C# stroke imagebrush windows应用商店应用程序

我在绘制多段线时使用imagebrush作为笔划

var bitmapImage = new Windows.UI.Xaml.Media.Imaging.BitmapImage(new Uri("ms-appx:/Assets/br1.png"));
        img = new Windows.UI.Xaml.Media.ImageBrush()
        {
            ImageSource = bitmapImage,
            Stretch = Stretch.None,
        };

并创建多段线：

this._polyline = new Polyline() { 
            StrokeThickness = 23,                
              Stroke = img,                  

        };

但当我在画布上绘制多段线时，图像会拉伸。我不能重复了，你能帮我吗