I make DXGI adapters and monitors enumeration. The second monitor connected to my computer is Dell P2715Q, which has 3840*2160 resolution. However, the program reports it as 2560*1440, the second available resolution. Minimal code to reproduce:

#include "stdafx.h"
#include <Windows.h>
#include <stdio.h>
#include <tchar.h>
#include <iostream>
#include <DXGI.h>
#pragma comment(lib, "DXGI.lib")
using namespace std;

int main()
{
    IDXGIFactory1* pFactory1;

    HRESULT hr = CreateDXGIFactory1(__uuidof(IDXGIFactory1), (void**)(&pFactory1));

    if (FAILED(hr))
    {
        wcout << L"CreateDXGIFactory1 failed. " << endl;
        return 0;
    }

    for (UINT i = 0;; i++)
    {
        IDXGIAdapter1* pAdapter1 = nullptr;

        hr = pFactory1->EnumAdapters1(i, &pAdapter1);

        if (hr == DXGI_ERROR_NOT_FOUND)
        {
            // no more adapters
            break;
        }

        if (FAILED(hr))
        {
            wcout << L"EnumAdapters1 failed. " << endl;
            return 0;
        }

        DXGI_ADAPTER_DESC1 desc;

        hr = pAdapter1->GetDesc1(&desc);

        if (FAILED(hr))
        {
            wcout << L"GetDesc1 failed. " << endl;
            return 0;
        }

        wcout << L"Adapter: " << desc.Description << endl;

        for (UINT j = 0;; j++)
        {
            IDXGIOutput *pOutput = nullptr;

            HRESULT hr = pAdapter1->EnumOutputs(j, &pOutput);

            if (hr == DXGI_ERROR_NOT_FOUND)
            {
                // no more outputs
                break;
            }

            if (FAILED(hr))
            {
                wcout << L"EnumOutputs failed. " << endl;
                return 0;
            }

            DXGI_OUTPUT_DESC desc;

            hr = pOutput->GetDesc(&desc);

            if (FAILED(hr))
            {
                wcout << L"GetDesc1 failed. " << endl;
                return 0;
            }

            wcout << L"  Output: " << desc.DeviceName <<
                L"  (" << desc.DesktopCoordinates.left << L"," << desc.DesktopCoordinates.top << L")-(" <<
                (desc.DesktopCoordinates.right - desc.DesktopCoordinates.left) << L"," << 
                (desc.DesktopCoordinates.bottom - desc.DesktopCoordinates.top) << L")" << endl;

        }
    }

    return 0;
}

Program output:

Adapter: Intel(R) Iris(TM) Pro Graphics 6200
  Output: \\.\DISPLAY1  (0,0)-(1920,1200)
  Output: \\.\DISPLAY2  (1920,0)-(2560,1440)

DISPLAY2 is reported with low resolution.

Environment:

Windows 10 x64

Intel(R) Iris(TM) Pro Graphics 6200

DELL P2715Q

What can cause this behavior: DirectX restrictions, video memory, display adapter, driver, monitor? How can I fix this and get full available resolution?

The desktop coordinate are based on the actual resolution your monitor is set, not the physical capabilities. It may also be influenced by the DPI setting ( not sure on this one, DPI management in windows sucks ).

You have to call IDXGIOutput::GetDisplayModeList to obtain the list of supported modes.

EDIT: Confirmed, it is DPI dependent ( https://msdn.microsoft.com/en-us/library/windows/desktop/bb173068(v=vs.85).aspx ) :

DesktopCoordinates

Type: RECT

A RECT structure containing the bounds of the output in desktop coordinates. Desktop coordinates depend on the dots per inch (DPI) of the desktop. For info about writing DPI-aware Win32 apps, see High DPI.

galop1n, I played with DPI by using SetProcessDpiAwareness API and setting the text size to 100% in Windows Display Settings. This helped to solve the problem, thank you for pointing in right direction.

I got onto the same problem and that's true it depends on DPI but there is a soluition to this:

		adapterOutput->GetDesc(&desc);
		std::wcout << desc.DeviceName << std::endl;

		std::cout << "Monitor X: " << desc.DesktopCoordinates.right - desc.DesktopCoordinates.left << std::endl;
		std::cout << "Monitor Y: " << desc.DesktopCoordinates.bottom - desc.DesktopCoordinates.top << std::endl;

		DEVMODE displayMode{};
		displayMode.dmSize = sizeof(DEVMODE);

		EnumDisplaySettings(desc.DeviceName, ENUM_CURRENT_SETTINGS, &displayMode);

		float proc = (float)displayMode.dmPelsHeight / (desc.DesktopCoordinates.bottom - desc.DesktopCoordinates.top);
		std::cout << "DPI percentage: " << proc << "\n";
		std::cout << "Screen Start position: " << displayMode.dmPosition.x << " x " << displayMode.dmPosition.y << std::endl;