DPI and screen resolution on OS X

Chris Sherlock chris.sherlock79 at gmail.com
Wed Feb 3 10:32:02 UTC 2016

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). 

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!


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