Form background with per-pixel alpha?

Discussion in 'C#' started by ShadowEvil, Jan 7, 2012.

  1. ShadowEvil

    ShadowEvil New Member

    Jan 7, 2012
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    Is it possible to create an irregular window with per pixel alpha so that it
    blends in to the desktop at the edges? I've managed to create an irregular
    from using the colour-keyed TransparentColour property, but this leaves it
    with a hard aliased edge.

    I found something, but this is what I get:

    Of course I have buttons, and other things behind what you see there. But It doesn't want to show correctly. Any fix for this, would be amazing. I think it's because I'm only alpha'ing the background image, which loads it ontop of everything. But again, I have no clue on how to alpha JUST the background, and keep it as a background.
    My code for Per-Pixel-Alpha:

            class Win32
                public enum Bool
                    False = 0,
                public struct Point
                    public Int32 x;
                    public Int32 y;
                    public Point(Int32 x, Int32 y) { this.x = x; this.y = y; }
                public struct Size
                    public Int32 cx;
                    public Int32 cy;
                    public Size(Int32 cx, Int32 cy) { = cx; = cy; }
                [StructLayout(LayoutKind.Sequential, Pack = 1)]
                struct ARGB
                    public byte Blue;
                    public byte Green;
                    public byte Red;
                    public byte Alpha;
                [StructLayout(LayoutKind.Sequential, Pack = 1)]
                public struct BLENDFUNCTION
                    public byte BlendOp;
                    public byte BlendFlags;
                    public byte SourceConstantAlpha;
                    public byte AlphaFormat;
                public const Int32 WS_EX_LAYERED = 0x80000;
                public const Int32 ULW_COLORKEY = 0x00000001;
                public const Int32 ULW_ALPHA = 0x00000002;
                public const Int32 ULW_OPAQUE = 0x00000004;
                public const byte AC_SRC_OVER = 0x00;
                public const byte AC_SRC_ALPHA = 0x01;
                [DllImport("user32.dll", ExactSpelling = true, SetLastError = true)]
                public static extern Bool UpdateLayeredWindow(IntPtr hwnd, IntPtr hdcDst, ref Point pptDst, ref Size psize, IntPtr hdcSrc, ref Point pprSrc, Int32 crKey, ref BLENDFUNCTION pblend, Int32 dwFlags);
                [DllImport("user32.dll", ExactSpelling = true, SetLastError = true)]
                public static extern IntPtr GetDC(IntPtr hWnd);
                [DllImport("user32.dll", ExactSpelling = true)]
                public static extern int ReleaseDC(IntPtr hWnd, IntPtr hDC);
                [DllImport("gdi32.dll", ExactSpelling = true, SetLastError = true)]
                public static extern IntPtr CreateCompatibleDC(IntPtr hDC);
                [DllImport("gdi32.dll", ExactSpelling = true, SetLastError = true)]
                public static extern Bool DeleteDC(IntPtr hdc);
                [DllImport("gdi32.dll", ExactSpelling = true)]
                public static extern IntPtr SelectObject(IntPtr hDC, IntPtr hObject);
                [DllImport("gdi32.dll", ExactSpelling = true, SetLastError = true)]
                public static extern Bool DeleteObject(IntPtr hObject);
            protected void OnPaintBackground(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 = Win32.GetDC(IntPtr.Zero);
                IntPtr memDc = Win32.CreateCompatibleDC(screenDc);
                IntPtr hBitmap = IntPtr.Zero;
                IntPtr oldBitmap = IntPtr.Zero;
                    hBitmap = bitmap.GetHbitmap(Color.FromArgb(0));  // grab a GDI handle from this GDI+ bitmap
                    oldBitmap = Win32.SelectObject(memDc, hBitmap);
                    Win32.Size size = new Win32.Size(bitmap.Width, bitmap.Height);
                    Win32.Point pointSource = new Win32.Point(0, 0);
                    Win32.Point topPos = new Win32.Point(Left, Top);
                    Win32.BLENDFUNCTION blend = new Win32.BLENDFUNCTION();
                    blend.BlendOp = Win32.AC_SRC_OVER;
                    blend.BlendFlags = 0;
                    blend.SourceConstantAlpha = opacity;
                    blend.AlphaFormat = Win32.AC_SRC_ALPHA;
                    Win32.UpdateLayeredWindow(Handle, screenDc, ref topPos, ref size, memDc, ref pointSource, 0, ref blend, Win32.ULW_ALPHA);
                    Win32.ReleaseDC(IntPtr.Zero, screenDc);
                    if (hBitmap != IntPtr.Zero)
                        Win32.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.
            protected override CreateParams CreateParams
                    // Add the layered extended style (WS_EX_LAYERED) to this window
                    CreateParams createParams = base.CreateParams;
                    createParams.ExStyle |= Win32.WS_EX_LAYERED;
                    return createParams;
            private void main_Load(object sender, EventArgs e)
                OnPaintBackground(Properties.Resources.Background, 255);

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