Question/feedback regarding Windows Hi DPI support and how it will affect applications

Jim Graham james.graham at
Wed Feb 18 19:52:31 UTC 2015

On 2/18/2015 2:13 AM, Werner Lehmann wrote:
> Finally, it would be nice to get information about the actual screen
> DPI. In my tests Screen.getDpi always returns 96, regardless of what it
> actually is...

I'm not sure if there is a legacy API for that value in Windows 
pre-Win8.1, but I know that we have been using "::GetDeviceCaps(hDC, 
LOGPIXELSX)" to get our value for Screen.getDPI().  That method returns 
the effective scale of a screen, which is what they want you to use to 
compute a scale factor that is relative to the users preferences.

If the user chooses "I want to make things bigger or smaller on the 
screen" in the Displays CP, then they can request that the default font 
be 100%, 125%, 150% and sometimes 200% (and, on a Yoga Pro, they can 
even ask for 300%).  Depending on that setting you will see 96, 120, 
144, 192, or 288 from the LOGPIXELSX/Y and you are supposed to scale 
yourself by N/96.0.  *NOTE* that you this setting does not change 
between logout/reboots so if the user changes this setting in the CP, it 
will continue to return the old information until they reboot or log out 
and log back in.  Then the new settings will be retrieved.  (I can see 
why they wouldn't want it to change dynamically when an app is running, 
but I'm disappointed that they don't allow it to change when an app is 
quit and restarted...)

In Win8.1 they have a new method "GetDPIForMonitor(DPI_TYPE)" where you 
can ask for "Effective DPI" which is roughly the LOGPIXELSXY information 
that represents "scale yourself by N/96.0", or you can ask for "Raw DPI" 
which is the actual physical pixels per physical inch measurement.  This 
value can also change dynamically if the user modifies their Display CP 
preferences.  I plan to support the use of that new API soon, but it 
will only be effective in Windows 8.1 or later since the API was 
introduced in 8.1...


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