DPI and screen resolution on OS X

SOS sos at pmg.be
Wed Feb 3 08:24:44 UTC 2016

On 3/02/2016 3:55, Kohei Yoshida wrote:
> On Wed, 2016-02-03 at 10:52 +1100, Chris Sherlock wrote:
>> The other question is: why would we not want to the actual DPI and
>> screen resolution?
> My understanding is that, historically, the OS provided a function to
> query DPI but what gets returned from such function was not always
> accurate (or always not accurate depending on who you ask).  So, the
> workaround at the time was to assume that DPI is always 96 (and
> hard-code that value) regardless of what the OS told you, which worked
> just fine because the monitors used back in the day had the same screen
> resolution.
Mostly DPI is found in the header of a pixelfile (taken by camera). 
Unfortunately it's not the photographer who gets to decide about the 
needed DPI.
DPI is actually a wrong definition for documents, Dots Per Inch is a 
definition used by output devices. Screens need a PIXEL par DOT but for 
print devices there is no precise correlation between the number of dots 
used by the device and the pixels needed in  the image for having a 
maximum image-view quality.
The print industry has come to some standards by trial and error.
- monitor screens need 96 - (220-retina) pixels per inch
- laser printers need 150 pixels per inch (up tot 2000 + dots)
- offset printers need 254 -300 pixels per inch (up to 3000 dots)

For a document we must use Pixels Per Inch which are calculated 
regarding the DPI needed by the final output device and represented in 
each document by  a "Print Intention" .
When producing docs for printing on an office laser printer we need less 
Pixels Per Inch than docs (magazines , books) which are printed on 
offset machines.
When an image is loaded, then the system can calculate the viewing size 
using the number of pixels needed by the "Print Intention". The user can 
then see the maximum size the image can have in his document without 
losing image  quality.

Hope it helps

> I'm not sure if that's a non-issue today.  I don't know enough about
> this topic to tell you that with confidence.
> Kohei
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