Ben Straub

Native Win32 for fun and profit

February 02, 2011

[Note: this is ported from my old blog, and there’s more discussion there.]

All the cool kids these days are playing with awesome dynamic languages, or on cool frameworks. I’m stuck with c++ at work, but every now and then I get to do something cool with it.

radial menu

That’s the Wacom radial menu, which is implemented as a fully alpha-blended window in native Win32. Something like this is dead simple in WPF, but with native code it’s a bit trickier. I used WTL, GDI+, and a handy, little-known Windows feature to get it done, and I’m going to share my secrets with you, dear reader.



Windowing frameworks are thick on the ground, and I’ve been mostly dissatisfied with the abilities of the Win32-wrapping category. However, they make something like this reusable, so what the heck.

You can grab WTL at the project home on SourceForge. For this project, I’m just taking the files in the include directory and putting them under wtl in my project directory, so I don’t get the Windows SDK versions instead.

I’ve found this to be the best way to include the WTL headers:

#define _SECURE_ATL 1

// These are required to be included first
#include "atlbase.h"
#include "atlwin.h"
#include "wtl/atlapp.h"

#include "wtl/atlgdi.h" // For WTL::CDC
#include "wtl/atlframe.h" // For WTL::CFrameWindowImpl

Those defines specify that the ATL and WTL classes should stay safely ensconced in their own namespaces. This means you have to reference them as WTL::CFrameWndImpl, but it keeps the global namespace clean, which is a major failing of windows.h.


GDI+ is an immediate-mode drawing API that has shipped with Windows since XP, so I can use it without needing to ship yet another redistributable installer. Here’s all you need to do:

#pragma comment(lib, "gdiplus.lib")
#include <gdiplus.h>

While GDI+ is written in c++ and uses classes, it’s initialization isn’t RAII-friendly, so I wrote a little wrapper class:

class ScopedGdiplusInitializer
    Gdiplus::GdiplusStartupInput gdisi;
    Gdiplus::GdiplusStartup(&mGdipToken, &gdisi, NULL);
  ULONG_PTR mGdipToken;

Now I can write my main function like this:

int main()
  ScopedGdiplusInitializer gdiplusinit;
  // ...


The production code for this feature uses boost (specifically shared_ptr), but in the interest of simplicity I’ve left it out. If you use boost, or your compiler supports the new std::shared_ptr introduced with TR1, I highly recommend you use that instead of raw pointers whenever possible.

A window class

Here’s where it all comes together. Meet me after the code, and I’ll explain more fully.

class AlphaWindow
  : public WTL::CFrameWindowImpl< AlphaWindow, ATL::CWindow,
    ATL::CWinTraits< WS_POPUP, WS_EX_LAYERED > >

  virtual ~AlphaWindow()
    if (IsWindow())

  void UpdateWithBitmap(Gdiplus::Bitmap *bmp_I, POINT *windowLocation_I = NULL)
    // Create a memory DC
    HDC screenDC = ::GetDC(NULL);
    WTL::CDC memDC;
    ::ReleaseDC(NULL, screenDC);

    // Copy the input bitmap and select it into the memory DC
    WTL::CBitmap localBmp;
      bmp_I->GetHBITMAP(Gdiplus::Color(0,0,0,0), &localBmp.m_hBitmap);
    HBITMAP oldBmp = memDC.SelectBitmap(localBmp);

    // Update the display
    POINT p = {0};
    SIZE s = {bmp_I->GetWidth(), bmp_I->GetHeight()};
      ::UpdateLayeredWindow(m_hWnd, NULL, windowLocation_I, &s, memDC,
        &p, RGB(0,255,255), &bf, ULW_ALPHA);

    // Cleanup

Layered Windows

The magic ingredients for this class are the WS_EX_* styles and the UpdateLayeredWindow call.

First, the styles. These are specified on line 3, as part of the base class. That’s just how you declare your window’s styles in WTL. There are two:

  • WS_POPUP means this is a square window with no decorations around the outside. No title bar, no close button, nothing.
  • WS_EX_LAYERED tells Windows that this window is different, and that it can do per-pixel alpha blending with other windows. This was available in Windows 2000, but starting with Vista the window’s face could be cached and composited by the GPU, which made it much more useful.

The call to UpdateLayeredWindow on line 37 is what tells Windows what the contents of the display are. There’s some clunky interop code here, since the GDI+ Bitmap object can’t be used directly with the GDI-oriented layered window API. I’m sure there’s a better way, but in my case the overhead of copying my smallish Bitmap into another smallish HBITMAP wasn’t a problem.

WTL complains rather loudly if a window object is destroyed before the HWND it’s wrapping is closed, so the destructor on line 7 takes care of that.

Pretty Pictures

That UpdatedLayeredWindow call is wrapped in a method that takes a GDI+ bitmap, so now all we need to do is provide it with one. GDI+ makes this pretty easy, especially when compared to GDI code:

using namespace Gdiplus;
// Create a bitmap buffer
Bitmap bmp(400,400);
// Context for drawing on the bitmap
Graphics g(&bmp);
// ...

All together now

Here’s the main function of my little test program.

int main()
  ScopedGdiplusInitializer init;

    // Create the display window
    AlphaWindow wnd;
    wnd.SetWindowPos(NULL, 200,200, 0,0, SWP_NOSIZE | SWP_NOREPOSITION);

    // Create a backbuffer
    Gdiplus::Bitmap bmp(400,400);

    // Clear the background of the buffer to translucent black
    Gdiplus::Graphics g(&bmp);

    // This tells GDI+ to anti-alias when it draws shapes

    // Draw two semi-transparent ellipses
    Gdiplus::Pen redPen(Gdiplus::Color(100,255,0,0), 10.);
    Gdiplus::Pen bluePen(Gdiplus::Color(100,0,0,255), 10.);
    g.DrawEllipse(&redPen, 50,50, 200,300);
    g.DrawLine(&bluePen, 175,10, 175,390);
    g.DrawEllipse(&redPen, 100,50, 200,300);

    // Update the window's display

    // Wait to exit

I know, programmer demos of this are always ugly. Maybe one day I’ll write about how to store a PNG as a resource, and load it in for use with this. For now, you get an ugly screenshot:

ugly test image

Ben Straub lives and works in Portland building useful things. You should follow him on Twitter.