为何BitBlt()复制同一位图自身像素时出现放大？

我正在开发一个可复制桌面并对像素进行操作的窗口，通过BitBlt从桌面句柄向窗口句柄复制图像，此步骤可正常生成与桌面完全一致的窗口。但当再次调用BitBlt将窗口内某段像素复制到窗口另一区域时，像素出现放大效果。请问该现象的原因是什么？如何修复？

#include <windows.h>

int myWidth, myHeight;

LRESULT CALLBACK WndProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
    switch (uMsg)
    {
    case WM_CREATE:
    {
        HDC DhWnd = GetDC(HWND_DESKTOP);
        HDC MhWnd = GetDC(hwnd);
        BitBlt(MhWnd, 0, 0, myWidth, myHeight, DhWnd, 0, 0, SRCCOPY);
        ShowWindow(hwnd, SW_SHOW);
        BitBlt(MhWnd, 300, 0, 100, 500, MhWnd, 300, 0, SRCCOPY);

        /*^^^^ 这段代码明明是把相同坐标的位图复制到自身，却出现了像素放大的效果 ^^^^*/


        ReleaseDC(hwnd, DhWnd);
        ReleaseDC(hwnd, MhWnd);
        return 0;
    }
    case WM_PAINT:
        ValidateRect(hwnd, NULL);
        return 0;

    case WM_CLOSE:
    case WM_DESTROY:
        DestroyWindow(hwnd);
        PostQuitMessage(0);
        return 0;
    default:
        return DefWindowProc(hwnd, uMsg, wParam, lParam);
    }

}



int APIENTRY WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PWSTR pCmdLine, int nCmdShow)
{
    RECT Drc;
    MSG msg;
    HWND DhWnd = GetDesktopWindow();
    HWND MyhWnd;
    GetWindowRect(DhWnd, &Drc);
    myWidth = Drc.right - Drc.left;
    myHeight = Drc.bottom - Drc.top;


    WNDCLASS wc;
    wc.style = 0;
    wc.lpfnWndProc = WndProc;
    wc.cbClsExtra = 0;
    wc.cbWndExtra = 0;
    wc.hInstance = hInstance;
    wc.hIcon = NULL;
    wc.hCursor = LoadCursor(NULL,IDC_ARROW);
    wc.hbrBackground = NULL;
    wc.lpszMenuName = NULL;
    wc.lpszClassName = "MyWindow";


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

    MyhWnd = CreateWindow("MyWindow", NULL, WS_POPUP, 0, 0, myWidth, myHeight, NULL, NULL, hInstance, NULL);
    if (MyhWnd == NULL)
    {
        MessageBox(NULL, "Window Creation Failed!", "Error!",
            MB_ICONEXCLAMATION | MB_OK);
        return 0;
    }


    while (GetMessage(&msg, NULL, 0, 0) > 0)
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
    return 0;

}

1. 高DPI缩放不匹配：如果系统开启了高DPI缩放（比如125%、150%），窗口的逻辑坐标和物理像素坐标会错位。你通过GetWindowRect获取的桌面尺寸是物理像素，但创建窗口时使用的是逻辑坐标，导致窗口实际物理尺寸和桌面不一致，BitBlt操作时自然会出现拉伸放大。
2. 同一DC的重叠复制冲突：当源DC和目标DC为同一个时，直接调用BitBlt若源区域与目标区域有重叠（高DPI下实际坐标已错位，看似相同的坐标其实对应不同物理区域），会导致像素被重复覆盖复制，产生类似放大的异常效果。

1. 适配高DPI环境 #

在WinMain函数开头添加DPI感知声明，让系统按物理像素处理窗口：

SetProcessDpiAwarenessContext(DPI_AWARENESS_CONTEXT_PER_MONITOR_AWARE_V2);

2. 避免同一DC的直接复制 #

需要在窗口内部复制像素时，先将源区域内容复制到内存DC中，再从内存DC复制到目标区域，绕过同一DC的重叠冲突：

case WM_CREATE:
{
    HDC DhWnd = GetDC(HWND_DESKTOP);
    HDC MhWnd = GetDC(hwnd);
    // 复制桌面到窗口
    BitBlt(MhWnd, 0, 0, myWidth, myHeight, DhWnd, 0, 0, SRCCOPY);
    
    // 创建内存DC和兼容位图
    HDC memDC = CreateCompatibleDC(MhWnd);
    HBITMAP memBmp = CreateCompatibleBitmap(MhWnd, 100, 500);
    HBITMAP oldBmp = (HBITMAP)SelectObject(memDC, memBmp);
    
    // 先把窗口目标区域内容复制到内存DC
    BitBlt(memDC, 0, 0, 100, 500, MhWnd, 300, 0, SRCCOPY);
    // 再从内存DC复制回窗口目标区域
    BitBlt(MhWnd, 300, 0, 100, 500, memDC, 0, 0, SRCCOPY);
    
    // 清理资源
    SelectObject(memDC, oldBmp);
    DeleteObject(memBmp);
    DeleteDC(memDC);
    
    ShowWindow(hwnd, SW_SHOW);
    
    ReleaseDC(hwnd, DhWnd);
    ReleaseDC(hwnd, MhWnd);
    return 0;
}

3. 修正WM_PAINT处理逻辑 #

当前WM_PAINT直接调用ValidateRect会导致窗口重绘时内容丢失，应该在WM_PAINT中重新执行绘制逻辑：

case WM_PAINT:
{
    PAINTSTRUCT ps;
    HDC hdc = BeginPaint(hwnd, &ps);
    HDC DhWnd = GetDC(HWND_DESKTOP);
    // 重新复制桌面到窗口
    BitBlt(hdc, 0, 0, myWidth, myHeight, DhWnd, 0, 0, SRCCOPY);
    
    // 再次执行内部像素复制（使用内存DC方式）
    HDC memDC = CreateCompatibleDC(hdc);
    HBITMAP memBmp = CreateCompatibleBitmap(hdc, 100, 500);
    HBITMAP oldBmp = (HBITMAP)SelectObject(memDC, memBmp);
    BitBlt(memDC, 0, 0, 100, 500, hdc, 300, 0, SRCCOPY);
    BitBlt(hdc, 300, 0, 100, 500, memDC, 0, 0, SRCCOPY);
    
    SelectObject(memDC, oldBmp);
    DeleteObject(memBmp);
    DeleteDC(memDC);
    
    ReleaseDC(hwnd, DhWnd);
    EndPaint(hwnd, &ps);
    return 0;
}

内容的提问来源于stack exchange，提问作者bugger