public static extern Int32 AlphaBlend(IntPtr hdcDest,
Int32 xDest, Int32 yDest, Int32 cxDest, Int32 cyDest, IntPtr hdcSrc,
Int32 xSrc, Int32 ySrc, Int32 cxSrc, Int32 cySrc, BlendFunction blendFunction);
/// <summary>
/// Draws an image with transparency. Source must be 32bpp (or 24bpp)
/// </summary>
/// <param name="gx">Graphics to drawn on.</param>
/// <param name="image">Image to draw.</param>
/// <param name="transparency">Transparency constant</param>
/// <param name="x">X location</param>
/// <param name="y">Y location</param>
public static void DrawAlpha(this Graphics gx, Bitmap image, byte transparency, int x, int y)
{
using (Graphics gxSrc = Graphics.FromImage(image))
{
IntPtr hdcDst = gx.GetHdc();
IntPtr hdcSrc = gxSrc.GetHdc();
BlendFunction blendFunction = new BlendFunction();
blendFunction.BlendOp = (byte)BlendOperation.AC_SRC_OVER; // Only supported blend operation
blendFunction.BlendFlags = (byte)BlendFlags.Zero; // Documentation says put 0 here
blendFunction.SourceConstantAlpha = transparency; // Constant alpha factor
blendFunction.AlphaFormat = (byte)0; // Don't look for per pixel alpha
Win32Helper.AlphaBlend(hdcDst, x, y, image.Width, image.Height,
hdcSrc, 0, 0, image.Width, image.Height, blendFunction);
gx.ReleaseHdc(hdcDst); // Required cleanup to GetHdc()
gxSrc.ReleaseHdc(hdcSrc); // Required cleanup to GetHdc()
}
}