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copied!<p>The documentation for <a href="http://msdn.microsoft.com/en-us/library/system.windows.media.imaging.writeablebitmap.pixels%28v=VS.95%29.aspx" rel="nofollow">WritableBitmap.Pixels</a> states that the "format used by the Silverlight WriteableBitmap is ARGB32 (premultiplied RGB)". Perhaps then the live tiles expect a non-premultiplied pixel format.</p> <p>I could not find any API in Silverlight to change the format, but I think the method in this article might be what you need:</p> <p><a href="http://nokola.com/blog/post/2010/01/27/The-Most-Important-Silverlight-WriteableBitmap-Gotcha-Does-It-LoseChange-Colors.aspx" rel="nofollow">http://nokola.com/blog/post/2010/01/27/The-Most-Important-Silverlight-WriteableBitmap-Gotcha-Does-It-LoseChange-Colors.aspx</a></p> <p>Edit:</p> <p>From my testing it seems like the problem is with JPEG compression artifacts after all, since SaveJpeg saves files in JPEG format even if you name them with a .png extension.</p> <p>My example code below has a commented out call to MakeNonPremultiplied(bitmap.Pixels) that shows how you would call the filter to modify pixel format to non-premultiplied if you used some library to save it to a file format that works with transparencies and expects non-premultiplied format.</p> <pre><code>using System; using System.IO; using System.IO.IsolatedStorage; using System.Linq; using System.Windows; using System.Windows.Media.Imaging; using Microsoft.Phone.Shell; namespace LiveTilePlayground { public partial class LiveTileGenerator { /// <summary> /// Renders a FrameworkElement (control) to a bitmap /// the size of a live tile or a custom sized square. /// </summary> /// <param name="element">The element.</param> /// <param name="size"> /// The size of the bitmap (in each dimension). /// </param> /// <returns></returns> public static WriteableBitmap RenderBitmap( FrameworkElement element, double size = 173.0) { element.Measure(new Size(size, size)); element.Arrange(new Rect(0, 0, size, size)); return new WriteableBitmap(element, null); } /// <summary> /// Updates the primary tile with specific title and background image. /// </summary> /// <param name="title">The title.</param> /// <param name="backgroundImage">The background image.</param> public static void UpdatePrimaryTile(string title, Uri backgroundImage) { ShellTile primaryTile = ShellTile.ActiveTiles.First(); StandardTileData newTileData = new StandardTileData { Title = title, BackgroundImage = backgroundImage }; primaryTile.Update(newTileData); } /// <summary> /// Saves the tile bitmap with a given file name and returns the URI. /// </summary> /// <param name="bitmap">The bitmap.</param> /// <param name="fileName">Name of the file.</param> /// <returns></returns> public static Uri SaveTileBitmap( WriteableBitmap bitmap, string fileName) { //MakeNonPremultiplied(bitmap.Pixels); using (var store = IsolatedStorageFile.GetUserStoreForApplication()) { if (!store.DirectoryExists(@"Shared\ShellContent")) { store.CreateDirectory(@"Shared\ShellContent"); } using ( var stream = store.OpenFile( @"Shared\ShellContent\" + fileName, FileMode.OpenOrCreate)) { bitmap.SaveJpeg(stream, 173, 173, 0, 100); } } return new Uri( "isostore:/Shared/ShellContent/" + fileName, UriKind.Absolute); } /// <summary> /// Transforms bitmap pixels to a non-alpha premultiplied format. /// </summary> /// <param name="bitmapPixels">The bitmap pixels.</param> public static void MakeNonPremultiplied(int[] bitmapPixels) { int count = bitmapPixels.Length; // Iterate through all pixels and // make each semi-transparent pixel non-premultiplied for (int i = 0; i < count; i++) { uint pixel = unchecked((uint)bitmapPixels[i]); // Decompose ARGB structure from the uint into separate channels // Shift by 3 bytes to get Alpha double a = pixel >> 24; // If alpha is 255 (solid color) or 0 (completely transparent) - // skip this pixel. if ((a == 255) || (a == 0)) { continue; } // Shift 2 bytes and filter out the Alpha byte to get Red double r = (pixel >> 16) & 255; // Shift 1 bytes and filter out Alpha and Red bytes to get Green double g = (pixel >> 8) & 255; // Filter out Alpha, Red and Green bytes to get Blue double b = (pixel) & 255; // Divide by normalized Alpha to get non-premultiplied values double factor = 256 / a; uint newR = (uint)Math.Round(r * factor); uint newG = (uint)Math.Round(g * factor); uint newB = (uint)Math.Round(b * factor); // Compose back to ARGB uint bitmapPixels[i] = unchecked((int)( (pixel & 0xFF000000) | (newR << 16) | (newG << 8) | newB)); } } } } </code></pre>

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