#include <windows.h>

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam);
HBITMAP CreateLayeredWindowBitmap(void);
BOOL UpdateLayeredWindowByBitmapWithPerPixelAlpha(HWND hWnd, HBITMAP hBmp);

HINSTANCE hInstance;

const char szWindowClass[] = "LayeredWindowSampleClass";
const char szWindowName[] = "LayeredWindowSample";

#define WINDOW_WIDTH  512
#define WINDOW_HEIGHT 384

INT WINAPI WinMain( HINSTANCE hInstance, HINSTANCE, LPSTR, INT nCmdShow)
{
	WNDCLASSEX wcex;
	HWND hWnd;
	MSG msg;

	::hInstance = hInstance;

	wcex.cbSize        = sizeof(WNDCLASSEX);
	wcex.style         = 0;
	wcex.lpfnWndProc   = WndProc;
	wcex.cbClsExtra    = 0;
	wcex.cbWndExtra    = 0;
	wcex.hInstance     = hInstance;
	wcex.hIcon         = NULL;
	wcex.hCursor       = LoadCursor(NULL, IDC_ARROW);
	wcex.hbrBackground = NULL;
	wcex.lpszMenuName  = NULL;
	wcex.lpszClassName = szWindowClass;
	wcex.hIconSm       = NULL;
	RegisterClassEx(&wcex);

	hWnd = CreateWindowEx(
		WS_EX_LAYERED, szWindowClass, szWindowName, WS_OVERLAPPEDWINDOW,
		CW_USEDEFAULT, CW_USEDEFAULT, WINDOW_WIDTH, WINDOW_HEIGHT,
		NULL, NULL, hInstance, NULL);
	ShowWindow(hWnd, nCmdShow);

	while(GetMessage(&msg, NULL, 0, 0))
	{
		TranslateMessage(&msg);
		DispatchMessage(&msg);
	}

	return 0;
}

LRESULT CALLBACK WndProc( HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam )
{
	switch (message) 
	{
	case WM_CREATE:
		{
			HBITMAP hBmp;
			HRGN hRgnWnd;

			hRgnWnd = CreateRectRgn(0, 0, WINDOW_WIDTH, WINDOW_HEIGHT);
			SetWindowRgn(hWnd, hRgnWnd, FALSE);

			hBmp = CreateLayeredWindowBitmap();
			UpdateLayeredWindowByBitmapWithPerPixelAlpha(hWnd, hBmp);
		}
		return 0;

	case WM_DESTROY:
		PostQuitMessage(0);
		return 0;
	}

	return DefWindowProc(hWnd, message, wParam, lParam);
}

// レイヤードウィンドウを描画するためのビットマップを作成する
HBITMAP CreateLayeredWindowBitmap(void)
{
	DWORD *pdwBmp;
	BITMAPINFO bmi;
	HDC hdcScreen;
	HBITMAP hBmp;

	memset(&bmi, 0, sizeof(BITMAPINFO));
	bmi.bmiHeader.biSize     = sizeof(BITMAPINFOHEADER);
	bmi.bmiHeader.biWidth    = WINDOW_WIDTH;
	bmi.bmiHeader.biHeight   = WINDOW_HEIGHT;
	bmi.bmiHeader.biPlanes   = 1;
	bmi.bmiHeader.biBitCount = 32;

	hdcScreen = GetDC(NULL);
	hBmp = CreateDIBSection(hdcScreen, &bmi, DIB_RGB_COLORS, (void **)&pdwBmp, 0, 0);
	ReleaseDC(NULL, hdcScreen);

	for (int y = 0; y < WINDOW_HEIGHT; y++)
	{
		// 縦方向に色が変化する
		DWORD r = (y * 255) / WINDOW_HEIGHT;
		DWORD b = ((WINDOW_HEIGHT - y - 1) * 255) / WINDOW_HEIGHT;

		for (int x = 0; x < WINDOW_WIDTH; x++)
		{
			// 横方向にアルファ値が変化する
			DWORD a = (x * 255) / WINDOW_WIDTH;
			// 乗算済みアルファに変換する
			DWORD rr = r * a / 255;
			DWORD bb = b * a / 255;
			pdwBmp[x + y*WINDOW_WIDTH] = (a << 24) | (rr << 16) | bb;
		}
	}

	return hBmp;
}

// ビットマップをもとにレイヤードウィンドウを更新する
BOOL UpdateLayeredWindowByBitmapWithPerPixelAlpha(HWND hWnd, HBITMAP hBmp)
{
	HDC hdcScreen, hdcBmp;
	HGDIOBJ hBmpPrev;
	RECT rc;
	POINT ptDst, ptSrc;
	SIZE size;
	BLENDFUNCTION bf;
	BOOL ret;

	hdcScreen = GetDC(NULL);
	hdcBmp = CreateCompatibleDC(hdcScreen);
	hBmpPrev = SelectObject(hdcBmp, hBmp);

	GetWindowRect(hWnd, &rc);
	ptDst.x = rc.left;
	ptDst.y = rc.top;
	ptSrc.x = 0;
	ptSrc.y = 0;
	size.cx = WINDOW_WIDTH;
	size.cy = WINDOW_HEIGHT;

	bf.BlendOp             = AC_SRC_OVER;
	bf.BlendFlags          = 0;
	bf.AlphaFormat         = AC_SRC_ALPHA;
	bf.SourceConstantAlpha = 255;

	ret = UpdateLayeredWindow(hWnd, hdcScreen, &ptDst, &size, hdcBmp, &ptSrc, 0, &bf, ULW_ALPHA);

	ReleaseDC(NULL, hdcScreen);
	SelectObject(hdcBmp, hBmpPrev);
	DeleteDC(hdcBmp);

	return ret;
}