Re: ScanROM 4E

[P3D Peter F Davis <pd@xxxxxxxxxxxxx>]

> Peter Davis wrote:
> 
> >Consider the black and white case:  If you printed a 600dpi scan on a
> >600dpi laser printer, you would only get pure blacks and whites ... no
> >grays.  However, if your printer resolution was twice the scan
> >resolution, you'd get effectively 4 gray levels, 4 times the
> >resolution gives you 16 gray levels, etc.  So your printer resolution
> >needs to be HIGHER than your scanner resolution, not the other way
> >around.
> 
> Be careful with terminology here.  A printer that uses 4 dots (2 in each
> axis) to make a 5-level "pixel" (0, 1, 2, 3 and 4 dots on) is printing at
> 1/2 the resolution, not 4 times. 

Be careful of reading here.  Reread my sentence.  I said the printer
resolution (actually physical resolution) must be higher than that of
the scanned image, and the ratio determines the number of intensity
levels the printer can reproduce.  Roughly, the number of intensity
levels = (printer res./scan res.) squared + 1.

> I've written drivers for HP inkjets that
> do exactly this and you must divide the printer resolution down by the
> number of dots per pixel. 

We're in agreement.

> >Also, when resolution is specified for a color printer, are they
> >counting each color as a separate dot?  I don't think so.  Typically,
> >the colors are layed down over each other, so they occupy the same
> >pixel space.  Over years of traditional printing, conventions have
> >evolved about rotating the screens (the grid of possible dots) for
> >each color to avoid moire patterns, etc.
> 
> Don't confuse offset printing techniques with digital printing.  The
> inks used in inkjet printers aren't opaque like printer's inks are.
> In order for printer's inks to blend, they MUST be printed using grid
> patterns (screens) that don't overlay each other.  Inkjet inks can be
> mixed on the paper and the result is a blended color.

Printer's inks aren't opaque.  This is why designers have to worry
about overprinting, trapping, and related issues.  In any case,
neither offset presses nor digital printers reserve separate grid
locations for different colors of ink.  They are printing in multiple
passes on top of each other.  The use of halftone screens at various
angles is to minimize moire effects.

-pd

--------
                                Peter Davis
                        http://world.std.com/~pd

            "If you can't behave yourself, who can you behave?"


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