头文件

主要声明
DECLARE_MESSAGE_MAP()
VOID OnPaint();

cpp文件

#include <gdiplus.h>

using namespace Gdiplus;

BEGIN_MESSAGE_MAP(animateWindow, CDialog)
	ON_WM_PAINT()
END_MESSAGE_MAP()

void transparentWindow::OnPaint()
{

	initTransparent();

	CDialog::OnPaint();
}

//OnPaint只会响应一遍，设置了UpdateLayeredWindow之后就不响应了，不过如果只是做一个背景的话，不用响应OnPaint，
void transparentWindow::initTransparent()
{
	if (0 == m_ImagePath.GetLength())
	{
		return;
	}

	DWORD dwExStyle = GetWindowLong(m_hWnd, GWL_EXSTYLE);
	if ((dwExStyle & WS_EX_LAYERED) != WS_EX_LAYERED)
	{
		SetWindowLong(GetSafeHwnd(), GWL_EXSTYLE, dwExStyle | WS_EX_LAYERED);
		//SetLayeredWindowAttributes(m_BkColor, 128, LWA_COLORKEY);
        //SetLayeredWindowAttributes和UpdateLayeredWindow互斥，只能一个有效
	}

	Bitmap *_pImage = Bitmap::FromFile(m_ImagePath);
    //Bitmap::FromFile返回的对象可以用DeleteObject释放掉
	//透明度由PNG图片的Alpha通道决定  

	if (NULL == dcSrc)
	{
		dcSrc = new CDC();
		//bmp = new CBitmap();
	}

	RECT rt;
	GetClientRect(&rt);

	CDC *pDCDst = GetDC();
	dcSrc->CreateCompatibleDC(pDCDst);
	//bmp->CreateCompatibleBitmap(pDCDst, rt.right, rt.bottom);
	//dcSrc->SelectObject(&bmp);

	Gdiplus::Color color;//默认black
	HBITMAP hBitmap;
	_pImage->GetHBITMAP(color, &hBitmap);
	dcSrc->SelectObject(hBitmap);

	BLENDFUNCTION _Blend;
	_Blend.BlendOp = 0;
	_Blend.BlendFlags = 0;
	_Blend.AlphaFormat = 1;
	_Blend.SourceConstantAlpha = 255;

	//此时，鼠标会穿透窗体中Alpha值为0的区域  
	GetWindowRect(&rt);
	CPoint wndPoint;
	wndPoint.x = rt.left;
	wndPoint.y = rt.top;

	CPoint srcPoint;
	srcPoint.x = 0;
	srcPoint.y = 0;

	CSize clientSize;
	clientSize.cx = _pImage->GetWidth();
	clientSize.cy = _pImage->GetHeight();

	UpdateLayeredWindow(pDCDst, &wndPoint, &clientSize,
		dcSrc, &srcPoint,
		0, &_Blend, ULW_ALPHA);

	//bmp->DeleteObject();
	dcSrc->DeleteDC();
	ReleaseDC(pDCDst);
}

void transparentWindow::SetBkImage(CString filePath)
{
	m_ImagePath = filePath;
}

调用的时候需要提前调用SetBkImage一面OnPaint中找不到文件

效果如下