Digital BW, The Print

On Mon, Feb 10, 2003 at 09:09:39PM -0500, Austin Franklin wrote:
> Hi Tony,
> 
> > On Mon, Feb 10, 2003 at 07:37:50PM -0500, Austin Franklin wrote:
> > > What happens when you resample the image to accommodate some
> > mythical PPI to
> > > the printer is you degrade the image.  The printer dithering
> > algorithm is
> > > going to already resample the image simply to convert it to
> > printer dots,
> > > that's what dithering algorithms do.  Resampling the image yet
> > another time,
> > > as I said, degrades it.
> > >
> >
> > I know little about these things but is it not possible that the printer
> > driver resamples an image to a specific resolution before
> > performing its own
> > conversion to CMYK (or whatever inks it is using)?
> 
> I don't know what you are saying/asking.  It's a fact, the print driver HAS
> TO resample the image to convert from pixels to dots, that's what a print
> driver specifically does, in our case.
> 

I know that. I think, as usual, I am not making myself clear. Let my try
again. What I am saying is that prior to the driver doing its own
conversion from pixels to dots it may first do its own conversion to
resample the image to a fixed pixel resolution. 

Let's say the code routine in the driver which perfroms the pixels->dots
conversion has been written so that it can only accept an input resolution
of 360ppi. There will be a routine in the driver which would then first 
check to see whether the image resolution passed to it by the imaging
application/print spooler is at the correct resolution to be passed to the
pixels->dots function, and if not, it will do this resampling. Note there
are two separate things happening here in the print driver:

1. Resampling to a fixed PPI.
2. Conversion from PPI to whatever dpi/dither algorithm is used by the
printer and generation of the printer commands for the actual print.

It is this first step which I am contending could be sidestepped simply by
sending the correct PPI image to the printer.

> > In such an
> > instance is it
> > not also possible that if one presents an image to the printer driver
> > already at that dpi then that resampling does not have to be done?
> 
> You are NOT presenting an image in DPI (Dots Per Inch) to the print driver.
> You are presenting it in PPI (Pixels Per Inch).  Printers print in dots,
> images are in pixels.  There is a conversion that has to happen to get from
> pixels to dots.
> 

Yep I know. Apologies if my terminology was loose. See above.


> By your comment I can conclude you don't understand how the pixel to dot
> conversion is done (or that it even needs to be done!), and what dithering
> is?  You might want to read up on it, and there are some very excellent
> resources on the web that could explain it.  Adobe actually has a good
> write-up on this, and if I can find it, I'll pass the URL along.  If you
> would like me to explain somewhat I can, and possibly someone else can give
> you some explanation as well.
> 

I do understand pixel to dot conversion and dithering to a certain extent.
Of course if you do have any detailed URLs I'd be happy to earn more.

My question really was "where does the PPI resampling of an image happen, if
indeed it happens at all"? I'm not talking about the conversion of the pixel
data into the cmyk dots that go down as a dither pattern, just the pixel
resampling of the image. I am, above all asking whether it is an atomic
transformation which happens at exactly the same time that the pixel-dot 
conversion is done, whether is it a step which occurs before the pixel to
dot transformation although still within the driver, or whether there is
actually no resampling of the image data by the print driver prior to the
conversion to actual printer dots.

> > Given the choice of supplying my own resampled image to the printer driver
> > and knowing the driver will not resample further
> 
> It doesn't work that way.  As I said above, you are sending PIXELS to the
> printer driver, and it is converting to DOTS by the driver, then these dots
> are sent to the printer.
> 

Again Austin, please see my comments above and please let me know what, if
any of it, is correct. 

Tony