Computer Graphics with Photoshop - "dpi" vs "ppi" (was Re: 72dpi to 300dpi)

Author

"dpi" vs "ppi" (was Re: 72dpi to 300dpi)

Tacit

2004-05-29, 12:14 pm

>Eventually the widespread use of including dpi in the measurement of an
>image's size and quality will become the norm and traditionalists will
>forever morn the passing of yet another dead pixel.

True story: When i was working professional prepress at a trade shop called
Dimension, we had a job order come in one day where the client had asked for a
transparency to be scanned such that the scan would be 9 by 12 inches at 300
dpi.

Now, for the past thirty years, Dimension's clients had been art directors and
serious advertising professionals at big-name advertising agencies, so they
never made that particular mistake. Lately, as more and more companies attempt
(usually with dismal results) tosave money by bringing their advertising
in-house, the experience and skill of Dimension's client has dropped
significantly.

Anyway, the job bag was passed to the scanner operator. This particular scanner
operator, with 28 years' experience running a drum scanner, was completely
flummoxed; he had no idea what to do. He came to me and said "How can I make a
scan at 300 dpi? That doesn't make any sense! Do they want a prescreened
halftone at 300 lines per inch? What the hell are they talking about?"

I laughed and explained, "Don't sweat it, just make a 300 pixel per inch scan.
The job's for an ignoramous amateur who thinks "dpi" and "ppi" mean the same
thing."

Sadly, five years later, the ignoramous amateurs seem to outnumber the skilled
professionals even in the professional design and advertising industries.

--
Biohazard? Radiation hazard? SO last-century.
Nanohazard T-shirts now available! http://www.villaintees.com
Art, literature, shareware, polyamory, kink, and more:
http://www.xeromag.com/franklin.html

Xalinai

2004-05-30, 12:14 pm

On 29 May 2004 14:59:34 GMT, tacitr@aol.com (Tacit) wrote:

>
>True story: When i was working professional prepress at a trade shop called
>Dimension, we had a job order come in one day where the client had asked for a
>transparency to be scanned such that the scan would be 9 by 12 inches at 300
>dpi.
>
>Now, for the past thirty years, Dimension's clients had been art directors and
>serious advertising professionals at big-name advertising agencies, so they
>never made that particular mistake. Lately, as more and more companies attempt
>(usually with dismal results) tosave money by bringing their advertising
>in-house, the experience and skill of Dimension's client has dropped
>significantly.
>
>Anyway, the job bag was passed to the scanner operator. This particular scanner
>operator, with 28 years' experience running a drum scanner, was completely
>flummoxed; he had no idea what to do. He came to me and said "How can I make a
>scan at 300 dpi? That doesn't make any sense! Do they want a prescreened
>halftone at 300 lines per inch? What the hell are they talking about?"
>
>I laughed and explained, "Don't sweat it, just make a 300 pixel per inch scan.
>The job's for an ignoramous amateur who thinks "dpi" and "ppi" mean the same
>thing."
>
>Sadly, five years later, the ignoramous amateurs seem to outnumber the skilled
>professionals even in the professional design and advertising industries.

The problem is that manufacturers on the input and on the output side
use the same expression "DPI" for their very different purposes.

The scanner manufacturer calls full color pixels "Dots" and wouldn't
mind to use PPI for the same thing. The printer manufacturer uses the
same "Dots" for the density his devices can place ink drops and he
would be really unhappy if he were forced to advertise the actual PPI
the printers can achieve on various sorts of paper. And as many
companies make both, printers and scanners, the reason why there is no
change in units should be obvious.

Michael

-xiray-

2004-06-03, 7:14 pm

On 29 May 2004 14:59:34 GMT, tacitr@aol.com (Tacit) wrote:

>
>I laughed and explained, "Don't sweat it, just make a 300 pixel per inch scan.
>The job's for an ignoramous amateur who thinks "dpi" and "ppi" mean the same
>thing."
>

For ignoramous amateurs? Guess you've not noticed that many of the
companies that sell scanners (both low and high end) rate them with a
DPI number and only some companies use the PPI term.

Seems to me that the fine distinction has really only come up in
arguments in the last couple of years.

If I were told that a tiff file is 300 dpi, I'd know exactly what that
means. If I were told that a halftone was a 150 line screen I'd know
exactly what that means.

Why some people think that the same term can't mean different things
in different contexts is beyond me.

PPI, after all, also means "pages per inch" and nobody quibbles about
that.

Tacit

2004-06-03, 7:14 pm

>For ignoramous amateurs? Guess you've not noticed that many of the
>companies that sell scanners (both low and high end) rate them with a
>DPI number and only some companies use the PPI term.

Scanner copanies sell to...people outside the prepress and graphic arts
industry. Who do not know what "pixels per inch" means.

>Seems to me that the fine distinction has really only come up in
>arguments in the last couple of years.

It's come up since people with little formal training or education have begun
entering the design business, bringing their errors and their misconceptions
with them. This is both a good and a bad thing--good, because there are more
people than ever before doing design; bad, because there is more bad design in
the world that would be fixed by just a little bit of education.

>Why some people think that the same term can't mean different things
>in different contexts is beyond me.

The same term can mean different things--but the fact is, referring to a
continuous-tone image in terms of "dots" is simply wrong, just as referring to
a car as a "horse" would be wrong.

We think in language; fuzzy language leads to fuzzy thought.

In this particular case, fuzzy grasp of the difference between "dots" and
"pixels" leads to people coming on this newsgroup and saying "I have a 28,800
dpi Epson printer, but when I scan my pictures at 28,800 dpi to print them out,
the files are huge!"

--
Biohazard? Radiation hazard? SO last-century.
Nanohazard T-shirts now available! http://www.villaintees.com
Art, literature, shareware, polyamory, kink, and more:
http://www.xeromag.com/franklin.html

Brian

2004-06-03, 7:14 pm

> For ignoramous amateurs? Guess you've not noticed that many of the
> companies that sell scanners (both low and high end) rate them with a
> DPI number

And they are quite wrong in doing so. Anyone who measures digital image
resolution in "dots" is using incorrect terminology, period; regardless
of who the company or person in question may be, they're still 100% wrong.

> Why some people think that the same term can't mean different things
> in different contexts is beyond me.

Because they are NOT the same thing, and the only reason this has come
up more and more often over the last few years is the deluge of
neophytes calling themselves graphic artists who don't understand what
the terminology means or what it refers to, and for whatever reason
don't care to.

A digital image is constructed of pixels - there are no dots in a
digital image, it's impossible. Conversely, a printer, imagesetter,
platesetter, etc. outputs using dots - there are no pixels involved. See
how nicely the proper terminology works when the difference is made
clear? The problem is that far too many people are far too unwilling to
learn anything new; "my scanner calls it DPI so therefore it is" seems
to be the prevailing - but woefully incorrect - attitude.

> PPI, after all, also means "pages per inch" and nobody quibbles about
> that.

That doesn't matter one iota, because that use of PPI is not wrong - PPI
legitimately is used to mean both pixels per inch, and pages per inch,
depending on context. DPI is not a legitimate measurement of image
resolution under any circumstances.

jjs

2004-06-03, 7:14 pm

Would it be appropriate, or even feasible for an ISO, ANSI, ASA, DIN,
LSMFT or _someone_ to specify, for example, _exactly_ what Samples Per
Inch (_not_ "SPI" which is taken) means for scanners, digicams and other
image capture devices? Or have they? (Of course, I'd rather Samples Per
Centimeter, but I'll take what we can get.)

It would be a start.

Xalinai

2004-06-04, 12:14 pm

On Thu, 03 Jun 2004 17:08:41 -0500, john@xyzzy.stafford.net (jjs)
wrote:

>
>Would it be appropriate, or even feasible for an ISO, ANSI, ASA, DIN,
>LSMFT or _someone_ to specify, for example, _exactly_ what Samples Per
>Inch (_not_ "SPI" which is taken) means for scanners, digicams and other
>image capture devices? Or have they? (Of course, I'd rather Samples Per
>Centimeter, but I'll take what we can get.)
>
>It would be a start.

Samples per inch seems to be a fine definition as it restricts itself
to input devices.

But you still have to specify whether you measure the CCD chip (like
some camera manufacturers do) or the object size (like for scanning
someting).

And then: If there is interpolation involved, how do you convert
samples per inch into pixels per inch?

Wouldn't you just add another element to existing confusion?

Michael

Brian

2004-06-04, 7:14 pm

> Wouldn't you just add another element to existing confusion?

As long as the terminology was clearly understood and used properly, not
at all. The PPI vs DPI issue is confusing solely because far too many
people insist on using the two terms interchangeably, refusing to
understand the difference and to use each in its proper context.

Uni

2004-06-05, 7:14 pm

jjs wrote:
> Would it be appropriate, or even feasible for an ISO, ANSI, ASA, DIN,
> LSMFT or _someone_ to specify, for example, _exactly_ what Samples Per
> Inch (_not_ "SPI" which is taken) means for scanners, digicams and other
> image capture devices? Or have they? (Of course, I'd rather Samples Per
> Centimeter, but I'll take what we can get.)

It's a concern of no one. Who cares if you state DPI, LPI, PPI, Dogs Per
Inch, Lambs Per Inch or Parakeets Per Inch? That MAIN thing, they're all
Per Inch!

:-)

Uni

>
> It would be a start.