DPI and screen resolution on OS X

SOS sos at pmg.be
Wed Feb 3 12:15:47 UTC 2016

On 3/02/2016 13:01, Chris Sherlock wrote:
> On 3 Feb 2016, at 10:35 PM, SOS <sos at pmg.be> wrote:
>> On 3/02/2016 11:32, Chris Sherlock wrote:
>>> On 3 Feb 2016, at 7:24 PM, SOS <sos at pmg.be> wrote:
>>>> 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)
>>> Definitely true :-) Only in OS X’s case, it doesn’t actually report back the correct resolution unless you ask for the backing coordinate system.
>>> The PPI business is a red herring I think I’ve introduced into this discussion I’m afraid. We calculate the PPI ourselves (and call it DPI) based on the reported pixels, and the size of the screen in mm (which we obviously convert to inches).
>> its a bit the wrong discussion: what we see on screen has no relevance: the user can "zoom" the document until he is happy with the image quality on screen
>> But in the current situation, LO users has no idea how big (size) he can place a image in a document.
>> When the doc is intented for online use (email and Web) then there is a minimum of 96 pixels par inch needed. More is no problem but is in many cases a overkill.
>> Who is editing a "book" or a "magazine" need minimal 254 pixels par inch to has a good image quality after printing.
>> When using less pixels the book pages  are looking fine on screen put shall have a creepy print quality
>> So having a new "DocumentProperty" indicating the needed pixels (for printing)  make it possible to make the "size" calculations before inserting.
> We are actually detecting the PPI… with the greatest of respect, I’ve actually implemented some testing changes to detect the correct PPI and on my Mac is should actually be just over 200PPI…
ok but how did you detected the PPI ?
You need the "fysical" dots given by the manufactor off the screen. Then 
you cab devide the total dots by the size off the screen
Standard Led and LCD screens have around 100 dots par inch
A Retina screen on a tablet or a smartphone could have 150-300 dots par 

> I think this is going in the wrong direction. I worked for Epson about 13 years ago, so I have some knowledge of printing :-) I could talk your ear off on colour management and halftoning, and I probably know a bit too much about piezo-electric crystal technology…
> I’m really trying to understand what is relying on the resolution and what sort of impact fixing the resolution detection might be having on OS X systems.
> Chris

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