wxWindow::GetDPIScaleFactor() returns invalid values

[awi]

Under wxGTK (3.24.5) wxWindow::GetDPIScaleFactor() returns invalid values - both for standard and HiDPI screens:

24" HD          Scale:  DPI=0.989583   Content=1
27" UHD 100%    Scale:  DPI=1.67708   Content=1
27" UHD 200%    Scale:  DPI=1.67708   Content=2

According to the ​documentation either 1 or 2 should be returned.
Tested with minimal sample:

samples/minimal/minimal.cpp

@@ -178 +178 @@
     CreateStatusBar(2);
     SetStatusText("Welcome to wxWidgets!");
 #endif // wxUSE_STATUSBAR
+    double sc1 = GetDPIScaleFactor();
+    double sc2 = GetContentScaleFactor();
+    wxLogDebug("Scale:  DPI=%g   Content=%g", sc1, sc2);
 }
 
 

[vadz]

Thanks, this is indeed wrong. And it's easy to fix by just making GetDPIScaleFactor() return the same thing as GetContentScaleFactor() does under GTK.

But I wonder if we shouldn't be instead tweaking the return value of GetDPI() to just return GetContentScaleFactor()*BASELINE_DPI instead. Because currently the value we return from GetDPI() is correct in theory (it's really pixels per inch, according to the current resolution and display physical size), but it's arguably not useful.

So maybe a white lie here would be better than useless truth? I.e. who needs the actual DPI (which seems to be indeed ~280 in my case, even though DPI is only 192). What do you think?

[csomor]

I also think that GetDPI should return the 'theoretical' DPI, not the precise on the physical display size, like eg on msw where a 150% display returns 144 ... however small the real display is ...

[vadz]

I've created ​PR 2015 to fix this, as mentioned there it doesn't work under Mac in the current state, but does work for wxGTK (and hopefully doesn't break wxMSW, although I haven't tested this yet).

[Vadim Zeitlin <vadim@…>]

In 3df9d772/git-wxWidgets:

Merge branch 'display-ppi-from-scaling-factor'

Determine display DPI from scaling factor instead of trying to compute
it from the physical display dimensions.

See ​https://github.com/wxWidgets/wxWidgets/pull/2015

Closes #18855.