DPI and screen resolution on OS X

Armin Le Grand armin_le_grand at me.com
Wed Feb 3 13:10:50 UTC 2016


comments inline

Am 03.02.2016 um 12:35 schrieb SOS:
> 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.

It is relevant. If you have a vector graphic and it gets converted to 
bitmap, the DPI from the system is used to define the resulting pixel 
size. Conversion to bitmap happens more often than it might seem. Examples:
- user chooses to do so (context menu, convert to bitmap)
- some exporters who are not capable using vector graphics
- PDF, e.g. PDF/1A which is not allowed to use transprencies and solves 
by creating bitmaps where graphics and transparent parts overlap
- 3D renderer which targets to bitmaps (chart, 3D objects)
Thus, the system DPI is essential. If on Mac, the bigger DPI will be 
used, it will enlarge all these conversions.


>> I guess I’m curious as to what is relying on the screen resolution 
>> and PPI.
>> Although… it’s funny that we have the function 
>> SalGraphics::GetResolution, but that returns the PPI!
>> Chris
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