DPI and screen resolution on OS X

Armin Le Grand armin_le_grand at me.com
Thu Feb 4 16:10:58 UTC 2016


Am 03.02.2016 um 16:46 schrieb SOS:
>> 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.
> Thats the problem withn this "system DPI"
> for screen viewing is 96 DPI OK  but far to less when the document 
> needs to be printed
> we need a replacement for the system DPI who is a value who must can 
> differ par document

+1, DPI to use should be available at the target device, be on a 
appropritate value (96 for display, 300 for print, whatever, similar for 
PDF export, and obviously something over 200 for mac display).

At best case in no way should display DPI be used to permanently change 
model data and write it back to the file. No idea how to do that best.

You have painted an ellipse (vector data). User chooses 'convert to 
Bitmap' and saves the file. On Mac you will have a >200 DPI bitmap, on 
all other systems a 96 DPI one. If you do the change on a non-mac and 
load later at mac, it may be visibly 'pixelated' on the display.

What to do?

Not allow user to convert to bitmap?
This is not an option, the user's will should always be respected.

Use a very high DPI always (the highest currently known (what is already 
guessing), so mac one).
Also not an option, will make files much bigger (ressource need of 
bitmap raises quadratic)

Just mark the graphic as 'being a Bitmap', but save the vector data. At 
reload, re-create bitmap data.
Way to complicated, would give small file, needs re-creation of bitmap 
at load time


>> HTH! ibreOffice at lists.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/libreoffice

ALG (PGP Key: EE1C 4B3F E751 D8BC C485 DEC1 3C59 F953 D81C F4A2)

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