Re: ScanROM 4E

[P3D John Ohrt <johrt@xxxxxxx>]

P3D Peter F Davis wrote:
> 
> 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.


Nope.  The way halftoning works is to vary the dotsize.  Ask any
printer.


> 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.


Nope. You are confusing spatial resoltution with intensity resolution.

You can simulate an enhanced intensity resolution, but the pixel is the
area needed to encompus all those dots!  So if it takes 16 dots to give
you sixteen grey scale levels and the printer is 600x600 dpi, it is
150x150 pixels per inch, requiring a scan of 300 pixels per inch.

The Nyquist theorem relates solely to spatial resolution and roughly
states that if you do not sample the input at an optical spatial
resolution in excess of twice the final spatial resolution, then
aliasing will occur.  The only way to prevent aliasing when you violate
this theorem is to destroy information by filtering.  Sometimes though,
the aliasing isn't noticable/objectionable, but that is a judgement
call.

 
> Also, when resolution is specified for a color printer, are they
> counting each color as a separate dot?

For an inkjet or colour electrostatic, or a press yes!

Because they are using a CYMK process requiring separate dots.


For a dyesub no, because one dot on a dyesub can assume a nominal 16M
colours as opposed to 256 intensity levels of the same hue of an inkjet.


>  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.

You are correct in your description of the technique, but a 400 lpi
colour image only has 133 lpi spatial resolution on a press.

Regards,
John

ps. I have worked on prepress work.  Also lpi is tricky to compare to
pixels per inch because you have to understand the technologies and the
intracies and assess the quality of each particular image.

--
John Ohrt,  Regina, SK, Canada
johrt@xxxxxxx



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