public static extern bool UpdateLayeredWindow(IntPtr hWnd, IntPtr hdcDst, ref POINT pptDst, ref SIZE psize, IntPtr hdcSrc, ref POINT ppSrc, Int32 crKey, ref BLENDFUNCTION pblend, Int32 dwFlags);
/// <summary>
/// 设置图片为窗体，透明区域根据 opacity 的值决定透明度
/// </summary>
/// <param name="bitmap">透明位图</param>
/// <param name="opacity">透明度的值0~255</param>
public void SetBitmap(Bitmap bitmap, byte opacity)
{
if (bitmap.PixelFormat != PixelFormat.Format32bppArgb)
throw new ApplicationException("The bitmap must be 32ppp with alpha-channel.");
// The ideia of this is very simple,
// 1. Create a compatible DC with screen;
// 2. Select the bitmap with 32bpp with alpha-channel in the compatible DC;
// 3. Call the UpdateLayeredWindow.
IntPtr screenDc = NativeMethods.GetDC(IntPtr.Zero);
IntPtr memDc = NativeMethods.CreateCompatibleDC(screenDc);
IntPtr hBitmap = IntPtr.Zero;
IntPtr oldBitmap = IntPtr.Zero;
try
{
hBitmap = bitmap.GetHbitmap(Color.FromArgb(0)); // grab a GDI handle from this GDI+ bitmap
oldBitmap = NativeMethods.SelectObject(memDc, hBitmap);
SIZE size = new SIZE(bitmap.Width, bitmap.Height);
POINT pointSource = new POINT(0, 0);
POINT topPos = new POINT(Left, Top);
BLENDFUNCTION blend = new BLENDFUNCTION();
blend.BlendOp = Consts.AC_SRC_OVER;
blend.BlendFlags = 0;
blend.SourceConstantAlpha = opacity;
blend.AlphaFormat = Consts.AC_SRC_ALPHA;
NativeMethods.UpdateLayeredWindow(Handle, screenDc, ref topPos, ref size, memDc, ref pointSource, 0, ref blend, Consts.ULW_ALPHA);
}
finally
{
NativeMethods.ReleaseDC(IntPtr.Zero, screenDc);
if (hBitmap != IntPtr.Zero)
{
NativeMethods.SelectObject(memDc, oldBitmap);
//Windows.DeleteObject(hBitmap); // The documentation says that we have to use the Windows.DeleteObject... but since there is no such method I use the normal DeleteObject from Win32 GDI and it's working fine without any resource leak.
NativeMethods.DeleteObject(hBitmap);
}
NativeMethods.DeleteDC(memDc);
}
}