Sur Windows Phone 7, tout comme Silverlight d’ailleurs, la classe Bitmap n’existe pas (bon il faut être lucide, la classe Bitmap est une encapsulation d’une bitmap GDI+, donc ça parait logique que ce ne soit pas disponible sinon ce serait trop ancré avec Windows :)). Cela peut poser problème dans des plusieurs cas concrets, comme notre application TranslateIt présenté sur ce blog et pour le concours de Windows Phone 7 organisé par Microsoft. Dans notre cas, nous avons besoin de découper une image et avec la classe Bitmap, cela est très simple avec la méthode Clone().
Il a donc fallu trouver une alternative à cette classe. Nous sommes donc passer sur WriteableBitmap avec ses méthodes d’extensions disponibles dans le projet WriteableBitmapEx sur Codeplex.

La description du projet est très claire :

The WriteableBitmapEx library is a collection of extension methods for Silverlight's WriteableBitmap. The WriteableBitmap class that was added in Silverlight 3, allows the direct manipulation of a bitmap and could be used to generate fast procedural images by drawing directly to a bitmap. The WriteableBitmap API is very minimalistic and there's only the raw Pixels array for such operations. The WriteableBitmapEx library tries to compensate that with extensions methods that are easy to use like built in methods. The library extends the WriteableBitmap class with elementary and fast (2D drawing) functionality, supporting common shapes like point, line, ellipse, polyline, quad, rectangle, triangle. Conversion methods and functions to combine WriteableBitmaps (Blitting) are part of it too.

Comme on peut le constater, c’est très complet :).

Pour reprendre les exemples de Codeplex :

// Initialize the WriteableBitmap with size 512x512 and set it as source of an Image control
WriteableBitmap writeableBmp = new WriteableBitmap(512, 512);
ImageControl.Source = writeableBmp;

// Load an image from the calling Assembly's resources only by passing the relative path
writeableBmp = new WriteableBitmap(0, 0).FromResource("Data/flower2.png");

// Clear the WriteableBitmap with white color
writeableBmp.Clear(Colors.White);

// Set the pixel at P(10, 13) to black
writeableBmp.SetPixel(10, 13, Colors.Black);

// Get the color of the pixel at P(30, 43)
Color color = writeableBmp.GetPixel(30, 43);

// Green line from P1(1, 2) to P2(30, 40)
writeableBmp.DrawLine(1, 2, 30, 40, Colors.Green);

// Line from P1(1, 2) to P2(30, 40) using the fastest draw line method with the color as
// integer
int[] pixels = writeableBmp.Pixels;
int w = writeableBmp.PixelWidth;
int h = writeableBmp.PixelHeight;
WriteableBitmapExtensions.DrawLine(pixels, w, h, 1, 2, 30, 40, myIntColor);

// Black triangle with the points P1(10, 5), P2(20, 40) and P3(30, 10)
writeableBmp.DrawTriangle(10, 5, 20, 40, 30, 10, Colors.Black);

// Red rectangle from the point P1(2, 4) that is 10px wide and 6px high
writeableBmp.DrawRectangle(2, 4, 12, 10, Colors.Red);

// Filled blue ellipse with the center point P1(2, 2) that is 8px wide and 5px high
writeableBmp.FillEllipseCentered(2, 2, 8, 5, Colors.Blue);

// Closed green polyline with P1(10, 5), P2(20, 40), P3(30, 30) and P4(7, 8)
int[] p = new int[] { 10, 5, 20, 40, 30, 30, 7, 8, 10, 5 };
writeableBmp.DrawPolyline(p, Colors.Green);

// Cubic Beziér curve from P1(5, 5) to P4(20, 7) with the control points P2(10, 15)
// and P3(15, 0)
writeableBmp.DrawBezier(5, 5, 10, 15, 15, 0, 20, 7,  Colors.Purple);

// Cardinal spline through the points P1(10, 5), P2(20, 40) and P3(30, 30) with a 
// tension of 0.5
int[] pts = new int[] { 10, 5, 20, 40, 30, 30};
writeableBmp.DrawCurve(pts, 0.5,  Colors.Yellow);

// A filled Cardinal spline through the points P1(10, 5), P2(20, 40) and P3(30, 30) 
// with a tension of 0.5
writeableBmp.FillCurveClosed(pts, 0.5,  Colors.Green);

// Blit a bitmap using the additive blend mode at P1(10, 10)
writeableBmp.Blit(new Point(10, 10), bitmap, sourceRect, Colors.White, 
                                              WriteableBitmapExtensions.BlendMode.Additive);

// Override all pixels with a function that changes the color based on the coordinate
writeableBmp.ForEach((x, y, color) => Color.FromArgb(color.A, (byte)(color.R / 2), 
                                                                       (byte)(x * y), 100));

// Present the WriteableBitmap!
writeableBmp.Invalidate();

// Take snapshot
var clone = writeableBmp.Clone();

// Save to a TGA image stream (file for example)
writeableBmp.WriteTga(stream);

// Crops the WriteableBitmap to a region starting at P1(5, 8) and 10px wide and 10px high
var cropped = writeableBmp.Crop(5, 8, 10, 10);

// Resizes the WriteableBitmap to 200px wide and 300px high using a bilinear
// interpolation method
var resized = writeableBmp.Resize(200, 300, 
                                          WriteableBitmapExtensions.Interpolation.Bilinear);

A bientôt !