Need to use Canvas instead Grid control for group of rectangles:
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);
}
and save Canvas as WriteableBitmap:
var img=new Image();
var wrBit = new WriteableBitmap(canvas, null);
img.Source=wrBit;
Problem was solved!