Austin & Keith,
Posting 2
"I have not read the article, but it sounds as if you are reading the
article correctly because that is exactly what the Epson driver does -
however your concerns are not particularly sound, since this upsampling
is merely one stage in several which together make up the half-toning
algorithms which Epson use to convert from the pixels per inch that you
choose to send it and the dots per inch which it requires to print.
You can find the resampling resolution which Epson use by a simple test,
printing blocks of single pixel wide lines with single pixel gaps
between them at different ppi. As the ppi increases, the lines become
finer and the blocks become smaller, as you would expect. As you get to
higher ppi figures you will need a loupe to see the lines distinctly,
since the resolution of the printer exceeds that of the naked eye.
However, as you approach 720ppi you will find that not only do you see
the original lines on the printed page, but you see faint images of
broader lines at resolutions which correspond to the difference between
720ppi and the input resolution you are printing at. These are alias
images - false lines caused by the resampling of the input data. So if
you print lines at, say 650ppi, then you will not only see lines that
are on average 325line pairss per inch (you need one pixel width for the
line and one for the gap, so that the number of line pairs is always
half the ppi) but also "ghost" lines at 35line pairs per inch, since
720-650=70ppi, which is the aliased image.
Eventually, at exactly 720ppi, the block will print either completely
solid colour or completely white and shifting the image by a single
pixel will change between these two states. What has happened is that
the line pattern has aliased to 0 line pairs per inch, so each "ghost"
line is infinitely wide (clearly limited in size by the input size of
the test image.
This in itself is a clear indication of resampling to 720ppi or an
integer multiple thereof, but increasing the ppi of the test image even
further will result in "ghost" lines which reduce in width. So that if
you print your test image at 790ppi you will find the lines look
identical to the test image printed at 650ppi - the only difference
being that the test patch itself is smaller. Once again, the "ghost"
lines are at 35line pairs per inch with fine detail at, on average,
325line pairs per inch.
It is the existence of these two "mirror" images centred around 720ppi
which proves that the Epson driver is resampling to 720ppi. What is
also worth noting is that this 720ppi resampling occurs in both axes
whether the printer is set to print at 720ppi, 1440ppi or 2880ppi -
720ppi is the MAXIMUM resolution that the printer can reproduce on the
page. I haven't checked to see if the resampling resolution is reduced
when printing at, say, 360dpi - but I suspect it is.
After resampling to 720ppi, the Epson driver then applies a stochastic
dither to reduce the bit depth of the image to match the tonal
capability of the particular printer, which is defined in terms of the
number of ink colours, the dot sizes which is capable of depositing on
the page and the dot resolution. A 6 colour, 2880dpi, 6 dot size
printer is capable of finer tonal reproduction than a 4 colour, 720dpi,
1 dot size printer for example, and so the 720ppi image requires a
coarser dither with the latter than the former. The actual algorithm
used is quite complex, however it has the property of spreading the
dither over several pixels of the original image where smooth tone
variations occur and also printing individual pixels at reduced colour
accuracy where fine detail occurs in the image. Generally this yields
good results on the page, with near photographic tonal range and fine
detail reproduction, but occasionally (as with some cases of low level
film grain) results in the contrast of fine detail being exaggerated
when high resolution input files are used.
I suspect that the author of the page you are reading has some
alternative algorithm for both the resampling and the dither which
outperform the standard Epson algorithms in some, perhaps all,
situations. It could, for example, be possible to resample at the
native resolution of the printer itself - eg. 1440ppi or 2880ppi - and
obtain some marginal improvements over the standard Epson algorithm, but
I would not like to guess at how significant the difference would be.
Certainly the difference would be detectable in specific test images,
such as that discussed above, but for general photographic images I am
not entirely convinced the difference would be worthwhile.
I don't think you should be overly concerned about the resampling issue.
As above, using highly synthetic test images, it is possible to
determine that resampling has occurred and what the frequency of that
resampling is. However, for general images of less than 360ppi this is
usually imperceptible.
Why should you be worried about resampling on a printer, whether by
pixel replication or some interpolation algorithm, when the resampled
pixels are beyond the resolution of your eye? This is not the same
thing as resampling on a scanner - where the image can be printed out at
any size and, hence, the effect of resampling can be clearly visible.
Effectively, your eye is resampling every image you see by the structure
of the rods and cones on your retina - whilst more random than the
orthogonal matrix used in the Epson printer the resampling in your eye
is significantly coarser, which is why the images from these printers
look acceptable at all! :-)
In short:
Do Epson printer drivers resample to 720ppi? : Yes.
Is the resampling perceptible? : In around 99.99% of cases, no.
How much should you be concerned? : Around 0.01%. ;-)
--
Kennedy"
Bob Frost