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c++winapigdi

Transparency in GDI DCs

I have the "simple" goal of drawing a bitmap with some transparency around it on the screen. That bit wasn't so hard:

#include <windows.h>
#include "BBKG.h"

HINSTANCE hInst;
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
static int wH = 156;
static int wW = 166;

HBITMAP CreateBitmapMask(HBITMAP hbmColour, COLORREF crTransparent)
{
    HDC mem0, mem1;
    HBITMAP hbmMask;
    BITMAP bm;
    GetObject(hbmColour, sizeof(BITMAP), &bm);
    hbmMask = CreateBitmap(bm.bmWidth, bm.bmHeight, 1, 1, NULL);
    mem0 = CreateCompatibleDC(0);
    mem1 = CreateCompatibleDC(0);
    SelectObject(mem0, hbmColour);
    SelectObject(mem1, hbmMask);
    SetBkColor(mem0, crTransparent);
    BitBlt(mem1, 0, 0, bm.bmWidth, bm.bmHeight, mem0, 0, 0, SRCCOPY);
    BitBlt(mem0, 0, 0, bm.bmWidth, bm.bmHeight, mem1, 0, 0, SRCINVERT);
    DeleteDC(mem0);
    DeleteDC(mem1);

    return hbmMask;
}
int WINAPI wWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PWSTR pCmdLine, int nCmdShow)
{
    hInst = hInstance;
    MSG  msg;
    HWND hwnd;
    WNDCLASSW wc;

    wc.style = 0;
    wc.cbClsExtra = 0;
    wc.cbWndExtra = 0;
    wc.lpszClassName = L"nope";
    wc.hInstance = hInst;
    wc.hbrBackground = NULL;
    wc.lpszMenuName = NULL;
    wc.lpfnWndProc = WndProc;
    wc.hCursor = LoadCursor(NULL, IDC_ARROW);
    wc.hIcon = LoadIcon(NULL, IDI_APPLICATION);

    RegisterClassW(&wc);
    hwnd = CreateWindowW(wc.lpszClassName, L"",
        WS_VISIBLE | WS_POPUP|  WS_EX_TRANSPARENT,
        100, 100, wW, wH, NULL, NULL, hInst, NULL);

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

    while (GetMessage(&msg, NULL, 0, 0)) {
        //Workaround for focusables stealing my Esc key
        if (msg.message == WM_KEYDOWN){
            if (msg.wParam == VK_ESCAPE) {
                SendMessage(hwnd, WM_CLOSE, 0, 0);
            }
        }
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }

    return (int)msg.wParam;
}

LRESULT CALLBACK WndProc(HWND hwnd, UINT msg,
    WPARAM wParam, LPARAM lParam)
{
    static int px;
    static int py;
    static HBITMAP bhbm;
    static RECT nRect = { 0, 0, wW, wH };

    switch (msg)
    {
    case WM_CREATE:
    {
        HWND bb = CreateWindowW(L"STATIC", L"",
            WS_VISIBLE | WS_CHILD ,
            0, 0, wW, wH,
            hwnd, (HMENU)11, hInst, NULL);
        //SetTimer(hwnd, 1, 80, NULL);

        return 0;
    }
    case WM_PAINT: {
        //Vars
        RECT wRect;
        if (GetUpdateRect(hwnd, &wRect, FALSE) == 0) {
            return 0; //Nothing to paint
        }
        PAINTSTRUCT gps;
        PAINTSTRUCT ps;
        BeginPaint(hwnd, &gps);
        HWND bb = GetDlgItem(hwnd, 11);
        HDC bbhdc = BeginPaint(bb, &ps);
        HDC mdc = CreateCompatibleDC(bbhdc);

        //Load Image
        BITMAP pBM;
        HBITMAP pHBM = (HBITMAP)LoadImage(NULL, L"twi00.bmp", 0, 0, 0, LR_LOADFROMFILE);
        HBITMAP pMBM = CreateBitmapMask((HBITMAP)pHBM, 0x00000000);
        GetObject(pHBM, sizeof(pBM), &pBM);

        //Paint
        HBITMAP oldBM = (HBITMAP)SelectObject(mdc, pMBM);
        BitBlt(bbhdc, 0, 0, pBM.bmWidth, pBM.bmHeight, mdc, 0, 0, SRCAND);
        SelectObject(mdc, pHBM);
        BitBlt(bbhdc, 0, 0, pBM.bmWidth, pBM.bmHeight, mdc, 0, 0, SRCPAINT);

        //Cleanup
        SelectObject(mdc, oldBM);
        DeleteObject(pHBM);
        DeleteObject(pMBM);
        DeleteDC(mdc);
        EndPaint(bb, &ps);
        EndPaint(hwnd, &gps);
        return 1;
    }
    case WM_ERASEBKGND: {
        return 0;
    }
    case WM_DESTROY:
    {
        DeleteObject(bhbm);
        PostQuitMessage(0);
        return 0;
    }
    case WM_LBUTTONDOWN:
        SetCapture(hwnd);
        px = LOWORD(lParam);
        py = HIWORD(lParam);
        return 1;
    case WM_LBUTTONUP:
    {
        ReleaseCapture();
        return 1;
    }
    case WM_MOUSEMOVE:
    {
        if (GetCapture() == hwnd)
        {
            RECT rcWindow;
            GetWindowRect(hwnd, &rcWindow);
            SetWindowPos(hwnd, NULL, rcWindow.left + LOWORD(lParam) - px, rcWindow.top + HIWORD(lParam) - py, 0, 0, SWP_NOSIZE | SWP_NOZORDER);
        }
        break;
    }
    }
    return DefWindowProcW(hwnd, msg, wParam, lParam);
}

Use any generic bmp with a black border will do, I used this:

pony

Now the question is, how can I make it so that when I move the window (click/drag) the background updates? I was hoping for something like putting the bitmap into a transparent window so that it's overlayed on top of things but it seems to just grab the pixels of what ever is behind it.

I'm attempting to do this without GDI+ or other libraries, if possible.

Solution

  • CreateWindow() does not accept extended window styles, such as WS_EX_TRANSPARENT (which is why it has EX in its name). You have to use CreateWindowEx() instead:

    hwnd = CreateWindowExW(WS_EX_TRANSPARENT,
    wc.lpszClassName, L"",
    WS_VISIBLE | WS_POPUP,
    100, 100, wW, wH, NULL, NULL, hInst, NULL);

    A better option is to create a layered window (see also this) by using the WS_EX_LAYERED extended style). Then you can use the UpdateLayeredWindow() function to provide the window with the bitmap and the transparent color (you can also specify alpha as well). Let the window manage all of the hard work of drawing the bitmap transparently for you.

    Your WndProc() can also respond to the WM_NCHITTEST message to tell the OS that all clicks on the window should be treated as if the user were clicking on the window's titlebar. Let the window handle the mouse tracking and auto-positioning for you.

    Try something more like this:

    #include <windows.h>

HINSTANCE hInst;
static int wH = 156;
static int wW = 166;

LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);

int WINAPI wWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PWSTR pCmdLine, int nCmdShow)
{
    hInst = hInstance;

    WNDCLASSW wc = {0};    
    wc.lpszClassName = L"nope";
    wc.hInstance = hInst;
    wc.lpfnWndProc = WndProc;
    wc.hCursor = LoadCursor(NULL, IDC_ARROW);
    wc.hIcon = LoadIcon(NULL, IDI_APPLICATION);

    RegisterClassW(&wc);
    HWND hwnd = CreateWindowEx(WS_EX_LAYERED,
        wc.lpszClassName, L"",
        WS_POPUP, 100, 100, wW, wH, NULL, NULL,
        hInst, NULL);

    HBITMAP hBmp = (HBITMAP) LoadImage(NULL, L"twi00.bmp", 0, 0, 0, LR_LOADFROMFILE);
    HDC hdcScreen = GetDC(0);
    HDC hdcBmp = CreateCompatibleDC(hdcScreen);
    HBITMAP oldBM = (HBITMAP) SelectObject(hdcBmp, hBmp);

    POINT pt = {0};
    UpdateLayeredWindow(hwnd,
        hdcScreen,
        NULL, NULL,
        hdcBmp, &pt,
        RGB(0, 0, 0), // black
        NULL, ULW_COLORKEY
    );

    SelectObject(hdcBmp, oldBM);
    DeleteDC(hdcBmp);
    ReleaseDC(0, hdcScreen);
    DeleteObject(hBmp);

    ShowWindow(hwnd, nCmdShow);

    MSG msg;
    while (GetMessage(&msg, NULL, 0, 0) > 0) {
        //Workaround for focusables stealing my Esc key
        if ((msg.message == WM_KEYDOWN) && (msg.wParam == VK_ESCAPE) {
            SendMessage(hwnd, WM_CLOSE, 0, 0);
        }
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }

    return (int) msg.wParam;
}

LRESULT CALLBACK WndProc(HWND hwnd, UINT msg,
    WPARAM wParam, LPARAM lParam)
{    
    switch (msg)
    {
        case WM_DESTROY:
        {
            PostQuitMessage(0);
            return 0;
        }

        case WM_NCHITTEST:
        {
            return HTCAPTION;
        }
    }

    return DefWindowProcW(hwnd, msg, wParam, lParam);
}