ScaleTransform transforms non-linearly - wpf

Question

I am using scale transform to allow a user to resize a control. What happens though is that when you start to move the mouse the control jumps to a new size, and then scales oddly. The further you move your mouse from the starting location the larger the increase in size becomes.
I expect its the way I calculate the scale to be applied. Here is the code:
private void ResizeGrip_MouseDown(object sender, MouseButtonEventArgs e)
{
ResizeHandle.CaptureMouse();
//Get the initial coordinate cursor location on the window
initBtmX = e.GetPosition(this).X;
bottomResize = true;
}
private void ResizeGrip_MouseUp(object sender, MouseButtonEventArgs e)
{
bottomResize = false;
ResizeHandle.ReleaseMouseCapture();
}
private void ResizeGrip_MouseMove(object sender, MouseEventArgs e)
{
if( bottomResize == true)
{
//Get the new Y coordinate cursor location
double newBtmX = e.GetPosition(this).X;
//Get the smallest change between the initial and new cursor location
double diffX = initBtmX - newBtmX;
// Let our rectangle capture the mouse
ResizeHandle.CaptureMouse();
double newWidth = e.GetPosition(this).X - diffX;
double scaler = newWidth / ResizeContainer.ActualWidth;
Console.WriteLine("newWidth: {0}, scalar: {1}", newWidth, scaler);
if (scaler < 0.75 || scaler > 3)
return;
ScaleTransform scale = new ScaleTransform(scaler, scaler);
ResizeContainer.LayoutTransform = scale;
}
}
Update: Now with XAML
<wtk:IToolDialog x:Name="VideoPlayer" ParentControl="{Binding ElementName=Stage}" DialogTitle="Video Player" Margin="90,5,0,0">
<Grid>
<Grid x:Name="ResizeContainer" ClipToBounds="True" Width="320" Height="240" HorizontalAlignment="Center" VerticalAlignment="Bottom" Margin="0,0,0,1">
<!-- The video frame -->
<Image Stretch="Fill" Source="{Binding CurrentFrameImage}" x:Name="VideoImage" />
<Grid>
<pplcontrols:VideoGroundPlane Foreground="Black" GridSize="20" GroundPlane="{Binding GroundPlane}">
</pplcontrols:VideoGroundPlane>
</Grid>
<Grid x:Name="HitMask" IsHitTestVisible="False"/>
</Grid>
<ResizeGrip Cursor="SizeNWSE" x:Name="ResizeHandle" VerticalAlignment="Bottom" HorizontalAlignment="Right" Mouse.MouseDown="ResizeGrip_MouseDown" Mouse.MouseUp="ResizeGrip_MouseUp" Mouse.MouseMove="ResizeGrip_MouseMove"></ResizeGrip>
</Grid>
</wtk:IToolDialog>

Answer 1

Any reason why you are not using ResizeContainer.RenderTransfrom instead of ResizeContainer.LayoutTransform? I.e. use
ResizeContainer.LayoutTransform = scale;
If you want the scale to be linear I think you should use
double scaler = 1 - diff / ResizeContainer.ActualWidth;
EDIT:
There is a bug in the code that causes the scaled control to "jump" in size if you try to resize more than once. I suggest you do the following:
Add a RenderTransform to your ResizeContainer grid:
<Grid.RenderTransform>
<ScaleTransform x:Name="transform" ScaleX="1" ScaleY="1" />
</Grid.RenderTransform>
Change the code in your MouseMove event handler to the following:
if (bottomResize)
{
double newBtmX = e.GetPosition(this).X;
double scaler = -(initBtmX - newBtmX) / grid1.ActualWidth;
initBtmX = newBtmX;
transform.ScaleX += scaler;
transform.ScaleY += scaler;
}
This way you change the scale by whatever small amount the mouse has moved while dragging. All child controls within the grid should scale as well.