Question

我想在给定窗口中绘制一个缓冲区（带有alpha信息！）。绘图是在WM_PAINT之外完成的（它是在从Chromium-Embedded-Framework调用的CefRenderHandler :: OnPaint方法中完成的。）

我遇到的问题是：

窗口的旧内容未被清除（如果缓冲区已更改，我会绘制旧内容并绘制新内容）。
错误地解释了alpha通道 - 我认为，即使像素具有alpha信息，它也会被绘制，因为它没有alpha信息

这是我到目前为止所做的：

OnPaint（...）：

HDC screen_dc = GetDC(windowHandle);
RECT rcWin;
GetClientRect(windowHandle, &rcWin);

BITMAPINFO info;
ZeroMemory(&info, sizeof(BITMAPINFO));
info.bmiHeader.biBitCount = 32;
info.bmiHeader.biWidth = width;
info.bmiHeader.biHeight = -height;
info.bmiHeader.biPlanes = 1;
info.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
info.bmiHeader.biSizeImage = width*height * 4;
info.bmiHeader.biCompression = BI_RGB;

void *buf;
HBITMAP hDib = CreateDIBSection(screen_dc, &info, DIB_RGB_COLORS, (void **)&buf, 0, 0);
memcpy(buf, buffer, width * height * 4); //buffer contains bitmap to draw
HDC hDibDC = CreateCompatibleDC(screen_dc);
HGDIOBJ hOldObj = SelectObject(hDibDC, hDib);
BLENDFUNCTION blendFunction_;
blendFunction.BlendOp = AC_SRC_OVER;
blendFunction.BlendFlags = 0;
blendFunction.SourceConstantAlpha = 255;
blendFunction.AlphaFormat = AC_SRC_ALPHA;
AlphaBlend(screen_dc, 0, 0, width, height, hDibDC, 0, 0, width, height, blendFunction);

SelectObject(hDibDC, hOldObj);
ReleaseDC(windowHandle, screen_dc);
DeleteObject(hDib);
DeleteDC(hDibDC);

创建窗口：

WNDCLASSEX wcex = {0};
wcex.cbSize = sizeof(wcex);
wcex.style = CS_HREDRAW | CS_VREDRAW;
wcex.lpfnWndProc = BrowserWindowWndProc;
wcex.hInstance = hinstance;
wcex.hCursor = LoadCursor(NULL, IDC_ARROW);
wcex.hbrBackground = WHITE_BRUSH;
wcex.lpszClassName = BROWSER_WINDOW_CLASS;
wcex.cbClsExtra = 0;
wcex.cbWndExtra = 0;
RegisterClassEx(&wcex);

DWORD exStyle{0};
exStyle |= WS_EX_TOOLWINDOW;
exStyle |= WS_EX_LAYERED;

DWORD style {0};
style |= WS_SYSMENU;
style |= WS_VISIBLE;

HWND hWnd = CreateWindowEx(
    exStyle,
    BROWSER_WINDOW_CLASS, 
    BROWSER_WINDOW_CLASS,
    style,
    100,
    100,
    300,
    300,
    nullptr,
    nullptr, 
    hinstance,
    nullptr
);
...
SetLayeredWindowAttributes(hWnd, RGB(255, 255, 255), 255, LWA_COLORKEY);

你能帮我解决这些问题吗？

提前谢谢。

Answer 1

没有必要同时使用AlphaBlend和分层窗口。仅使用分层窗口：

void OnPaint(HDC hdc, int width, int height, HBITMAP hbitmap)
{
    HDC memdc = CreateCompatibleDC(hdc);
    auto oldbmp = SelectObject(memdc, hbitmap);

    BITMAP bm;
    GetObject(hbitmap, sizeof(bm), &bm);
    BitBlt(hdc, 0, 0, bm.bmWidth, bm.bmHeight, memdc, 0, 0, SRCCOPY);

    SelectObject(memdc, oldbmp);
    DeleteDC(memdc);
}

其中hbitmap是先前创建的位图的句柄。使用SetLayeredWindowAttributes(hwnd, RGB(255,255,255), 255, LWA_COLORKEY);

时，位图的白色区域应显示为透明

或使用LWA_COLORKEY | LWA_ALPHA调整透明度和Alpha级别。

假设OnPaint是对WM_PAINT的回复，请使用BeginPaint/EndPaint代替GetDC/ReleaseDC

请注意WHITE_BRUSH为零，因此wcex.hbrBackground = WHITE_BRUSH;将背景画笔设置为零。改为指定画笔手柄。

或者，您可以在同一窗口中使用TransparentBlt：

HDC memdc = CreateCompatibleDC(hdc);
auto oldbmp = SelectObject(memdc, hbitmap);

BITMAP bm;
GetObject(hbitmap, sizeof(bm), &bm);
TransparentBlt(hdc, 0, 0, width, height, 
    memdc, 0, 0, bm.bmWidth, bm.bmHeight, RGB(255, 255, 255));

SelectObject(memdc, oldbmp);
DeleteDC(memdc);

带有缓冲区用法的Windows AlphaBlend

1 个答案: