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Re: [css3-values] define 'dot' to be a device pixel

From: Liam R E Quin <liam@w3.org>
Date: Thu, 03 May 2012 16:10:05 -0400
To: Lea Verou <leaverou@gmail.com>
Cc: Edward O'Connor <eoconnor@apple.com>, www-style@w3.org
Message-ID: <1336075805.7472.25.camel@localhost.localdomain>
On Thu, 2012-05-03 at 16:58 +0300, Lea Verou wrote:

> Love the idea, and the name. How do you picture it working in print or 
> other media?

In digital printing the term "dot" refers to the hardware resolution;
for commercial offset lithography with plates the term refers to the
resolution used by the raster image processor (RIP).

[longer answer below - tl;dr is that yes, more than one device dot is
used per pixel in print too, although not for exactly the same reason]

One image pixel value in a given colour (cyan/magenta/yellow/black/spot)
is usually represented as a blob of dots (the device dots), with more
dots causing more ink to be used at that location. The process is called
screening, for historical reasons - but it makes an effect a bit like a
screen used for screen-printing, or like a window-screen to keep out
insects or gauze to sift flour. The screen process is needed because
each device dot can either have ink or no-ink, and the ink is always the
same colour, so you simulate grey by using smallr or larger blobs of
ink, and the eye combines the blobs with the unprinted paper between the
blobs to make grey.

For commercial printing the colours have to be processed separately,
with screens at carefully-chosen angles to avoid moiré patterns, and
with attention given to avoid having too much ink at any one place (or
it might flake off or build up on the roller causing mechanical
problems, or make pages stick together too much before the ink is dry,
for example). In a laser printer or inkjet the dots are in essentially
fixed locations.

So the dot concept is actually already in use, but one image pixel in
black (let's stick to greyscale to make it easier) might correspond to
over 100 device dots - a 10x10 "blob" gives about 100 levels of gray,
although since the ink spreads during printing you get slightly fewer in
practice.  (blob is not a technical term, I'm using it to avoid some
overloaded terminology) The number of blobs per inch is measured as
"lines per inch", so on a 1,000 dot-per-inch device with 10x10 blobs you
have 100 blobs to the inch, and it's a 100 line screen.

Fine art printing uses a 150-line-per-inch screen or sometimes
175-lines, and newspapers generally use 75 lines per inch or sometimes
as low as 50, which is why you can see the blobs more easily, especially
on black-and-white photos where there's only one layer of ink.

This is the difference between pixels per inch and dots per inch when
printing, and "dot" thus already has a technical meaning in printing.

Hope that helps.


Liam Quin - XML Activity Lead, W3C, http://www.w3.org/People/Quin/
Pictures from old books: http://fromoldbooks.org/
Received on Thursday, 3 May 2012 20:14:29 UTC

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