Laurie,
Kennedy McEwan published the 'proof' and Epson also state the facts in one
of their technical support documents -
. This says quite
clearly that the large-format printers resample to 360 ppi and the desktop
printers to 720 ppi, and that that is the reason for the maximum print sizes
from images in Photoshop (which has a 30,000 pixel size limit).
Here is Kennedy's posting on the Epsonx7x list:-
"What you appear to describe is FM modulation of an alias component due
to changes of the actual dpi achieved by the printer due to transport
irregularities on that final edge of the sheet leaving the printer.
You might think that the more resolution that you throw at the printer
the better the results, and that is true - up to a point. Although the
1270 has the capability to print 1440x720dpi, like most Epson printers
it cannot print more than 720x720ppi. (Note the difference here between
dpi & ppi). Every image that you send to the printer is first resampled
to 720ppi before being converted into ink dots which are placed on the
page at the dpi setting you have selected.
Resampling has the same restrictions as sampling - if the original image
has sufficiently fine detail it will be corrupted by the resampling
process. This is called aliasing and the effect is very predictable in
both the characteristic corruption and its intensity. Different types
of resampling algorithms change the intensity of this corruption, but
not its characteristic.
Since the Epson printer resamples the image at exactly 720ppi then you
should, in theory, send it data which is at an integral division of
720ppi - this means 720ppi, 360ppi, 240ppi, 180ppi etc. since all other
resolutions can, in theory, produce aliasing. In practice, the printer
appears to use a reasonable (but not perfect) interpolation algorithm
which means that even for other resolutions the intensity of the aliased
signal is moderated. This is also helped by the fact that few scanners
reproduce 100% modulation at their resolution limit (indeed, for reasons
I won't go into at the moment, if a scanner did this the results would
be VERY POOR). Royce or Chris Bair posted a set of synthetic images on
their site a few months back which had 100% modulation and showed the
effect of aliasing quite clearly at non integer divisions of the
fundamental resampling, but generally this is not an issue in scanned
photographic images.
If you print at 513ppi on an Epson then, in theory, there is an aliased
component of the image at 153ppi - in itself this is not enough to cause
a visible artefact unless the image contains high contrast repetitive
patterns.
However the alias will be highly FM modulated by paper transport
irregularities, and I guess this is what you might be seeing. For
example, if the paper is not being transported at exactly 720ppi, but
740ppi, then the alias will shift to 143ppi. Its a lot more complicated
than this because the paper is moved in much coarser steps than
1/720ppi, the 720ppi is achieved by the placement of the nozzles in the
head, so the shift in alias can be much greater in bands on the final
part of the image.
If you are printing with the driver set to Maximum print area (ie.
minimum margins all round, instead of the 1" or so margin on the
trailing edge of the sheet) then the final inch or so of the print will
have poorer transport stability than the rest of the sheet, and Epson
warn of this in the user manual for the 1270. It might be sufficiently
unstable to reduce the alias to a resolution that would be easily
visible.
If you then resample the image to 240ppi using bicubic resampling then
you significantly reduce the amplitude of the aliased component in the
image itself (since bicubic reproduces aliasing with little amplitude).
Furthermore, when resampled by the printer, there is no alias component
of this 240ppi image to be FM modulated. In short, you have removed the
fine detail (by resampling with a better algorithm than available in the
printer) that is being modulated by paper transport irregularities.
The best solution is not to print within that final 1" edge of the sheet
- as recommended by Epson in the printer handbook. If you must, then
you could (as you have done) resample to a lower resolution that will
avoid an alias component that can be FM modulated by the transport
instability. If you print on high resolution paper, such as Premium
Glossy, then you will still see some banding due to the transport in
this final edge on the print.
--
Kennedy"
Bob Frost.
----- Original Message -----
From: "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.
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