Screen size in inches on Windows

Question

I am developing a multi-platform game that runs on iOS as well as desktops (Windows, Mac, Linux). I want the game to be able to resize certain UI elements depending on the resolution of the screen in inches. The idea is that if a button should be, say, around 1/2 inch across in any interface, it will be scaled automatically that size.

Now for iOS devices this problem is reasonably well solvable using brute force techniques. You can look up the type of the device and use a hard-coded table to determine the screen size in inches for each device. Not the most elegant solution, but sufficient.

Desktops are the tricky ones. What I wish and hope exists is a mechanism by which (some?) monitors report to operating systems their actual screen size in inches. If that mechanism exists and I can access it somehow, I can get good numbers at least for some monitors. But I've never come across any such concept in any of the major OS APIs.

Is there a way to ask for the screen size in inches in Win32? If so, are there monitors that actually provide this information?

(And if the answer is no: Gosh, doesn't this seem awfully useful?)

Answer 1

Here is a method I found at the web address "https://ofekshilon.com/2011/11/13/reading-monitor-physical-dimensions-or-getting-the-edid-the-right-way/".

Authour says that measurement is in millimeters.

Does not give you the precise width and height but better approximation than HORSIZE and VERTSIZE. In which I tried on two different monitors and got a max difference of 38 cm (measured screen size - calculated screen size).

#include <SetupApi.h>
#pragma comment(lib, "setupapi.lib")

#define NAME_SIZE 128

const GUID GUID_CLASS_MONITOR = {0x4d36e96e, 0xe325, 0x11ce, 0xbf, 0xc1, 0x08, 0x00, 0x2b, 0xe1, 0x03, 0x18};

// Assumes hDevRegKey is valid
bool GetMonitorSizeFromEDID(const HKEY hDevRegKey, short& WidthMm, short& HeightMm)
{
    DWORD dwType, AcutalValueNameLength = NAME_SIZE;
    TCHAR valueName[NAME_SIZE];

    BYTE EDIDdata[1024];
    DWORD edidsize=sizeof(EDIDdata);

    for (LONG i = 0, retValue = ERROR_SUCCESS; retValue != ERROR_NO_MORE_ITEMS; ++i)
    {
        retValue = RegEnumValueA ( hDevRegKey, i, &valueName[0],
            &AcutalValueNameLength, NULL, &dwType,
            EDIDdata, // buffer
            &edidsize); // buffer size

        if (retValue != ERROR_SUCCESS || 0 != strcmp(valueName,"EDID"))
            continue;

        WidthMm  = ((EDIDdata[68] & 0xF0) << 4) + EDIDdata[66];
        HeightMm = ((EDIDdata[68] & 0x0F) << 8) + EDIDdata[67];
        return true; // valid EDID found
    }

    return false; // EDID not found
}

// strange! Authour requires TargetDevID argument but does not use it
bool GetSizeForDevID(const char *TargetDevID, short& WidthMm, short& HeightMm)
{
    HDEVINFO devInfo = SetupDiGetClassDevsExA(
                &GUID_CLASS_MONITOR, //class GUID
                NULL, //enumerator
                NULL, //HWND
                DIGCF_PRESENT, // Flags //DIGCF_ALLCLASSES|
                NULL, // device info, create a new one.
                NULL, // machine name, local machine
                NULL);// reserved

    if (NULL == devInfo)  return false;

    bool bRes = false;
    for (ULONG i=0; ERROR_NO_MORE_ITEMS != GetLastError(); ++i)
    {
        SP_DEVINFO_DATA devInfoData;
        memset(&devInfoData,0,sizeof(devInfoData));
        devInfoData.cbSize = sizeof(devInfoData);

        if (SetupDiEnumDeviceInfo(devInfo,i,&devInfoData))
        {
            HKEY hDevRegKey = SetupDiOpenDevRegKey(devInfo,&devInfoData,DICS_FLAG_GLOBAL, 0, DIREG_DEV, KEY_READ);
            if(!hDevRegKey || (hDevRegKey == INVALID_HANDLE_VALUE)) continue;

            bRes = GetMonitorSizeFromEDID(hDevRegKey, WidthMm, HeightMm);
            RegCloseKey(hDevRegKey);
        }
    }

    SetupDiDestroyDeviceInfoList(devInfo);
    return bRes;
}

int main(int argc, CHAR* argv[])
{
    short WidthMm, HeightMm;

    DISPLAY_DEVICE dd;
    dd.cb = sizeof(dd);

    DWORD dev = 0; // device index
    int id = 1; // monitor number, as used by Display Properties > Settings

    char DeviceID[1024];
    bool bFoundDevice = false;

    while (EnumDisplayDevices(0, dev, &dd, 0) && !bFoundDevice)
    {
        DISPLAY_DEVICE ddMon = {sizeof(ddMon)};
        DWORD devMon = 0;

        while (EnumDisplayDevices(dd.DeviceName, devMon, &ddMon, 0) && !bFoundDevice)
        {
            if (ddMon.StateFlags & DISPLAY_DEVICE_ATTACHED_TO_DESKTOP &&
                !(ddMon.StateFlags & DISPLAY_DEVICE_MIRRORING_DRIVER))
            {
                sprintf(DeviceID,"%s", ddMon.DeviceID+8);

                for(auto it=DeviceID; *it; ++it)
                if(*it == '\\') { *it = 0; break; }
             
                bFoundDevice = GetSizeForDevID(DeviceID, WidthMm, HeightMm);
            }
            devMon++;

            ZeroMemory(&ddMon, sizeof(ddMon));
            ddMon.cb = sizeof(ddMon);
        }

        ZeroMemory(&dd, sizeof(dd));
        dd.cb = sizeof(dd);
        dev++;
    }
    
    return 0;
}

Answer 2

For Windows, first see SetProcessDPIAware() for a discussion on turning off automatic scaling, and then call GetDeviceCaps( LOGPIXELSX ) and GetDeviceCaps( LOGPIXELSY ) on your HDC to determine the monitor's DPI. Divide the screen resolution on your active monitor by those settings and you've got the size.

Also see this article for a similar discussion on DPI aware apps.