Re: ScanROM 4E

[P3D Peter F Davis <pd@xxxxxxxxxxxxx>]

John Ohrt wrote:

> In an ideal world, your scanner needs twice the resolution of your
> printer.  So if you have an inkjet which requires 4 dots to compose a
> pixel, you divide the inkjets resolution by 2 in each dimension (ie
> 720x720 dpi -> 360x360 pixels per inch and then double each dimension to
> get the minimum scanner resolution, 720x720 pixels per inch).  If you
> have a dyesub printer, 1 dot -> 1 pixel, so if you have a 300x300 dpi
> dyesub, ideally you want a 600x600 pixel per inch scanner.

This is absolutely not true.  Each printer "dot" is either on or off,
so you only get two values per color.  The way halftoning works to get
intermediate color values is by clustering dots to approximate the
effect of larger dots.

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.

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.

-pd

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

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


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