Delphi High DPI switch between own scaling and Windows scaling

Question

Some of my customers want to be able to scale my application manually (when Windows dpi is set to 96), so I had to implement scaling. Unfortunately these customers cannot go wit

Answer 1

Thanks to Sertac Akyuz I found a solution to my problem. In the Initialization part of the unit containing the scaling code I can switch between DPI-Awareness and Non-DPI-Awareness. It is important not to have this setting in the application manifest, which can be achieved by supplying a custom manifest like this (use control decoration and run with rights of current user):

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0">
 <dependency>
   <dependentAssembly>
     <assemblyIdentity
       type="win32"
       name="Microsoft.Windows.Common-Controls"
       version="6.0.0.0"
       publicKeyToken="6595b64144ccf1df"
       language="*"
       processorArchitecture="*"/>
   </dependentAssembly>
 </dependency>
 <trustInfo xmlns="urn:schemas-microsoft-com:asm.v3">
   <security>
     <requestedPrivileges>
       <requestedExecutionLevel
         level="asInvoker"
         uiAccess="false"/>
       </requestedPrivileges>
   </security>
 </trustInfo>
</assembly>

This is the actual code switching depending on a registry key:

// Set DPI Awareness depending on a registry setting
with TRegIniFile.create('SOFTWARE\' + SRegName) do
begin
  setting := readInteger('SETTINGS', 'scale', 0);
  Free;
end;
handle := LoadLibrary('shcore.dll');
if handle <> 0 then
begin
  setProcessDPIAwareness := GetProcAddress(handle, 'SetProcessDpiAwareness');
  if Assigned(setProcessDPIAwareness) then
  begin
    if setting < 2 then
      // setting <2 means no scaling vs Windows
      setProcessDPIAwareness(0)
    else
      // setting 2: 120%, 3: 140% vs. Windows
      // The actual used scaling factor multiplies by windows DPI/96
      setProcessDPIAwareness(1);
  end;
  FreeLibrary(handle);
  // Get windows scaling as Screen.PixelsPerInch was read before swiching DPI awareness
  // Our scaling routines now work with WinDPI instead of Screen.PixelsPerInch
  WinDPI:= Screen.MonitorFromWindow(application.handle).PixelsPerInch;
end;

The last line of this snippet retrieves the current DPI for the current monitor as screen.pixelsperinch seems to be initialized before and does always return 96 as for a non dpi aware application. I use the value of winDPI in all subsequent scaling calculations and it works perfect.