[Development] Qt 6 high-dpi support

[Henry Skoglund]

On 2020-05-13 16:53, Morten Sørvig wrote:
> We are indeed planning to addreess high-DPI support for Qt 6. You can test the implementation on
> Qt 5.14+ today by setting
>
>      QGuiApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
>      QGuiApplication::setHighDpiScaleFactorRoundingPolicy(Qt::HighDpiScaleFactorRoundingPolicy::PassThrough);
>
> This should make sure labels and line edits are correctly sized, also for 125% and 150%.
>
> Interestingly, Qt will then “lie” about the about the screen size the same way that dpiawareness=0
> does, although while rendering at the full resolution (so no blurriness).
>
> Morten
Thank you for this!

All of my apps had the blurriness problem, in fact recently one of my 
users on a brand new laptop (with a 150% scale screen) complained and 
said that my program looked like "East Germany" which (if you're old 
enough to realize) is not a compliment :-(

My old code (I wanted a fire-and-forget solution, i.e. no qt.conf which 
is easily forgotten, so C++) was this:
...
#if !defined(DPI_AWARENESS_CONTEXT_UNAWARE)
// (these lines are copied from a fresh windef.h)
     DECLARE_HANDLE(DPI_AWARENESS_CONTEXT);

     #define DPI_AWARENESS_CONTEXT_UNAWARE ((DPI_AWARENESS_CONTEXT)-1)
     #define DPI_AWARENESS_CONTEXT_SYSTEM_AWARE ((DPI_AWARENESS_CONTEXT)-2)
     #define DPI_AWARENESS_CONTEXT_PER_MONITOR_AWARE 
((DPI_AWARENESS_CONTEXT)-3)
     #define DPI_AWARENESS_CONTEXT_PER_MONITOR_AWARE_V2 
((DPI_AWARENESS_CONTEXT)-4)
     #define DPI_AWARENESS_CONTEXT_UNAWARE_GDISCALED 
((DPI_AWARENESS_CONTEXT)-5)
#endif
     static const int nnBuild1703 = 15063;   // can use 
DPI_AWARENESS_CONTEXT_UNAWARE
     static const int nnBuild1809 = 17763;   // can use 
DPI_AWARENESS_CONTEXT_UNAWARE_GDISCALED

// Windows 10 build 1703 or later?
     if (QOperatingSystemVersion::current() >= 
QOperatingSystemVersion(QOperatingSystemVersion::Windows,10,0,nnBuild1703))
     {
     // dynamically resolve the address to SetProcessDpiAwarenessContext()
         auto f = reinterpret_cast<bool (__stdcall 
*)(DPI_AWARENESS_CONTEXT)>(QLibrary("user32.dll").resolve("SetProcessDpiAwarenessContext"));
         auto value = DPI_AWARENESS_CONTEXT_UNAWARE;

     // on build 1809 (or later)? let's try GDISCALED
         if (QOperatingSystemVersion::current() >= 
QOperatingSystemVersion(QOperatingSystemVersion::Windows,10,0,nnBuild1809))
             value = DPI_AWARENESS_CONTEXT_UNAWARE_GDISCALED;

         bool bOk = f(value);
         if (!bOk)
             qWarning("SetProcessDpiAwarenessContext() failed.");
     }
...
I think very few of my users are on Windows 8.1 so I leapfrogged the 
DpiAware functions in SHCore.dll and went for the new ones in Windows 10 
(they are in user32.dll) but to no avail, it was East Germany anyway, 
here are 2 screenshots, first from a 100% screen for comparison, then 
from a 125% screen using my code above (I've anonymized patient names etc.):
https://tripleboot.org/Pictures/Scale100Percent.png
https://tripleboot.org/Pictures/Scale125PercentDpiAwareness0.png

On the 100% screen all is fine and dandy but on the 125% screen -> 
fuzziness galore.

So I tossed the code above and tried your 2 liner from today:
...
QGuiApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
QGuiApplication::setHighDpiScaleFactorRoundingPolicy(Qt::HighDpiScaleFactorRoundingPolicy::PassThrough);
...

Voila: https://tripleboot.org/Pictures/Scale125PercentPassThrough.png
Goodbye East Germany!

Thank you again /Henry

P.S. Now this wouldn't be complete without some small complaint, I'm 
thinking of the horizontal lines for separating the different rows of 
patients (they are QFrames, or "Horizontal Line" as they are called in 
Qt Creator's widget designer).
I insert them manually on top of the QTableWidget, they have a lineWidth 
of 1 and are rendered as such on the 100% or the 200% screens. But on 
125%, 150% etc. screens they are rendered a bit uneven using the new 2 
liner/passthrough code, you can see it on my last/3rd screenshot. I 
suspect it's some kind of rounding problem. This is *not* a big deal, 
just me being pedantic.