Nomenclature.

  • Thread starter Thread starter rodney
  • Start date Start date
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rodney

I am a little perplexed in the language used by scanners, eg:
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)
Click to expand...

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
 
rodney said:
I am a little perplexed in the language used by scanners, eg:
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)
Click to expand...

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
Click to expand...
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.
 
"Colin_D"
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.
Click to expand...

Thanks Colin,
just to labour the point,
I can understand the differentiation, but would that remain the case
when the scanner is talking to the printer?

When the scanner's resolution is selected, is it not the "dpi" the scanner
selects to give a given print size?

Say I scan at 500dpi, isn't the scanner assuming a <suitable> pixel rate
to give the scanner the possibility to print at 500dpi.
That's what I thought was going on.

Trust you may understand what I am getting at.
Cheers
Rodney
 
rodney said:
"Colin_D"

Thanks Colin,
just to labour the point,
I can understand the differentiation, but would that remain the case
when the scanner is talking to the printer?

When the scanner's resolution is selected, is it not the "dpi" the scanner
selects to give a given print size?

Say I scan at 500dpi, isn't the scanner assuming a <suitable> pixel rate
to give the scanner the possibility to print at 500dpi.
That's what I thought was going on.

Trust you may understand what I am getting at.
Cheers
Rodney
Click to expand...


There is great confusion on scanning a image and the printer resolution.


It works like this.

8 1/2 X 11 inch document scanned at 300 ppi, print at 300 DPI, gets the same
size document.
It gets more complicated when you want to change the one to one
relationship.

If you want about the best image you can get on paper, you want an image
that prints at 300 DPI or PPI. In this case the two are interchangeable.

In order to get the 300 DPI print, you have to know the size of the original
document, and the size of the printed document.

There are two web pages that explains all of that.
http://scantips.com/basics02.html
 
CSM1 said:
There is great confusion on scanning a image and the printer resolution.


It works like this.

8 1/2 X 11 inch document scanned at 300 ppi, print at 300 DPI, gets the same
size document.
Click to expand...

Sorry to contradict, but that statement is wrong. An image scanned at
300ppi and printed at 300 *ppi* will be the same size as the original.

But, the printer will be printing at something like 4800*2400 DPI (a
Canon specification). DPI is Dots Per Inch, a printer function that has
no fixed relationship to the pixels of the image it is printing. It is
the measure of how many ink dots per inch of paper the printer can
print. Even if you printed an image at 1 ppi ( a pretty big image!) the
printer will still print at the specified DPI.
It gets more complicated when you want to change the one to one
relationship.
Click to expand...

The one-to-one relationship refers to image size only. A 300 ppi image
printed at 150 ppi will be twice the linear size of the original. The
printer will still print at the specified DPI.
If you want about the best image you can get on paper, you want an image
that prints at 300 DPI or PPI. In this case the two are interchangeable.
Click to expand...

No, they are not. DPI is not a measure of print size at all.
In order to get the 300 DPI print, you have to know the size of the original
document, and the size of the printed document.
Click to expand...

A very confusing - and wrong - statement. You specify the image ppi in
the image editor (Photoshop, PSP, whatever). Depending on the original
ppi of the image, you may need to resample the image to a new, specified
ppi depending on several factors; original image size, required printing
size, original ppi, etc.

Summarizing: PPI is pixels per inch, and is a measure of the digital
resolution of the image (not to be confused with the optical
resolution). PPI also refers to the number of pixels per inch the image
is printed at, called the 'native' ppi of the printer. In Canon
printers it is 600 ppi, and Epson, 720 ppi. The image sent to the
printer is resampled to the native ppi by the printer driver software.
If you send, say, a 100 ppi image to the printer, the driver will
resample to the native ppi.

DPI is dots of ink or toner laid down on the paper by the printer. Most
good printers have a DPI of 4800*2400 or thereabouts, for quality
printing. Regardless of the image ppi sent to the printer, the driver
will convert to the native ppi, and the printer will lay down 4800*2400
(nominal) DPI on the paper.

A lot of the confusion is caused by software programmers using the wrong
term. One needs to understand thoroughly the relationships so that
incorrect usage of PPI and DPI does not cause problems.

Colin D.
 
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