Creating a transparent window in C++ Win32

Use the SetLayeredWindowAttributes^archive function, this allows you to set a mask color that will become transparent, thus allowing the background to show through.

You will also need to configure your window with the layered flag, e.g.:

SetWindowLong(hwnd, GWL_EXSTYLE, GetWindowLong(hwnd, GWL_EXSTYLE) | WS_EX_LAYERED);

After that it's fairly simple:

// Make red pixels transparent:
SetLayeredWindowAttributes(hwnd, RGB(255,0,0), 0, LWA_COLORKEY);

When your PNG contains semi-transparent pixels that you want to blend with the background, this becomes more complicated. You could try looking at the approach in this CodeProject article:

Cool, Semi-transparent and Shaped Dialogs with Standard Controls for Windows 2000 and Above

I was able to do exactly what I wanted by using the code from Part 1 and Part 2 of this series:

Displaying a Splash Screen with C++

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• Part 1: Creating a HBITMAP ^archive
• Part 2: Displaying the window ^archive

Those blog posts are talking about displaying a splash screen in Win32 C++, but it was almost identical to what I needed to do. I believe the part that I was missing was that instead of just painting the PNG to the window using GDI+, I needed to use the UpdateLayeredWindow function with the proper BLENDFUNCTION parameter. I'll paste the SetSplashImage method below, which can be found in Part 2 in the link above:

void SetSplashImage(HWND hwndSplash, HBITMAP hbmpSplash)
{
  // get the size of the bitmap
  BITMAP bm;
  GetObject(hbmpSplash, sizeof(bm), &bm);
  SIZE sizeSplash = { bm.bmWidth, bm.bmHeight };

  // get the primary monitor's info
  POINT ptZero = { 0 };
  HMONITOR hmonPrimary = MonitorFromPoint(ptZero, MONITOR_DEFAULTTOPRIMARY);
  MONITORINFO monitorinfo = { 0 };
  monitorinfo.cbSize = sizeof(monitorinfo);
  GetMonitorInfo(hmonPrimary, &monitorinfo);

  // center the splash screen in the middle of the primary work area
  const RECT & rcWork = monitorinfo.rcWork;
  POINT ptOrigin;
  ptOrigin.x = 0;
  ptOrigin.y = rcWork.top + (rcWork.bottom - rcWork.top - sizeSplash.cy) / 2;

  // create a memory DC holding the splash bitmap
  HDC hdcScreen = GetDC(NULL);
  HDC hdcMem = CreateCompatibleDC(hdcScreen);
  HBITMAP hbmpOld = (HBITMAP) SelectObject(hdcMem, hbmpSplash);

  // use the source image's alpha channel for blending
  BLENDFUNCTION blend = { 0 };
  blend.BlendOp = AC_SRC_OVER;
  blend.SourceConstantAlpha = 255;
  blend.AlphaFormat = AC_SRC_ALPHA;

  // paint the window (in the right location) with the alpha-blended bitmap
  UpdateLayeredWindow(hwndSplash, hdcScreen, &ptOrigin, &sizeSplash,
      hdcMem, &ptZero, RGB(0, 0, 0), &blend, ULW_ALPHA);

  // delete temporary objects
  SelectObject(hdcMem, hbmpOld);
  DeleteDC(hdcMem);
  ReleaseDC(NULL, hdcScreen);
}