在透明窗口中绘制半透​​明矩形

Question

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。但不知何故，我无法改变矩形的透明度。更准确地说，我可以，但它会变暗。就好像位图本身有黑色背景一样。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);
}

Answer 1

“好像位图本身有黑色背景一样。”

因为你的背景设置为：

windowClass.hbrBackground = (HBRUSH)GetStockObject(BLACK_BRUSH);

当然你可以设置成NULL_BRUSH让它看起来透明：

windowClass.hbrBackground = (HBRUSH)GetStockObject(NULL_BRUSH);

但是当你移动它的时候，你会发现这并不是让窗口透明，而是停止绘制背景：

正如@Ben 回答here：

您还没有使窗口透明，您刚刚停止绘制背景。您所看到的背景只是窗口第一次绘制时发生在窗口下方的任何内容。

而你需要参考Layered Windows

不能直接在窗口上添加矩形，应该使用分层窗口，然后添加你需要的。

这是一个示例，您可以参考它：

#include <Windows.h>
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
int WINAPI WinMain(_In_  HINSTANCE hInstance, _In_opt_ HINSTANCE hPrevInstance, _In_  LPSTR szCmdLine, _In_  int iCmdShow)
{
    static TCHAR szAppName[] = TEXT("test window");
    HWND hwnd;
    MSG msg;
    WNDCLASS wndclass;
    wndclass.style = CS_HREDRAW | CS_VREDRAW;
    wndclass.lpfnWndProc = WndProc;
    wndclass.cbClsExtra = 0;
    wndclass.cbWndExtra = 0;
    wndclass.hInstance = hInstance;
    wndclass.hIcon = LoadIcon(NULL, IDI_APPLICATION);
    wndclass.hCursor = LoadCursor(NULL, IDC_ARROW);
    wndclass.hbrBackground = (HBRUSH)GetStockObject(WHITE_BRUSH);
    wndclass.lpszMenuName = NULL;
    wndclass.lpszClassName = szAppName;
    if (!RegisterClass(&wndclass))
    {
        MessageBox(NULL, TEXT("This program requires Windows NT!"), szAppName, MB_ICONERROR);
    }

    hwnd = CreateWindowEx(WS_EX_LAYERED | WS_EX_TOPMOST | WS_EX_TOOLWINDOW, szAppName,
        TEXT("the hello program"),
        WS_POPUP,
        CW_USEDEFAULT,
        CW_USEDEFAULT,
        148,
        148,
        NULL,
        NULL,
        hInstance,
        NULL);
    SetLayeredWindowAttributes(hwnd, 0, 1, LWA_ALPHA);
    ShowWindow(hwnd, iCmdShow);
    UpdateWindow(hwnd);
    while (GetMessageW(&msg, NULL, 0, 0))
    {
        TranslateMessage(&msg);
        DispatchMessageW(&msg);
    }
    return msg.wParam;
}
VOID FadeRect(RECT* prc, HDC hdc)
{
    BOOL fFade = FALSE;
    static HWND hwnd;
    SIZE size;
    POINT ptSrc = { 0, 0 };
    BLENDFUNCTION blend;

    SystemParametersInfo(SPI_GETSELECTIONFADE, 0, &fFade, 0);
    if (!fFade)
        return;
    if (!hwnd) hwnd = CreateWindowEx(WS_EX_LAYERED | 
        WS_EX_TRANSPARENT | 
        WS_EX_TOPMOST | WS_EX_TOOLWINDOW,
        L"static", L"static", WS_POPUP | WS_VISIBLE, prc->left,
        prc->top, prc->right, prc->bottom, NULL, (HMENU)0, NULL, NULL);
    else MoveWindow(hwnd, prc->left, prc->top, prc->right, prc->bottom, TRUE);

    RECT rect{ prc->left,prc->top,prc->right,prc->bottom };
    size.cx = prc->right - prc->left;
    size.cy = prc->bottom - prc->top;

    blend.BlendOp = AC_SRC_OVER;
    blend.BlendFlags = 0;
    blend.AlphaFormat = 0;
    blend.SourceConstantAlpha = 150;
    UpdateLayeredWindow(hwnd, NULL, NULL, &size, hdc, &ptSrc, 0,
        &blend, ULW_ALPHA);
}
LRESULT CALLBACK WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    switch (message)
    {
    case WM_LBUTTONDOWN:
    {
        SendMessage(hwnd, WM_NCLBUTTONDOWN, HTCAPTION, 0);
        InvalidateRect(hwnd, NULL, TRUE);
    }
    break;
    case WM_MBUTTONDOWN:
    {
        PostQuitMessage(0);
    }

    break;

    case WM_PAINT:
    {
        PAINTSTRUCT ps;
        RECT rect;
        GetClientRect(hwnd, &rect);
        MapWindowPoints(hwnd, GetParent(hwnd), (LPPOINT)&rect, 2);
        HDC hDC = BeginPaint(hwnd, &ps);
        RECT rc{ rect.left,rect.top,rect.left + 48,rect.top + 48 };
        FadeRect(&rc, hDC);
        EndPaint(hwnd, &ps);
    }
    break;
    case WM_DESTROY:
        PostQuitMessage(0);
        break;
    }

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