Laurie,
The images were resized first. The 4000 dpi 35mm scan, at about 1.3x0.8
inches, was resized by reducing resolution to 400 without resampling,
yielding obviously a 13 x 8 in image. After saving, I then took that image
and resampled it to 360 dpi without changing dimensions.
Finally, I just cut out a small section of each image for printing purposes
but took care not to resample. Before printing, I confirmed that PS7 showed
the 400dpi and the 360dpi values were retained.
I believe the results are reliable.
Stan
-----Original Message-----
From: filmscanners_owner@halftone.co.uk
[mailto:filmscanners_owner@halftone.co.uk]On Behalf Of Laurie Solomon
Sent: Monday, October 28, 2002 11:59 AM
To: snsok@cox.net
Subject: [filmscanners] RE: Suggestions for scanning 4x5 transparencies
> The "400" was not resampled from the original 4000 dpi. This is a
> snippet. The "360" was bicubic resampled in PS7 before going to the
> printer.
Like Paul, I would not have expected you to find any difference in detail or
patterns in this case unless you scanned an already halftoned image. Such
differences really have to be exaggerated to be readily noticed even under a
loupe.
However, having said that, I will note that by merely snipping as you put it
from the original 4000 dpi image without resampling, you do not have a 400
dpi image but a 4000 dpi section of an image; and if you merely resized the
image without carrying out any resampling by changing the dimensions, then
you have also effectively changed the effective resolution from 4000 dpi to
something greater if you made the image smaller. Thus, we really have no
knowledge of the dpi of the so-called 400 dpi image that you sent to the
printer to be able to do an accurrate comparision to begin with for
putrposes of this discussion.
filmscanners_owner@halftone.co.uk <> wrote:
> I printed a small area from the same digital image at 360 and at 400
> dpi.
>
>
>
> The "400" was not resampled from the original 4000 dpi. This is a
> snippet. The "360" was bicubic resampled in PS7 before going to the
> printer. The difference in size is just a slight error in snipping.
>
> Under the loupe, I can't see any difference in detail or in the
> patterns within the azalea petals.
>
> Stan
>
> -----Original Message-----
> From: filmscanners_owner@halftone.co.uk
> [mailto:filmscanners_owner@halftone.co.uk]On Behalf Of Laurie Solomon
> Sent: Sunday, October 27, 2002 6:40 PM
> To: snsok@cox.net
> Subject: [filmscanners] RE: Suggestions for scanning 4x5
> transparencies
>
>
>> 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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