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copied!<p>You loop over the pixels and play only the alpha channel. If you do this with Bitmap.LockBits it will actually be very fast.</p> <pre><code> private const int bytesPerPixel = 4; /// <summary> /// Change the opacity of an image /// </summary> /// <param name="originalImage">The original image</param> /// <param name="opacity">Opacity, where 1.0 is no opacity, 0.0 is full transparency</param> /// <returns>The changed image</returns> public static Image ChangeImageOpacity(Image originalImage, double opacity) { if ((originalImage.PixelFormat & PixelFormat.Indexed) == PixelFormat.Indexed) { // Cannot modify an image with indexed colors return originalImage; } Bitmap bmp = (Bitmap)originalImage.Clone(); // Specify a pixel format. PixelFormat pxf = PixelFormat.Format32bppArgb; // Lock the bitmap's bits. Rectangle rect = new Rectangle(0, 0, bmp.Width, bmp.Height); BitmapData bmpData = bmp.LockBits(rect, ImageLockMode.ReadWrite, pxf); // Get the address of the first line. IntPtr ptr = bmpData.Scan0; // Declare an array to hold the bytes of the bitmap. // This code is specific to a bitmap with 32 bits per pixels // (32 bits = 4 bytes, 3 for RGB and 1 byte for alpha). int numBytes = bmp.Width * bmp.Height * bytesPerPixel; byte[] argbValues = new byte[numBytes]; // Copy the ARGB values into the array. System.Runtime.InteropServices.Marshal.Copy(ptr, argbValues, 0, numBytes); // Manipulate the bitmap, such as changing the // RGB values for all pixels in the the bitmap. for (int counter = 0; counter < argbValues.Length; counter += bytesPerPixel) { // argbValues is in format BGRA (Blue, Green, Red, Alpha) // If 100% transparent, skip pixel if (argbValues[counter + bytesPerPixel - 1] == 0) continue; int pos = 0; pos++; // B value pos++; // G value pos++; // R value argbValues[counter + pos] = (byte) (argbValues[counter + pos] * opacity); } // Copy the ARGB values back to the bitmap System.Runtime.InteropServices.Marshal.Copy(argbValues, 0, ptr, numBytes); // Unlock the bits. bmp.UnlockBits(bmpData); return bmp; } </code></pre>

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