How to Render a Transparent Cursor to Bitmap preserving alpha channel?

Question

I use the code below to render a transparent icon:

    private void button1_Click(object sender, EventArgs e)
    {
        // using LoadCursorFromFile from user32.dll
        var cursor = NativeMethods.LoadCustomCursor(@"d:\Temp\Cursors\Cursors\aero_busy.ani");

        // cursor -> bitmap
        Bitmap bitmap = new Bitmap(48, 48, PixelFormat.Format32bppArgb);
        Graphics gBitmap = Graphics.FromImage(bitmap);
        cursor.DrawStretched(gBitmap, new Rectangle(0, 0, 32, 32));

        // 1. Draw bitmap on a form canvas
        Graphics gForm = Graphics.FromHwnd(this.Handle);
        gForm.DrawImage(bitmap, 50, 50);

        // 2. Draw cursor directly to a form canvas
        cursor.Draw(gForm, new Rectangle(100, 50, 32, 32));

        cursor.Dispose();
    }

Unfortunately I am unable to render a transparent Cursor to Bitmap! It works when I draw Cursor directly to the form canvas, but there is a problem when I draw Cursor to bitmap. Any advice is highly appreciated.

alt text

Yeah, GDI+ has known problems preserving the alpha channel when drawing icons. Maybe someone knows of a workaround, but I suspect the best way to go here is using GDI instead. — Cody Gray - on strike, Dec 15 '10 at 16:06
Hmm... It's pity, because I'd like to stay with managed code. Do you know, if Windows 7 still uses GDI do render the transparency cursors? — Murat Korkmaz, Dec 15 '10 at 16:27
Looks familiar. Attribution is required here. You might actually get a better answer if you take care of this. — Hans Passant, Dec 15 '10 at 16:40

score 9 · Accepted Answer · answered Dec 15 '10 at 17:07

The solution you have right now doesn't stay completely with managed code. Your own comment says that you are P/Invoking LoadCursorFromFile from user32.dll. And regardless, using the Win32 API is really nothing that you should be afraid of.

As I mentioned in a comment, what you're trying to do is often problematic with GDI+ drawing functions, like most of those provided by the .NET Framework. The task is made much easier by using GDI instead. You can use the following code to create a bitmap from a cursor (or icon, they're basically interchangeable) that does respect the alpha channel:

[StructLayout(LayoutKind.Sequential)]    
private struct ICONINFO
{
    public bool fIcon;
    public int xHotspot;
    public int yHotspot;
    public IntPtr hbmMask;
    public IntPtr hbmColor;
}

[DllImport("user32")]
private static extern bool GetIconInfo(IntPtr hIcon, out ICONINFO pIconInfo);

[DllImport("user32.dll")]
private static extern IntPtr LoadCursorFromFile(string lpFileName);

[DllImport("gdi32.dll", SetLastError = true)]
private static extern bool DeleteObject(IntPtr hObject);

private Bitmap BitmapFromCursor(Cursor cur)
{
    ICONINFO ii;
    GetIconInfo(cur.Handle, out ii);

    Bitmap bmp = Bitmap.FromHbitmap(ii.hbmColor);
    DeleteObject(ii.hbmColor);
    DeleteObject(ii.hbmMask);

    BitmapData bmData = bmp.LockBits(new Rectangle(0, 0, bmp.Width, bmp.Height), ImageLockMode.ReadOnly, bmp.PixelFormat);
    Bitmap dstBitmap = new Bitmap(bmData.Width, bmData.Height, bmData.Stride, PixelFormat.Format32bppArgb, bmData.Scan0);
    bmp.UnlockBits(bmData);

    return new Bitmap(dstBitmap);
}

private void Form1_Paint(object sender, System.Windows.Forms.PaintEventArgs e)
{
    //Using LoadCursorFromFile from user32.dll, get a handle to the icon
    IntPtr hCursor = LoadCursorFromFile("C:\\Windows\\Cursors\\Windows Aero\\aero_busy.ani");

    //Create a Cursor object from that handle
    Cursor cursor = new Cursor(hCursor);

    //Convert that cursor into a bitmap
    using (Bitmap cursorBitmap = BitmapFromCursor(cursor))
    {
        //Draw that cursor bitmap directly to the form canvas
        e.Graphics.DrawImage(cursorBitmap, 50, 50);
    }
}

If this code doesn't compile, make sure that you've added using statements for System.Drawing, System.Drawing.Imaging, and System.Runtime.InteropServices. Also remember to wire up the Form1_Paint method as the handler for your form's Paint event.

It is tested to work:

cursor, drawn from bitmap complete with alpha channel

Hi awesome piece of code there. When I call BitmapFromCursor I get a "A generic error occurred in GDI+." on the line "Bitmap bmp = Bitmap.FromHbitmap(ii.hbmColor);". Anyone know what might be causing it and how to fix it? — Neaox, Dec 13 '12 at 23:30
Ok so I've figured out that ii.hbmColor is returning a null pointer. Anyone know why that might be and how to fix it? — Neaox, Dec 13 '12 at 23:45
Further details - Cursors.IBeam is the problem, what can I do to fix that? — Neaox, Dec 13 '12 at 23:51

score 3 · Answer 2 · answered Aug 23 '13 at 13:10

@Cody Gray, That does NOT work with cursors having low color bits.

An alternative is to use Icon.ExtractAssociatedIcon instead:

System.Drawing.Icon i = System.Drawing.Icon.ExtractAssociatedIcon(@"C:\Windows\Cursors\arrow_rl.cur");
System.Drawing.Bitmap b = i.ToBitmap();

Hope that helps someone ...

How to Render a Transparent Cursor to Bitmap preserving alpha channel?

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