"Mayayana" wrote:
| You can leave it to the printer driver to resample the image. All you
| do is feed it a file and tell the printer the desired output size
| width x breadth.
|

Feed it from where? Isn't there some software
that's showing you the printer dialog? In my
experience that Window is actually just feedback
for the software that has an image open. The software
then uses your choices to adjust what it sends to
the printer. There's usually an option to also open
the specific printer settings window, but my printer
settings don't include anything like print size. They only
include print-specific settings. I've never seen a printer
settings dialog that did offer anything like resampling.
Unless you're referring to specific high-end printers I
think you may be mistaken.

The dialog box is probably part of the print driver,
though some programs may usurp part or all of the
interface functionality. There is no circumstance where
a user actually interfaces directly with the printer to
make a print. It always goes through a print driver.

Every print driver "offers" resampling. When the user
sets a physical size dimension that sets the resampling
parameters. If the user then tells the program in use
to change the pixel dimensions, that will also change
the print driver's resampling.

That is because the image file has no physical
dimensions, only pixel dimensions. And the printer can
only print images at one specific pixel rate. (A given
printer might have a "high resolution" mode, but that is
for text and line drawings, not photographs. It also
may have a draft mode for fast prints, but again that is
not for photgraphs. There will be just one PPI rate,
e.g. 300 PPI for Canona and HP, and 360 PPI for Epson.)

Given that Epson as an example prints at 360 PPI, if
the user sets the print driver for a 10 inch wide print,
the *only* pixel dimension that can do that is 3600 pixels.
If the image being sent to the printer has 1800 pixels
across, it *must* be up sampled to 3600. If it has 7200
pixels across it *must* be down sampled to 3600 pixels.

That is what the print driver does for you. But, you
don't get to see what it looks like before being
printed. And it is a simple fact of life that when an
image is resampled to a different pixel dimension it
needs to be resharpened too, for best results. Down
sampling is usually done with an algorithm that has a
slight amount of "ringing", which produces a very faint
halo. That means less sharpening is needed. If the same
algorithm is used for up sampling it will not look good.
Instead a filter that has no ringing and might even
slight blur edge transitions is best; and that requires
a different kind of sharpen than does one that was down
sampled.

Hence you are far better off to do the size adjustment
locally, and the sharpen the results by looking at the
effect, before sending an image file to the print
driver.


--
Floyd L. Davidson http://www.apaflo.com/
Ukpeagvik (Barrow, Alaska)