如何將UIControl以不透明方式正確保存到WriteableBitmap？

Question

我就與不同的不透明度值的矩形組，並添加這些矩形網格中的WindowsPhone：

var grid=new Grid(); 
grid.Width = grid.Height = 200; 
var rectanglesCount=55; 
var rectangleSizeStep = grid.Height/rectanglesCount; 
var opacityStep = 1.0/rectanglesCount 
var rectangleSize = grid.Height; 
var opacity = 0.0; 
for (int i = 0; i <= rectanglesCount; i++) 
{ 
Rectangle rect = new Rectangle(); 
rect.Height = rect.Width = rectangleSize; 
rect.VerticalAlignment = VerticalAlignment.Center; 
rect.HorizontalAlignment = HorizontalAlignment.Center; 
rect.Fill = new SolidColorBrush(Colors.Yellow); 
rect.Opacity = opacity; 
opacity += opacityStep; 
rectangleSize -= rectangleSizeStep; 
grid.Children.Add(rect); 
} 

我能在網看到下面的圖片：

後，我試圖挽救這個組的矩形到WriteableBitmap的並顯示爲圖片：

var img=new Image(); 
var wrBit = new WriteableBitmap(grid, null); 
img.Source=wrBit; 

而且我看到的畫面：

什麼發生在頂部和左邊的不透明度？ 如何正確保存一組矩形？

Answer 1

需要使用帆布代替Grid控件的矩形組：

var canvas=new Canvas(); 
canvas.Width = canvas.Height = 200; 
var rectanglesCount=55; 
var rectangleSizeStep = canvas.Height/rectanglesCount; 
var opacityStep = 1.0/rectanglesCount 
var rectangleSize = canvas.Height; 
var opacity = 0.0; 
var rectMargin = 0.0; 
for (int i = 0; i <= rectanglesCount; i++) 
{ 
Rectangle rect = new Rectangle(); 
rect.Height = rect.Width = rectangleSize; 
rect.Margin=new Thickness(rectMargin,rectMargin,0,0); 
rectMargin += rectangleSizeStep/2; 
rect.Fill = new SolidColorBrush(Colors.Yellow); 
rect.Opacity = opacity; 
opacity += opacityStep; 
rectangleSize -= rectangleSizeStep; 
canvas.Children.Add(rect); 
} 

和Canvas保存爲WriteableBitmap的：

var img=new Image(); 
var wrBit = new WriteableBitmap(canvas, null); 
img.Source=wrBit; 

問題解決了！