Re: ScanROM 4E

[P3D Gregory J. Wageman <gjw@xxxxxxxxxx>]

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.  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.  A 3x3 matrix with a 300dpi printer gives
an effective resolution of 100dpi with 10 greyscale levels.  With a CMY
cartridge the same 3x3 matrix gives 1,000 colors.  This is not very
many and leads to very obvious "false contouring", which can be mitigated
to some extent with suitable dithering (which introduces problems of its
own, e.g. dithering "noise" [speckles]).  4x4 is better at 4K colors, but
you've now reduced your 300 dpi to screen resolution (75dpi typical).

>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.

        -Greg W.


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