Querying Windows display scaling

Question

I want to query the Windows display scaling setting programmatically: In this case, I want it to return 125 since I configured my display to be at 125% scaling. According to this article, the following Windows API C++ code can be used:

// Get desktop dc
desktopDc = GetDC(NULL);
// Get native resolution
horizontalDPI = GetDeviceCaps(desktopDc,LOGPIXELSX);
verticalDPI = GetDeviceCaps(desktopDc,LOGPIXELSY);

However, this code always return 96 and 96 for the horizontal as well as vertical DPI which translates to 100% scaling (according to the provided table):

This output is wrong since I still get the same result with 125% scaling. How can it be done? I'm programming in Java so I can execute C++ using JNA. Windows API solutions are preferred but everything else like a .bat script or registry query is also fine as long as it's reliable for all Windows versions from 7 to 10.

Note that `LOGPIXELSY` will always return 96 for compatibility unless you have your app flagged as dpi-aware. — Jonathan Potter, Feb 27 '19 at 20:32

score 5 · Accepted Answer · answered Feb 27 '19 at 18:26

This answer solved it:

#include "pch.h"
#include <iostream>
#include <windows.h>

int main()
{
    auto activeWindow = GetActiveWindow();
    HMONITOR monitor = MonitorFromWindow(activeWindow, MONITOR_DEFAULTTONEAREST);

    // Get the logical width and height of the monitor
    MONITORINFOEX monitorInfoEx;
    monitorInfoEx.cbSize = sizeof(monitorInfoEx);
    GetMonitorInfo(monitor, &monitorInfoEx);
    auto cxLogical = monitorInfoEx.rcMonitor.right - monitorInfoEx.rcMonitor.left;
    auto cyLogical = monitorInfoEx.rcMonitor.bottom - monitorInfoEx.rcMonitor.top;

    // Get the physical width and height of the monitor
    DEVMODE devMode;
    devMode.dmSize = sizeof(devMode);
    devMode.dmDriverExtra = 0;
    EnumDisplaySettings(monitorInfoEx.szDevice, ENUM_CURRENT_SETTINGS, &devMode);
    auto cxPhysical = devMode.dmPelsWidth;
    auto cyPhysical = devMode.dmPelsHeight;

    // Calculate the scaling factor
    auto horizontalScale = ((double) cxPhysical / (double) cxLogical);
    auto verticalScale = ((double) cyPhysical / (double) cyLogical);

    std::cout << "Horizonzal scaling: " << horizontalScale << "\n";
    std::cout << "Vertical scaling: " << verticalScale;
}

There is something counterintuitive: when I call `SetProcessDpiAwarenessContext(DPI_AWARENESS_CONTEXT_PER_MONITOR_AWARE_V2);` first in main, it reports a factor 1.0 for all monitors. If I don't, the factors match those of each monitor as shown in the control panel. — Cee McSharpface, Oct 06 '20 at 14:37

Querying Windows display scaling

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