Win32: Semi-Transparent Window

In order to make any window semi-transparent, you need to apply the following code to the window handle:

SetWindowLong(hWnd, GWL_EXSTYLE,
                      	GetWindowLong(hWnd, GWL_EXSTYLE) | WS_EX_LAYERED);
                      SetLayeredWindowAttributes(hWnd, 0, (255 * 70) / 100, LWA_ALPHA);

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The first line in this code modifies the extended window style - add WS_EX_LAYERED. The second line changes the window layered attribute - make this window with 70% alpha.

Of course the window could be created with this WS_EX_LAYERED style:

hWnd = CreateWindowEx(WS_EX_LAYERED, 
                      	s_szWndClassName, s_szWndClassName, 
                      	WS_OVERLAPPEDWINDOW,
                      	CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, 
                      	NULL, NULL, hInstance, NULL);

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This alphablended/layered window technique was introduced in Windows 2000, so the application code should contain the following definitions:

#define WINVER 0x0501
                      #define _WIN32_WINNT 0x0501

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I wrote a small application to show this technique in action:

#define WIN32_LEAN_AND_MEAN
                      
                      #define WINVER 0x0501
                      #define _WIN32_WINNT 0x0501
                      
                      #include <windows.h>
                      
                      LPCWSTR s_szWndClassName = L"Semi-Transparent Window";
                      LPCWSTR s_szFile = L"sunset.bmp";
                      HBITMAP s_hBitmap = NULL;
                      
                      ATOM RegisterWndClass(HINSTANCE, LPCWSTR);
                      LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
                      HBITMAP LoadPicture(LPCWSTR);
                      BOOL DrawPicture(HDC, LPRECT);
                      
                      int APIENTRY wWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, 
                      	LPWSTR lpCmdLine, int nCmdShow)
                      {
                      	HWND hWnd;
                      	MSG msg;
                      
                      	s_hBitmap = LoadPicture(L"sunset.bmp"); 
                      
                      	RegisterWndClass(hInstance, s_szWndClassName);
                      
                      	hWnd = CreateWindowEx(WS_EX_LAYERED, 
                      		s_szWndClassName, s_szWndClassName, 
                      		WS_OVERLAPPEDWINDOW,
                      		CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, 
                      		NULL, NULL, hInstance, NULL);
                      
                      	if (hWnd != NULL)
                      	{
                      		SetLayeredWindowAttributes(hWnd,  0,  
                      				(255 * 70) / 100, LWA_ALPHA);
                      
                      		ShowWindow(hWnd, nCmdShow);
                      		UpdateWindow(hWnd);
                      
                      		while (GetMessage(&msg, NULL, 0, 0))
                      		{
                      			TranslateMessage(&msg);
                      			DispatchMessage(&msg);
                      		}
                      	}
                      
                      	if (s_hBitmap != NULL)
                      		DeleteObject(s_hBitmap);
                      
                      	return (int)msg.wParam;
                      }
                      
                      ATOM RegisterWndClass(HINSTANCE hInstance, LPCWSTR lpszWndClassName)
                      {
                      	WNDCLASSEX wcex		= { 0 };
                      	wcex.cbSize		= sizeof(WNDCLASSEX);
                      	wcex.style		= CS_HREDRAW | CS_VREDRAW;
                      	wcex.lpfnWndProc		= WndProc;
                      	wcex.hInstance		= hInstance;
                      	wcex.hCursor		= LoadCursor(NULL, IDC_ARROW);
                      	wcex.hbrBackground	= (HBRUSH)(COLOR_WINDOW+1);
                      	wcex.lpszMenuName	= NULL;
                      	wcex.lpszClassName	= lpszWndClassName;
                      	return RegisterClassEx(&wcex);
                      }
                      
                      LRESULT CALLBACK WndProc(HWND hWnd, UINT message, 
                      	 WPARAM wParam, LPARAM lParam)
                      {
                      	PAINTSTRUCT ps;
                      	HDC hdc;
                      	RECT rect;
                      
                      	switch (message)
                      	{
                      	case WM_PAINT:
                      		hdc = BeginPaint(hWnd, &ps);
                      		GetClientRect(hWnd, &rect);
                      		DrawPicture(hdc, &rect);
                      		EndPaint(hWnd, &ps);
                      		break;
                      
                      	case WM_DESTROY:
                      		PostQuitMessage(0);
                      		break;
                      
                      	default:
                      		return DefWindowProc(hWnd, message, wParam, lParam);
                      	}
                      	return 0;
                      }
                      
                      HBITMAP LoadPicture(LPCWSTR lpszFileName)
                      {
                          HBITMAP hBitmap = (HBITMAP)LoadImage(NULL, lpszFileName,
                      	IMAGE_BITMAP, 0, 0, 				
                      	LR_CREATEDIBSECTION | LR_DEFAULTSIZE | LR_LOADFROMFILE); 
                          return hBitmap;
                      }
                      
                      BOOL DrawPicture(HDC hDC, LPRECT lpRect)
                      {
                      	BITMAP bitmap;
                      	HDC hMemDC;
                      	HGDIOBJ hBitmapOld;
                      	if (s_hBitmap == NULL)
                      		return FALSE;
                      	GetObject(s_hBitmap, sizeof(BITMAP), &bitmap);
                      	hMemDC = CreateCompatibleDC(hDC);
                      	hBitmapOld = SelectObject(hMemDC, s_hBitmap);
                      	StretchBlt(hDC, 
                      		lpRect->left, lpRect->top, 
                      		lpRect->right - lpRect->left,
                      		lpRect->bottom - lpRect->top, 
                      		hMemDC, 
                      		0, 0, bitmap.bmWidth, bitmap. bmHeight, 
                      		SRCCOPY);
                      	SelectObject(hMemDC, hBitmapOld);
                      	DeleteDC(hMemDC);
                      	return TRUE;
                      }

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This application creates a semi-transparent window and draws an image in it, as shown in the earlier screenshot.

More information you can find in MSDN:
Layered Windows