Clipping a low resolution image using Transforms and assigning it to my user control

I have been working on a Windows 8.1 RT app where the user loads an image with Stretch=Uniform. The image can be as small as possible and as big as possible. The clipping happens in my user control and my user control appears when I tap/press and hold on the screen/image. Clipping happens when when I tap and hold and move my finger/mouse around the image that is in the background.

The Problems I am facing are

Whenever the app is loaded for the first time on Emulator , and for the very first time when the tap /clicks and holding is performed , the user control appears on the top left on the screen and then it comes above my clicked/hold area. What I require is it should always appear wherever I click and hold and whenever I click and hold. On releasing the finger/mouse , it should collapse.
I am using center transform. I want the region(the pixel) where my mouse is currently to be displayed exactly in the center of the user control , If i am loading a small resolution image 480*800 or even smaller , the region of my mouse is not coming into the center. To be more clearer, Imagine I am tapping and holding on the CORNEA of the human eye.The cornea should be displayed in the center of the user control and area above and below should cover the rest of the part.
1. I don't want my control to go outside my image boundary, if my mouse is at the last pixel of the image , some part of image and some part of background should be displayed.
I need to rotate the control as shown in the video Find the complete code below.

MainPage.XAML

<Page
    x:Class="App78.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:App78"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">

    <Grid
        x:Name="LayoutGrid"
        Margin="0,0"
        Background="{ThemeResource ApplicationPageBackgroundThemeBrush}"
        Holding="LayoutGrid_Holding"
        PointerMoved="LayoutGrid_OnPointerMoved"
        PointerWheelChanged="LayoutGrid_OnPointerWheelChanged"
        PointerPressed="LayoutGrid_OnPointerPressed"
        PointerReleased="LayoutGrid_OnPointerReleased">
        <Image
            x:Name="BigImage"
            HorizontalAlignment="Center"
            VerticalAlignment="Center"
            Stretch="Uniform"
            Source="http://blog.al.com/space-news/2009/04/iss015e22574.jpg" />

        <local:Magnifier VerticalAlignment="Top" HorizontalAlignment="Left" x:Name="MagnifierTip" Visibility="Collapsed" />
    </Grid>
</Page>

MAINPAGE.XAML.CS

using System;
using System.Diagnostics;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Input;
using Windows.UI.Xaml.Media;
using Windows.UI.Xaml.Media.Imaging;

namespace App78
{
    public sealed partial class MainPage : Page
    {
        private double zoomScale = 2;
        private double pointerX = 0;
        private double pointerY = 0;
        private const double MinZoomScale = .25;
        private const double MaxZoomScale = 32;


        public MainPage()
        {
            this.InitializeComponent();

            var bi = (BitmapImage)BigImage.Source;
            bi.ImageOpened += bi_ImageOpened;
            this.SizeChanged += MainPage_SizeChanged;
        }

        void MainPage_SizeChanged(object sender, Windows.UI.Xaml.SizeChangedEventArgs e)
        {
            this.UpdateImageLayout();
        }

        void bi_ImageOpened(object sender, Windows.UI.Xaml.RoutedEventArgs e)
        {
            this.UpdateImageLayout();
        }

        private void UpdateImageLayout()
        {
            var bi = (BitmapImage)BigImage.Source;
            if (bi.PixelWidth < this.LayoutGrid.ActualWidth &&
                bi.PixelHeight < this.LayoutGrid.ActualHeight)
            {
                this.BigImage.Stretch = Stretch.None;
            }
            else
            {
                this.BigImage.Stretch = Stretch.Uniform;
            }

            this.UpdateMagnifier();
        }

        private void LayoutGrid_OnPointerMoved(object sender, PointerRoutedEventArgs e)
        {
            //DV: If pointer is not in contact we can ignore it
            if (!e.Pointer.IsInContact) { return; }

            var point = e.GetCurrentPoint(this.LayoutGrid);
            this.pointerX = point.Position.X;
            this.pointerY = point.Position.Y;
            this.UpdateMagnifier();
        }

        private void UpdateMagnifier()
        {
            var bi = (BitmapImage)BigImage.Source;

            double offsetX = 0;
            double offsetY = 0;
            double imageScale = 1;

            var imageRatio = (double)bi.PixelWidth / bi.PixelHeight;
            var gridRatio = this.LayoutGrid.ActualWidth / this.LayoutGrid.ActualHeight;

            if (bi.PixelWidth < this.LayoutGrid.ActualWidth &&
                bi.PixelHeight < this.LayoutGrid.ActualHeight)
            {
                offsetX = 0.5 * (this.LayoutGrid.ActualWidth - bi.PixelWidth);
                offsetY = 0.5 * (this.LayoutGrid.ActualHeight - bi.PixelHeight);
                //imageScale = 1; - remains
            }
            else if (imageRatio < gridRatio)
            {
                offsetX = 0.5 * (this.LayoutGrid.ActualWidth - imageRatio * this.LayoutGrid.ActualHeight);
                offsetY = 0;
                imageScale = BigImage.ActualHeight / bi.PixelHeight;
            }
            else
            {
                offsetX = 0;
                offsetY = 0.5 * (this.LayoutGrid.ActualHeight - this.LayoutGrid.ActualWidth / imageRatio);
                imageScale = BigImage.ActualWidth / bi.PixelWidth;
            }

            //DV: This is probably not need anymore
           //MagnifierTip.MagnifierTransform.X = this.pointerX;
           //MagnifierTip.MagnifierTransform.Y = this.pointerY;
            MagnifierTip.PositionTransform.X = (-this.pointerX + offsetX) / imageScale;
            MagnifierTip.PositionTransform.Y = (-this.pointerY + offsetY) / imageScale;

            //DV: I haven't tested the Scaling/Zoom
            MagnifierTip.ZoomTransform.ScaleX = imageScale * zoomScale;
            MagnifierTip.ZoomTransform.ScaleY = imageScale * zoomScale;

            MagnifierTip.CenterTransform.X = MagnifierTip.MagnifierEllipse.ActualWidth / 2 - MagnifierTip.MagnifierEllipse.StrokeThickness / 2;
            MagnifierTip.CenterTransform.Y = MagnifierTip.MagnifierEllipse.ActualHeight / 2 - MagnifierTip.MagnifierEllipse.StrokeThickness / 2;

            //DV: I added a GlobalGrid Transform which translates every children
            MagnifierTip.MagnifierTransformGrid.X = this.pointerX - (MagnifierTip.ActualWidth / 2);
            MagnifierTip.MagnifierTransformGrid.Y = this.pointerY - (MagnifierTip.ActualHeight); ;

        }

        private void LayoutGrid_OnPointerWheelChanged(object sender, PointerRoutedEventArgs e)
        {
            if (e.GetCurrentPoint(this.LayoutGrid).Properties.MouseWheelDelta > 0)
            {
                zoomScale = Math.Max(MinZoomScale, Math.Min(MaxZoomScale, zoomScale * 1.2));
            }
            else
            {
                zoomScale = Math.Max(MinZoomScale, Math.Min(MaxZoomScale, zoomScale / 1.2));
            }

            this.UpdateMagnifier();
        }


        //DV: Holding usually only works with touch https://msdn.microsoft.com/en-us/library/windows/apps/windows.ui.xaml.uielement.holding.aspx?f=255&MSPPError=-2147217396
        private void LayoutGrid_Holding(object sender, HoldingRoutedEventArgs e)
        {
            //
        }

        //DV: pointer pressed supports both mouse and touch but fires immeadiatley. You'll have to figure out a delay strategy or using holding for touch and right click for mouse
        private void LayoutGrid_OnPointerPressed(object sender, PointerRoutedEventArgs e)
        {
            MagnifierTip.Visibility = Windows.UI.Xaml.Visibility.Visible;
        }

        //DV: pointer released supports both mouse and touch.
        private void LayoutGrid_OnPointerReleased(object sender, PointerRoutedEventArgs e)
        {
            MagnifierTip.Visibility = Windows.UI.Xaml.Visibility.Collapsed;
        }
    }
}

Magnifier.XAML

<UserControl
    x:Class="App78.Magnifier"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:App78"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    Height="230"
    Width="170">


    <Grid Height="230" Width="170">

        <!-- DV: This is the global transform I added -->
        <Grid.RenderTransform>
            <TransformGroup>
                <TranslateTransform x:Name="MagnifierTransformGrid" x:FieldModifier="public"/>
            </TransformGroup>
        </Grid.RenderTransform>
        <Ellipse Opacity="1" Visibility="Visible" Fill="{ThemeResource ApplicationPageBackgroundThemeBrush}"  HorizontalAlignment="Center" VerticalAlignment="Top" IsHitTestVisible="False" Width="135" Height="128"  StrokeThickness="3" Margin="0,17,0,0" />
        <Ellipse x:Name="MagnifierEllipse" x:FieldModifier="public" Opacity="1" Visibility="Visible" HorizontalAlignment="Left" VerticalAlignment="Top" IsHitTestVisible="False" Width="150" Height="150" Stroke="White" StrokeThickness="3" Margin="11,8,0,0" >
            <Ellipse.Fill>
                <ImageBrush
                    ImageSource="http://blog.al.com/space-news/2009/04/iss015e22574.jpg"
                    Stretch="None"
                    AlignmentX="Left"
                    AlignmentY="Top">
                    <ImageBrush.Transform>
                        <TransformGroup>
                            <TranslateTransform x:FieldModifier="public"
                                x:Name="CenterTransform"/>
                            <TranslateTransform x:FieldModifier="public"
                                x:Name="PositionTransform"/>
                            <ScaleTransform x:FieldModifier="public"
                                x:Name="ZoomTransform"/>
                        </TransformGroup>
                    </ImageBrush.Transform>
                </ImageBrush>
            </Ellipse.Fill>
        </Ellipse>
        <Path Data="M25.533,0C15.457,0,7.262,8.199,7.262,18.271c0,9.461,13.676,19.698,17.63,32.338 c0.085,0.273,0.34,0.459,0.626,0.457c0.287-0.004,0.538-0.192,0.619-0.467c3.836-12.951,17.666-22.856,17.667-32.33 C43.803,8.199,35.607,0,25.533,0z M25.533,32.131c-7.9,0-14.328-6.429-14.328-14.328c0-7.9,6.428-14.328,14.328-14.328 c7.898,0,14.327,6.428,14.327,14.328C39.86,25.702,33.431,32.131,25.533,32.131z"
              Fill="#FFF4F4F5"
              Stretch="Fill"
              Stroke="Black"
              UseLayoutRounding="False"
              Height="227"
              Width="171" ></Path>
    </Grid>
</UserControl>

For ease , the project can be downloaded from here. For better understanding , I would like you to see this video. This is what exactly I need to implement.

I got it working with some changes and trigonometry here.

MainPage.xaml

<Page
    x:Class="MagnifierApp.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:MagnifierApp"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">

    <Grid
        x:Name="LayoutGrid"
        Margin="0,0"
        Background="{ThemeResource ApplicationPageBackgroundThemeBrush}"
        PointerMoved="LayoutGrid_OnPointerMoved"
        PointerWheelChanged="LayoutGrid_OnPointerWheelChanged"
        PointerPressed="LayoutGrid_OnPointerPressed"
        PointerReleased="LayoutGrid_OnPointerReleased">
        <Image
            x:Name="BigImage"
            HorizontalAlignment="Center"
            VerticalAlignment="Center"
            Stretch="Uniform"
            Source="http://blog.al.com/space-news/2009/04/iss015e22574.jpg" />
        <local:Magnifier
            x:Name="MagnifierTip"
            VerticalAlignment="Top"
            HorizontalAlignment="Left"
            Visibility="Collapsed" />
    </Grid>
</Page>

MainPage.xaml.cs

using System;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Input;
using Windows.UI.Xaml.Media;
using Windows.UI.Xaml.Media.Imaging;

namespace MagnifierApp
{
    public sealed partial class MainPage : Page
    {
        private double zoomScale = 2;
        private double pointerX = 0;
        private double pointerY = 0;
        private const double MinZoomScale = .25;
        private const double MaxZoomScale = 32;
        private bool isFirst = true;

        public MainPage()
        {
            this.InitializeComponent();

            var bi = (BitmapImage)BigImage.Source;
            bi.ImageOpened += bi_ImageOpened;
            this.SizeChanged += MainPage_SizeChanged;
        }

        void MainPage_SizeChanged(object sender, Windows.UI.Xaml.SizeChangedEventArgs e)
        {
            this.UpdateImageLayout();
        }

        void bi_ImageOpened(object sender, Windows.UI.Xaml.RoutedEventArgs e)
        {
            this.UpdateImageLayout();
        }

        private void UpdateImageLayout()
        {
            var bi = (BitmapImage)BigImage.Source;

            if (bi.PixelWidth < this.LayoutGrid.ActualWidth &&
                bi.PixelHeight < this.LayoutGrid.ActualHeight)
            {
                this.BigImage.Stretch = Stretch.None;
            }
            else
            {
                this.BigImage.Stretch = Stretch.Uniform;
            }

            this.UpdateMagnifier();
        }

        private void LayoutGrid_OnPointerMoved(object sender, PointerRoutedEventArgs e)
        {
            if (!e.Pointer.IsInContact)
            {
                return;
            }

            var point = e.GetCurrentPoint(this.LayoutGrid);
            this.pointerX = point.Position.X;
            this.pointerY = point.Position.Y;
            this.UpdateMagnifier();
        }

        private void LayoutGrid_OnPointerWheelChanged(object sender, PointerRoutedEventArgs e)
        {
            if (e.GetCurrentPoint(this.LayoutGrid).Properties.MouseWheelDelta > 0)
            {
                zoomScale = Math.Max(MinZoomScale, Math.Min(MaxZoomScale, zoomScale * 1.2));
            }
            else
            {
                zoomScale = Math.Max(MinZoomScale, Math.Min(MaxZoomScale, zoomScale / 1.2));
            }

            this.UpdateMagnifier();
        }

        private void LayoutGrid_OnPointerPressed(object sender, PointerRoutedEventArgs e)
        {
            if (!e.Pointer.IsInContact) { return; }

            var point = e.GetCurrentPoint(this.LayoutGrid);

            this.pointerX = point.Position.X;
            this.pointerY = point.Position.Y;
            this.UpdateMagnifier();
            MagnifierTip.Visibility = Windows.UI.Xaml.Visibility.Visible;
        }

        private void LayoutGrid_OnPointerReleased(object sender, PointerRoutedEventArgs e)
        {
            MagnifierTip.Visibility = Windows.UI.Xaml.Visibility.Collapsed;
        }

        private void UpdateMagnifier()
        {
            var bi = (BitmapImage)BigImage.Source;

            double offsetX;
            double offsetY;
            double imageScale;
            CalculateImageScaleAndOffsets(bi, out offsetX, out offsetY, out imageScale);

            MagnifierTip.PositionTransform.X = (-this.pointerX + offsetX) / imageScale;
            MagnifierTip.PositionTransform.Y = (-this.pointerY + offsetY) / imageScale;
            MagnifierTip.ZoomTransform.ScaleX = imageScale * zoomScale;
            MagnifierTip.ZoomTransform.ScaleY = imageScale * zoomScale;

            MagnifierTip.CenterTransform.X = MagnifierTip.MagnifierEllipse.ActualWidth / 2 - MagnifierTip.MagnifierEllipse.StrokeThickness / 2;
            MagnifierTip.CenterTransform.Y = MagnifierTip.MagnifierEllipse.ActualHeight / 2 - MagnifierTip.MagnifierEllipse.StrokeThickness / 2;

            MagnifierTip.MagnifierTranslateTransform.X = this.pointerX - (MagnifierTip.ActualWidth / 2);
            MagnifierTip.MagnifierTranslateTransform.Y = this.pointerY - (MagnifierTip.ActualHeight);

            bool tooHigh = MagnifierTip.MagnifierTranslateTransform.Y < 0;
            bool tooLeft = MagnifierTip.MagnifierTranslateTransform.X < 0;
            bool tooRight = MagnifierTip.MagnifierTranslateTransform.X + MagnifierTip.ActualWidth > this.LayoutGrid.ActualWidth;

            if (tooHigh || tooLeft || tooRight)
            {
                double angle = 0.0;

                if (tooLeft && !tooHigh)
                {
                    var dx = -MagnifierTip.MagnifierTranslateTransform.X;
                    var r = MagnifierTip.ActualHeight - MagnifierTip.ActualWidth / 2;
                    var arad = Math.Asin(dx / r);
                    angle = arad * 180 / Math.PI;
                }
                else if (tooLeft && tooHigh)
                {
                    var dx = -MagnifierTip.MagnifierTranslateTransform.X;
                    var dy = -MagnifierTip.MagnifierTranslateTransform.Y;
                    var r = MagnifierTip.ActualHeight - MagnifierTip.ActualWidth / 2;
                    var arad1 = Math.Asin(dx / r);
                    var arad2 = Math.Acos((r - dy) / r);
                    var arad = Math.Max(arad1, arad2);
                    angle = arad * 180 / Math.PI;
                }
                else if (tooHigh && !tooRight)
                {
                    var dy = -MagnifierTip.MagnifierTranslateTransform.Y;
                    var r = MagnifierTip.ActualHeight - MagnifierTip.ActualWidth / 2;
                    var arad = Math.Acos((r - dy) / r);

                    if (MagnifierTip.MagnifierTranslateTransform.X + Math.Sin(arad) * r + MagnifierTip.ActualWidth > this.LayoutGrid.ActualWidth)
                    {
                        arad = -arad;
                    }

                    angle = arad * 180 / Math.PI;
                }
                else if (tooHigh)
                {
                    var dy = -MagnifierTip.MagnifierTranslateTransform.Y;
                    var dx = MagnifierTip.MagnifierTranslateTransform.X + MagnifierTip.ActualWidth - this.LayoutGrid.ActualWidth;
                    var r = MagnifierTip.ActualHeight - MagnifierTip.ActualWidth / 2;
                    var arad1 = -Math.Acos((r - dy) / r);
                    var arad2 = -Math.Asin(dx / r);
                    var arad = Math.Min(arad1, arad2);
                    angle = arad * 180 / Math.PI;
                }
                else //if (tooRight)
                {
                    var dx = MagnifierTip.MagnifierTranslateTransform.X + MagnifierTip.ActualWidth - this.LayoutGrid.ActualWidth;
                    var r = MagnifierTip.ActualHeight - MagnifierTip.ActualWidth / 2;
                    var arad = -Math.Asin(dx / r);
                    angle = arad * 180 / Math.PI;
                }

                MagnifierTip.RotateTransform.Angle = angle;
                MagnifierTip.LensRotateTransform.Angle = -angle;
            }
            else
            {
                MagnifierTip.RotateTransform.Angle = 0;
                MagnifierTip.LensRotateTransform.Angle = 0;
            }
        }

        private void CalculateImageScaleAndOffsets(BitmapImage bi, out double offsetX, out double offsetY, out double imageScale)
        {
            var imageRatio = (double)bi.PixelWidth / bi.PixelHeight;
            var gridRatio = this.LayoutGrid.ActualWidth / this.LayoutGrid.ActualHeight;

            if (bi.PixelWidth < this.LayoutGrid.ActualWidth &&
                bi.PixelHeight < this.LayoutGrid.ActualHeight)
            {
                offsetX = 0.5 * (this.LayoutGrid.ActualWidth - bi.PixelWidth);
                offsetY = 0.5 * (this.LayoutGrid.ActualHeight - bi.PixelHeight);
                imageScale = 1;
            }
            else if (imageRatio < gridRatio)
            {
                offsetX = 0.5 * (this.LayoutGrid.ActualWidth - imageRatio * this.LayoutGrid.ActualHeight);
                offsetY = 0;
                imageScale = BigImage.ActualHeight / bi.PixelHeight;
            }
            else
            {
                offsetX = 0;
                offsetY = 0.5 * (this.LayoutGrid.ActualHeight - this.LayoutGrid.ActualWidth / imageRatio);
                imageScale = BigImage.ActualWidth / bi.PixelWidth;
            }
        }
    }
}

Magnifier.xaml

<UserControl
    x:Class="MagnifierApp.Magnifier"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:MagnifierApp"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    Height="227"
    Width="170">
    <Grid
        Height="227"
        Width="170"
        RenderTransformOrigin="0.5,1">
        <Grid.RenderTransform>
            <TransformGroup>
                <RotateTransform
                    x:FieldModifier="public"
                    x:Name="RotateTransform"
                    Angle="0" />
                <TranslateTransform
                    x:Name="MagnifierTranslateTransform"
                    x:FieldModifier="public" />
            </TransformGroup>
        </Grid.RenderTransform>
        <Ellipse
            HorizontalAlignment="Left"
            VerticalAlignment="Top"
            IsHitTestVisible="False"
            Width="152"
            Height="152"
            Fill="{ThemeResource ApplicationPageBackgroundThemeBrush}"
            Margin="9,10,0,0"
            RenderTransformOrigin="0.5,0.5"
            StrokeThickness="0">
        </Ellipse>
        <Ellipse
            x:Name="MagnifierEllipse"
            x:FieldModifier="public"
            HorizontalAlignment="Left"
            VerticalAlignment="Top"
            IsHitTestVisible="False"
            Width="152"
            Height="152"
            Stroke="White"
            Margin="9,10,0,0"
            RenderTransformOrigin="0.5,0.5"
            StrokeThickness="0">
            <Ellipse.RenderTransform>
                <RotateTransform
                    x:FieldModifier="public"
                    x:Name="LensRotateTransform"
                    Angle="0" />
            </Ellipse.RenderTransform>
            <Ellipse.Fill>
                <ImageBrush
                    ImageSource="http://blog.al.com/space-news/2009/04/iss015e22574.jpg"
                    Stretch="None"
                    AlignmentX="Left"
                    AlignmentY="Top">
                    <ImageBrush.Transform>
                        <TransformGroup>
                            <TranslateTransform
                                x:FieldModifier="public"
                                x:Name="PositionTransform" />
                            <ScaleTransform
                                x:FieldModifier="public"
                                x:Name="ZoomTransform" />
                            <TranslateTransform
                                x:FieldModifier="public"
                                x:Name="CenterTransform" />
                        </TransformGroup>
                    </ImageBrush.Transform>
                </ImageBrush>
            </Ellipse.Fill>
        </Ellipse>
        <Path
            Data="M85,11 C43.8548,11 10.5,44.3548 10.5,85.5 C10.5,126.645 43.8548,160 85,160 C126.145,160 159.5,126.645 159.5,85.5 C159.5,44.3548 126.145,11 85,11 z M85,0.5 C131.668,0.5 169.5,38.3319 169.5,85 C169.5,103.959 163.256,121.459 152.713,135.558 L151.895,136.625 L152,136.625 L84.2999,226 L18,136.625 L18.1054,136.625 L17.2872,135.558 C6.74375,121.459 0.5,103.959 0.5,85 C0.5,38.3319 38.3319,0.5 85,0.5 z"
            Margin="0,0.5,0,0"
            Stretch="Fill"
            Stroke="Black"
            UseLayoutRounding="False"
            StrokeThickness="0"
            Fill="White" />
    </Grid>
</UserControl>

The fix for 1 here is to Update the magnifier even when within

LayoutGrid_OnPointerPressed

So the updated method looks like

 private void LayoutGrid_OnPointerPressed(object sender, PointerRoutedEventArgs e)
        {
            MagnifierTip.Visibility = Windows.UI.Xaml.Visibility.Visible;

            var point = e.GetCurrentPoint(this.LayoutGrid);
            this.pointerX = point.Position.X;
            this.pointerY = point.Position.Y;
            this.UpdateMagnifier();
        }

This gives you the desired behavior that you are talking about.

For resolving the bounds issue

calculate the pointer values above with respect to BitImage instead of LayoutGrid.

if (this.pointerX < 0.0 || this.pointerY < 0.0) return; if (this.pointerX > BigImage.ActualWidth || this.pointerY > BigImage.ActualHeight) return; var bi = (BitmapImage)BigImage.Source;

The rotation when reaching close the edge is all about carefully calculating the rotation angle as it reaches the edge and applying that Rotation transform.

MagnifierTip.RenderTransform = new RotateTransform{Angle=<CalculatedValue>, CenterX=<CalculatedValue>, CenterY=<CalculatedValue>};

Hope this helps.

来源：https://stackoverflow.com/questions/29703191/clipping-a-low-resolution-image-using-transforms-and-assigning-it-to-my-user-con

标签

image-processing

winrt-xaml

windows-8.1

windows-rt