在透明窗口中绘制一个半透明矩形

我在stackoverflow中找到了这段代码，它确实使用位图在透明窗口上绘制了一个矩形。但是以某种方式我无法更改矩形的透明度。更确切地说，我可以，但是它变得更暗了。好像位图本身具有黑色背景。如何使矩形透明？

void paintRect(HDC hdc, RECT dim, COLORREF penCol, COLORREF brushCol, int opacity)
    {
        HDC tempHDC = CreateCompatibleDC(hdc);
    
        BITMAPINFO bitmapInfo;
        ZeroMemory(&bitmapInfo, sizeof(BITMAPINFO));
    
        bitmapInfo.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
        bitmapInfo.bmiHeader.biWidth = dim.right - dim.left;
        bitmapInfo.bmiHeader.biHeight = dim.bottom - dim.top;
        bitmapInfo.bmiHeader.biPlanes = 1;
        bitmapInfo.bmiHeader.biBitCount = 32;
        bitmapInfo.bmiHeader.biCompression = BI_RGB;
        bitmapInfo.bmiHeader.biSizeImage = (dim.right - dim.left) * (dim.bottom - dim.top) * 4;
    
        HBITMAP hBitmap = CreateDIBSection(tempHDC, &bitmapInfo, DIB_RGB_COLORS, NULL, NULL, 0x0);
    
        SelectObject(tempHDC, hBitmap);
        SetDCPenColor(tempHDC, RGB(0, 0, 255));
        SetDCBrushColor(tempHDC, RGB(0, 0, 255));
        FillRect(tempHDC, &dim, CreateSolidBrush(RGB(0, 0, 255)));
    
        BLENDFUNCTION blend = { AC_SRC_OVER, 0, 255, 0 };
        AlphaBlend(hdc, dim.left, dim.top, dim.right, dim.bottom, tempHDC, dim.left, dim.top, dim.right, dim.bottom, blend);
    }

完整代码

#include <Windows.h>

int wWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PWSTR pCmdLine, int nCmdShow);
LRESULT CALLBACK windowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam);

int APIENTRY wWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PWSTR pCmdLine, int nCmdShow)
{
    WNDCLASS windowClass {};

    windowClass.lpfnWndProc = windowProc;
    windowClass.hInstance = hInstance;
    windowClass.lpszClassName = L"Keystrokes";
    windowClass.style = CS_NOCLOSE;
    windowClass.hbrBackground = (HBRUSH)GetStockObject(BLACK_BRUSH);
    windowClass.hCursor = LoadCursor(NULL, IDC_ARROW);

    if (!RegisterClass(&windowClass))
    {
        return 0;
    }

    HWND hwnd = CreateWindowEx(WS_EX_LAYERED | WS_EX_TOPMOST | WS_EX_TOOLWINDOW, L"Keystrokes", L"Keystrokes", WS_POPUP, 0, 0, 148, 140, 0, 0, hInstance, 0);

    if (!hwnd)
    {
        return 0;
    }

    SetLayeredWindowAttributes(hwnd, RGB(0, 0, 0), 0, LWA_COLORKEY);

    ShowWindow(hwnd, nCmdShow);

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

    return 0;
}

void paintRect(HDC hdc, RECT dim, COLORREF penCol, COLORREF brushCol, int opacity)
{
    HDC tempHDC = CreateCompatibleDC(hdc);

    BITMAPINFO bitmapInfo;
    ZeroMemory(&bitmapInfo, sizeof(BITMAPINFO));

    bitmapInfo.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
    bitmapInfo.bmiHeader.biWidth = dim.right - dim.left;
    bitmapInfo.bmiHeader.biHeight = dim.bottom - dim.top;
    bitmapInfo.bmiHeader.biPlanes = 1;
    bitmapInfo.bmiHeader.biBitCount = 32;
    bitmapInfo.bmiHeader.biCompression = BI_RGB;
    bitmapInfo.bmiHeader.biSizeImage = (dim.right - dim.left) * (dim.bottom - dim.top) * 4;

    HBITMAP hBitmap = CreateDIBSection(tempHDC, &bitmapInfo, DIB_RGB_COLORS, NULL, NULL, 0x0);

    SelectObject(tempHDC, hBitmap);
    SetDCPenColor(tempHDC, RGB(0, 0, 255));
    SetDCBrushColor(tempHDC, RGB(0, 0, 255));
    FillRect(tempHDC, &dim, CreateSolidBrush(RGB(0, 0, 255)));

    BLENDFUNCTION blend = { AC_SRC_OVER, 0, 255, 0 };
    AlphaBlend(hdc, dim.left, dim.top, dim.right, dim.bottom, tempHDC, dim.left, dim.top, dim.right, dim.bottom, blend);
}

LRESULT CALLBACK windowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
    switch (uMsg)
    {
        case WM_LBUTTONDOWN:
        {
            SendMessage(hwnd, WM_NCLBUTTONDOWN, HTCAPTION, 0);
        }
        
        break;

        case WM_MBUTTONDOWN:
        {
            PostQuitMessage(0);
        }

        break;

        case WM_PAINT:
        {
            PAINTSTRUCT ps;
            HDC hDC = BeginPaint(hwnd, &ps);

            paintRect(hDC, { 0, 0, 48, 48 }, RGB(255, 0, 0), RGB(255, 255, 255), 255);

            EndPaint(hwnd, &ps);
        }
    }

    return DefWindowProc(hwnd, uMsg, wParam, lParam);
}