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POWindows Forms: Making a cursor bitmap partially transparent
 

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    <p>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. </p> <p>The following image describes the problem:</p> <p><img src="https://i.stack.imgur.com/AEPKZ.png" alt="Transparency problem"></p> <p>Figure a) is the original bitmap.</p> <p>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.</p> <p>Figure c) is the desired effect, image a) with 50% transparency (added to the composition with a drawing program).</p> <p>The code I use to produce the trasparent image is the following:</p> <pre><code>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; </code></pre> <p>Does anyone know why the alpha blending does not work?</p> <p><strong>Update I:</strong></p> <p>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.</p> <p><strong>Update II:</strong></p> <p>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.</p> <p>Here's the IconInfo structure:</p> <pre><code>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. } </code></pre> <p>And here is the code that actually creates the Cursor:</p> <pre><code> 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)); } </code></pre> <p>The two external functions are defined as follows:</p> <pre><code> [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); </code></pre>
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    1. COi don't know the answer, but this is a _very_ good question. :)
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    2. COThanks! It just seems *really* weird to me that even after fifteen years of drag/drop operations and custom cursors, this thing can not be resolved easily. I get *some* transparency so at least the system hints that it can be done. After three days of tapping hard on my keyboard and reading articles, I have a slight suspicion one might need to tap into the BITMAPV5HEADER structure. Here's a C++ article that might solve the problem: http://support.microsoft.com/default.aspx?scid=kb;en-us;318876. If anyone can port this I'll be glad to hand over the accepted answer vote.
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