Capturing wrong part of the screen

Question

I have created a small demo app which window looks as below:

When I run this demo app, and press any key, I want to capture the part of the screen bitmap.

The part of the screen I am interested in, is the one my window occupies, namely the content of the top rectangle in my window that holds letters. The captured screen bitmap should look like below:

The problem I face is that screen capturing code captures wrong part of the screen.

Below is the full code (bare in mind that I tried to keep things as minimal as possible):

#include 

void foo(HWND hWnd)
{
    HDC hdcScreen;
    HDC hdcWindow;

    hdcScreen = GetDC(NULL);
    hdcWindow = GetDC(hWnd);
    
    RECT rcClient;
    GetClientRect(hWnd, &rcClient);

    // map window's client coordinates to screen coordinates
    // HERE IS THE PROBLEM, SOMEHOW COORDINATES ARE NOT TRANSLATED CORRECTLY 
    // do not know how to fix this, but I am trying  :( 
    RECT rc1 = rcClient;
    MapWindowPoints(hWnd, NULL, (LPPOINT)&rc1, 2);  

    // capture desktop portion of the image
    // that corresponds to the window's top rectangle (the one that has letters in it)
    // and blit the result in the bottom rectangle
    // so result can be visually compared
    if (!BitBlt(hdcWindow, 
        rcClient.left + 50, // coordinates of the bottom rectangle 
        rcClient.top + 70,  // sorry for the "magic numbers" 
        75, 35,             // I am low on time :( 
        hdcScreen,           
        rc1.left + 50,      // screen coordinates of the top rectangle     
        rc1.top + 20,       // (the one that contains letters) 
        SRCCOPY))            
    {
        OutputDebugString(L"StretchBlt has failed");
        ReleaseDC(NULL, hdcScreen);
        ReleaseDC(hWnd, hdcWindow);
        return;
    }
    
    RECT rcBottomRect;                        // Frame again the bottom rectangle in the window,  
    rcBottomRect.left = rcClient.left + 50;   // to make visual comparing easier
    rcBottomRect.top = rcClient.top + 70;     // and to verify that I didn't screw up
    rcBottomRect.right = rcClient.left + 125; // the coordinates
    rcBottomRect.bottom = rcClient.top + 105;

    HBRUSH br = (HBRUSH)GetStockObject(BLACK_BRUSH);
    FrameRect(hdcWindow, &rcBottomRect, br);

    ReleaseDC(NULL, hdcScreen);
    ReleaseDC(hWnd, hdcWindow);
}

LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
    switch (msg)
    {
    case WM_KEYUP:      // easiest handler to add that keeps things minimal
        foo(hwnd);      // capture screen      
        break;
    case WM_PAINT:
    {
        PAINTSTRUCT ps;
        HDC hdc = BeginPaint(hwnd, &ps);

        RECT rcClient;
        GetClientRect(hwnd, &rcClient);

        RECT rcTopRect;            
        rcTopRect.left = rcClient.left + 50;
        rcTopRect.top = rcClient.top + 20;
        rcTopRect.right = rcTopRect.left + 75;
        rcTopRect.bottom = rcTopRect.top + 35;

        HBRUSH br = (HBRUSH)GetStockObject(BLACK_BRUSH);

        TextOut(hdc, 20, 30, L"Asdf ghj kkl oioio 4545 676767", ARRAYSIZE(L"Asdf ghj kkl oioio 4545 676767"));
        FrameRect(hdc, &rcTopRect, br);

        RECT rcBottomRect;
        rcBottomRect.left = rcClient.left + 50;
        rcBottomRect.top = rcClient.top + 70;
        rcBottomRect.right = rcClient.left + 125;
        rcBottomRect.bottom = rcClient.top + 105;

        FrameRect(hdc, &rcBottomRect, br);

        EndPaint(hwnd, &ps);
    }
    break;
    case WM_CLOSE:
        DestroyWindow(hwnd);
        break;
    case WM_DESTROY:
        PostQuitMessage(0);
        break;
    default:
        return DefWindowProc(hwnd, msg, wParam, lParam);
    }
    return 0;
}

// BOILERPLATE CODE...
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
    LPSTR lpCmdLine, int nCmdShow)
{
    WNDCLASSEX wc;
    HWND hwnd;
    MSG Msg;

    wc.cbSize = sizeof(WNDCLASSEX);
    wc.style = 0;
    wc.lpfnWndProc = WndProc;
    wc.cbClsExtra = 0;
    wc.cbWndExtra = 0;
    wc.hInstance = hInstance;
    wc.hIcon = LoadIcon(NULL, IDI_APPLICATION);
    wc.hCursor = LoadCursor(NULL, IDC_ARROW);
    wc.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1);
    wc.lpszMenuName = NULL;
    wc.lpszClassName = L"myWindowClass";
    wc.hIconSm = LoadIcon(NULL, IDI_APPLICATION);

    if (!RegisterClassEx(&wc))
    {
        MessageBox(NULL, L"Window Registration Failed!", L"Error!",
            MB_ICONEXCLAMATION | MB_OK);
        return 0;
    }

    // Step 2: Creating the Window
    hwnd = CreateWindowEx(
        WS_EX_CLIENTEDGE,
        L"myWindowClass",
        L"MVCE",
        WS_OVERLAPPEDWINDOW,
        CW_USEDEFAULT, CW_USEDEFAULT, 300, 170,
        NULL, NULL, hInstance, NULL);

    if (hwnd == NULL)
    {
        MessageBox(NULL, L"Window Creation Failed!", L"Error!",
            MB_ICONEXCLAMATION | MB_OK);
        return 0;
    }

    ShowWindow(hwnd, nCmdShow);
    UpdateWindow(hwnd);

    // Step 3: The Message Loop
    while (GetMessage(&Msg, NULL, 0, 0) > 0)
    {
        TranslateMessage(&Msg);
        DispatchMessage(&Msg);
    }
    return Msg.wParam;
}

QUESTION:

How to fix calculation "error" (maybe it is not an error, maybe I am misusing the API?) with MapWindowPoints?

UPDATE:

I have forgotten to mention that I have 2 monitors. After testing the app on second monitor everything worked fine.

After going through the settings for the first monitor, I have found out that it is set to scale text, apps and other items to 150%.

Reverting it to 100% made the code work, but now I need to find a solution for this case, as I may not force users to change their settings.

Any help would be appreciated.

Soonts · Accepted Answer

You can’t force user to change the DPI, but you can ask Windows to stop messing with the coordinates in your application. To do that, include manifest into the main .exe of your program. You probably need max. settings, true/pm and PerMonitorV2.

See this article for more info.

Capturing wrong part of the screen

QUESTION:

UPDATE:

Answers (2)

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