Re: thematic collecting - scanning images of stamps

Rodney,


It looks like it, that if I understood Colin, and I think I do, each apparatus will split up an image in whatever the resolution
that apparatus can handle. The scanner may have 1200 sample-dots/inch horizontally and 900 sample-lines/inch vertically as the scanner moves in both directions. The encoded result of the scan will be converted on a computer screen to pixels horizontally/vertically depending on the resolution of the screen; in a similar way the printer may use 600/300 dots/inch horizontally/vertically depending on the resolution of the printer head, etc.

I prefer to use to pixels when I refer to a computer or television screen; and to dots when talking about scanners of printers.

It becomes rather complicated when I am scanning a postage stamp that had been printed in let's say photogravure with a dpi of
300, and then using that scan in an article that should be printed in a rather rude offset-lithography. Sometimes all the details get lost ;)

For those who just need a nice picture printed it may not be relevant, but if you need to publish an already screened illustration, it is extremely important to know how to get the best results...

groetjes, Rein

Op Sat, 29 Mar 2008 13:19:34 +0100 schreef rodney <pookiethai@xxxxxxxxxxxxxx>:



G'day Rein,
I am still clarifying a point I still have
regarding exactly what you mention.

This was a reply from the scanning NG for your consideration
===============================================
rodney wrote:
I am a little perplexed in the language used by scanners, eg:

I wrote:

This is important to keep a standard scan size amongst all your images
(I scan all mine at 500dpi [dots per inch] which results in an image
around 160Kb for a standard machin stamp)

A respondent wrote:

I would agree with everything Rodney mentions, excepting the above,
There are no dots in scanning. You ae scanning an object into pixels, thus
ppi is the term.

--------------------

How is it my Epson shows "dpi" and Wiki siggests dpi is the language
sent to the printer re image size.
Was I incorrect?

Thanks

There is great confusion between the two terms. Pixels per inch - ppi -
is always used with images, since images are composed of pixels. Dots
per inch - dpi - is used only with regard to printers, since printers
print dots - of ink.

Most printers print at a fixed ppi, 600 ppi for Canon and 720 ppi for
Epson, and the printer drivers convert your image at whatever ppi to the
fixed ppi the printer requires.

But, a medium to high-end printer prints at something like 4800*2400
dpi. This is because the printer lays down multiple dots per image pixel
- using 600 ppi and 4800*2400 dpi, each image pixel is printed using 8
horizontal dots of ink in each of four rows; a total of 32 dots of ink
per image pixel, which can be of different colors to achieve the
required color in the print.

So, yes, it is incorrect to refer to image pixels as so many dpi. But
that doesn't stop programmers using the wrong term. Even Photoshop uses
dpi when referring to images, when it should be ppi.

Colin D.






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