Silverlight 3 - ScaleTransform or other method to zoom in a Canvas?

Question

I need to be able to zoom in and out of a Canvas using the mousewheel. I have successfully set up the mouse wheel handlers and am currently using a ScaleTransform to apply the zoom; however, the scaling is not done in an "intuitive" way.

I'm trying to accomplish the same style of "zooming" as you can see in MultiScaleImage, Google Maps/Earth, or Adobe Acrobat Reader--but NOT with an image, with a control. The transition doesn't need to be "smooth" or animated (unless it's an easier approach), but the functionality needs to be the same.

Any thoughts or ideas would be highly appreciated and thanks in advance!

Edit: I've managed to "smooth" the zoom using animation:

<Canvas.Resources>
            <Storyboard x:Name="ZoomStoryboard">
                <DoubleAnimation x:Name="ZoomAnimationX"
                                 Storyboard.TargetName="Workspace"
                                 Storyboard.TargetProperty="Canvas.RenderTransform.ScaleTransform.ScaleX"
                                 Duration="0:0:0.2"/>
                <DoubleAnimation x:Name="ZoomAnimationY"
                                 Storyboard.TargetName="Workspace"
                                 Storyboard.TargetProperty="Canvas.RenderTransform.ScaleTransform.ScaleY"
                                 Duration="0:0:0.2"/>
            </Storyboard>
        </Canvas.Resources>

with the following code:

_Zoom += (args.Delta / 7);
if (_Zoom < 0.15)
    _Zoom = 0.15;
ZoomAnimationX.To = _Zoom;
ZoomAnimationY.To = _Zoom;
ZoomStoryboard.Begin();
ZoomScale.Text = _Zoom.ToString("0.00") + "x";
_PreviousMousePosition = _CurrentMousePosition

However, the issue still arises that it is zooming out of the top-left corner, as opposed to sites like Google Maps where the zoom is "around" the mouse.

Answer 1

You need to use a weighted average as the zoom center based on the mouse position. In other words, keep the latest zoom center (or if you don't have one yet just set it to the current mouse position) and keep the number of times the zoom center was calculated (after the first zoom, that would be 1). Than each time you recalculate the zoom center, increment that var.

Example Code Follows - deltaZoom is how much you're zooming, centerX and centerY are the current zoom center, ZoomSteps is the number of times we've zoomed, and mouseX and mouseY are the current mouse position:

_Zoom += deltaZoom;
        if (_Zoom <= 0)
            _Zoom = 0.1;

        if (deltaZoom >= 0)
        {
            if (_ZoomSteps == -1)
            {
                _CenterX = 0;
                _CenterY = 0;
                _ZoomSteps = 0;
            }
            else
            {
                _CenterX = (_CenterX * Math.Abs(_ZoomSteps) + mouseX) / (Math.Abs(_ZoomSteps + 1));
                _CenterY = (_CenterY * Math.Abs(_ZoomSteps) + mouseY) / (Math.Abs(_ZoomSteps + 1));
                _ZoomSteps++;
            }
        }
        else
        {
            if (_ZoomSteps == 1)
            {
                _CenterX = 0;
                _CenterY = 0;
                _ZoomSteps = 0;
            }
            else
            {
                _CenterX = (_CenterX * Math.Abs(_ZoomSteps) - mouseX) / (Math.Abs(_ZoomSteps - 1));
                _CenterY = (_CenterY * Math.Abs(_ZoomSteps) - mouseY) / (Math.Abs(_ZoomSteps - 1));
                _ZoomSteps--;
            }
        }

        ZoomAnimationX.To = _Zoom;
        ZoomAnimationY.To = _Zoom;
        CenterAnimationX.To = Math.Abs(_CenterX);
        CenterAnimationY.To = Math.Abs(_CenterY);
        ZoomStoryboard.Begin();

Edited so that you can drop below 1.0 zoom level but there are still some problems (ZoomStep = -1, 0 or 1 sometimes cause weird shakes).