I want to use partially transparent images in drag/drop operations. This is all set up and works fine, but the actual transformation to transparency has a weird side effect. For some reason, the pixels seem to be blended against a black background.
The following image describes the problem:
Figure a) is the original bitmap.
Figure b) is what is produced after alpha blending has been performed. Obviously this is a lot darker than the intended 50% alpha filter intended.
Figure c) is the desired effect, image a) with 50% transparency (added to the composition with a drawing program).
The code I use to produce the trasparent image is the following:
Bitmap bmpNew = new Bitmap(bmpOriginal.Width, bmpOriginal.Height);
Graphics g = Graphics.FromImage(bmpNew);
// Making the bitmap 50% transparent:
float[][] ptsArray ={
new float[] {1, 0, 0, 0, 0}, // Red
new float[] {0, 1, 0, 0, 0}, // Green
new float[] {0, 0, 1, 0, 0}, // Blue
new float[] {0, 0, 0, 0.5f, 0}, // Alpha
new float[] {0, 0, 0, 0, 1} // Brightness
};
ColorMatrix clrMatrix = new ColorMatrix(ptsArray);
ImageAttributes imgAttributes = new ImageAttributes();
imgAttributes.SetColorMatrix(clrMatrix, ColorMatrixFlag.Default, ColorAdjustType.Bitmap);
g.DrawImage(bmpOriginal, new Rectangle(0, 0, bmpOriginal.Width, bmpOriginal.Height), 0, 0, bmpOriginal.Width, bmpOriginal.Height, GraphicsUnit.Pixel, imgAttributes);
Cursors.Default.Draw(g, new Rectangle(bmpOriginal.Width / 2 - 8, bmpOriginal.Height / 2 - 8, 32, 32));
g.Dispose();
imgAttributes.Dispose();
return bmpNew;
Does anyone know why the alpha blending does not work?
Update I:
For clarity, the code does work if I'm alphablending on top of a drawn surface. The problem is that I want to create a completely semitransparent image from an existing image and use this as a dynamic cursor during drag/drop operations. Even skipping the above and only painting a filled rectangle of color 88ffffff yields a dark grey color. Something fishy is going on with the icon.
Update II:
Since I've reseached a whole lot and believe this has got something to do with the Cursor creation, I'm gonna include that code below too. If I GetPixel-sample the bitmap just before the CreateIconIndirect call, the four color values seem to be intact. Thus I have a feeling the culprits might be the hbmColor or the hbmMask members of the IconInfo structure.
Here's the IconInfo structure:
public struct IconInfo { // http://msdn.microsoft.com/en-us/library/ms648052(VS.85).aspx
public bool fIcon; // Icon or cursor. True = Icon, False = Cursor
public int xHotspot;
public int yHotspot;
public IntPtr hbmMask; // Specifies the icon bitmask bitmap. If this structure defines a black and white icon,
// this bitmask is formatted so that the upper half is the icon AND bitmask and the lower
// half is the icon XOR bitmask. Under this condition, the height should be an even multiple of two.
// If this structure defines a color icon, this mask only defines the AND bitmask of the icon.
public IntPtr hbmColor; // Handle to the icon color bitmap. This member can be optional if this structure defines a black
// and white icon. The AND bitmask of hbmMask is applied with the SRCAND flag to the destination;
// subsequently, the color bitmap is applied (using XOR) to the destination by using the SRCINVERT flag.
}
And here is the code that actually creates the Cursor:
public static Cursor CreateCursor(Bitmap bmp, int xHotSpot, int yHotSpot) {
IconInfo iconInfo = new IconInfo();
GetIconInfo(bmp.GetHicon(), ref iconInfo);
iconInfo.hbmColor = (IntPtr)0;
iconInfo.hbmMask = bmp.GetHbitmap();
iconInfo.xHotspot = xHotSpot;
iconInfo.yHotspot = yHotSpot;
iconInfo.fIcon = false;
return new Cursor(CreateIconIndirect(ref iconInfo));
}
The two external functions are defined as follows:
[DllImport("user32.dll", EntryPoint = "CreateIconIndirect")]
public static extern IntPtr CreateIconIndirect(ref IconInfo icon);
[DllImport("user32.dll")]
[return: MarshalAs(UnmanagedType.Bool)]
public static extern bool GetIconInfo(IntPtr hIcon, ref IconInfo pIconInfo);
