic shot of a view across a forest, when "properly" resampled,
> might produce smooth green tones with some variations from tree to
> tree. But if the distant leaf detail is allowed to alias, it can
> produce graininess in the image that suggests more detail than is
> really there, but still looks good to the eye.
>
>> Thus, in light of your post, I would suggest that the user would have
>> to test for themselves to see if they needed to resample or not.
>> I,however, would suggest ( and I do not know if you would agree or
>> not) that the least amount of resampling one does the better in most
>> cases; so use interpolation and downsampling sparingly and only when
>> absolutely needed.
>
> I agree. Since aliasing is only really objectionable when there is
> _coherent_ high-frequency spatial energy (resulting in moire
> patterns), doing manual resampling to avoid it is only necessary on
> certain images. For instance, a distant picket fence, or a tweed
> pattern, or focusing on a window screen. These images are probably
> rare enough that you can just pretend this problem doesn't exist
> until you see some moire in a particular print, and then just redo
> that print. Personally, I've yet to see it crop up in any of my
> prints.

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