Using AlphaBlend to draw slightly transparent rectangle fails

c++, gdi, winapi

Solution

This will work:

bool paintRect(HDC hdc, RECT dim, COLORREF penCol, COLORREF brushCol, unsigned int opacity)
{
        HDC tempHdc         = CreateCompatibleDC(hdc);
        BLENDFUNCTION blend = {AC_SRC_OVER, 0, 127, 0};

        HBITMAP hbitmap;       // bitmap handle 
        BITMAPINFO bmi;        // bitmap header 
        // zero the memory for the bitmap info 
        ZeroMemory(&bmi, sizeof(BITMAPINFO));

        // setup bitmap info  
        // set the bitmap width and height to 60% of the width and height of each of the three horizontal areas. Later on, the blending will occur in the center of each of the three areas. 
        bmi.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
        bmi.bmiHeader.biWidth = dim.right-dim.left;
        bmi.bmiHeader.biHeight = dim.bottom-dim.top;
        bmi.bmiHeader.biPlanes = 1;
        bmi.bmiHeader.biBitCount = 32;         // four 8-bit components 
        bmi.bmiHeader.biCompression = BI_RGB;
        bmi.bmiHeader.biSizeImage = (dim.right-dim.left) * (dim.bottom-dim.top) * 4;

        // create our DIB section and select the bitmap into the dc 
        hbitmap = CreateDIBSection(tempHdc, &bmi, 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)));

        return bool(AlphaBlend(hdc, dim.left, dim.top, dim.right, dim.bottom, tempHdc, dim.left, dim.top, dim.right, dim.bottom, blend)); 
}

Problem

I am attempting to draw a slightly transparent blue rectangle in Native Win32 C++. I am using the function AlphaBlend() but its not drawing anything onto the window, nothing happens. My Problem: When I run my function to draw a slightly transparent rectangle, it doesn't get shown on my window. I have a feeling I am doing this wrong, maybe I should be using a HBITMAP? Can you tell me what I need to do to get my function to draw a slightly transparent rectangle on the window? Also I'm aware of GDI+ but I want to avoid it for now because I am getting alot of compile/include errors when I use that library plus I want to go as low/native as possible without the help of libraries that do everything for me. ``` bool paintRect(HDC hdc, RECT dim, COLORREF penCol, COLORREF brushCol, unsigned int opacity) { HDC tempHdc = CreateCompatibleDC(hdc); BLENDFUNCTION blend = {AC_SRC_OVER, 0, 127, AC_SRC_ALPHA}; SetDCPenColor(tempHdc, RGB(255,255,0)); SetDCBrushColor(tempHdc, RGB(255,255,0)); Rectangle(tempHdc, dim.left, dim.top, dim.right, dim.bottom); return bool(AlphaBlend(hdc, dim.left, dim.top, dim.right, dim.bottom, tempHdc, dim.left, dim.top, dim.right, dim.bottom, blend)); } // Usage case WM_PAINT: { HDC hdc; PAINTSTRUCT ps; hdc = BeginPaint(hwnd, &ps); RECT a = {0,0,100,100}; paintRect(hdc, a, RGB(255,255,0), RGB(255,255,0), 127); // 127 is 50% transparency right? EndPaint(hwnd, &ps); } break; ```

Original source