The documentation for WritableBitmap.Pixels states that the "format used by the Silverlight WriteableBitmap is ARGB32 (premultiplied RGB)". Perhaps then the live tiles expect a non-premultiplied pixel format.
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:
http://nokola.com/blog/post/2010/01/27/The-Most-Important-Silverlight-WriteableBitmap-Gotcha-Does-It-LoseChange-Colors.aspx
Edit:
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.
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.
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));
}
}
}
}
