UWP Parallax with offset

Question

I develop UWP application with listview of images with translucated items background like on Windows Phone Start screen (e.g. https://www.windowscentral.com/sites/wpcentral.com/files/styles/larger/public/field/image/2014/04/Clean_vs_Busy.jpg?itok=58NioLgB). I decided to base my solution on UWP Community toolkit parallax service. First I took left-top point of item:

var p = parallaxElement.TransformToVisual(scroller).TransformPoint(new Point(0, 0));

Where should I add this offset in animation expression? Also I did not find full documentation for this.

 ExpressionAnimation expression = compositor.CreateExpressionAnimation(
        "Matrix4x4.CreateFromTranslation(Vector3(HorizontalMultiplier * scroller.Translation.X, VerticalMultiplier * scroller.Translation.Y, 0.0f))");
    expression.SetReferenceParameter("scroller", scrollerViewerManipulation);
    expression.SetScalarParameter("offsetX", (float)p.X);
    expression.SetScalarParameter("offsetY", (float)p.Y);

In other words, I'd like to make effect "looking through items on shared large image"; items are wholes in canvas.

Answer 1

To animate all of the images of your list, you can follow the sample from the Windows Composition team.

Here's the tl/dr version:

Create the animation. Do this when the page is loaded:

private void SetupAnimation()
{
    // available with SDK version 15063
    Compositor compositor = Window.Current.Compositor;
    // available with previous SDK version
    //Compositor compositor = ElementCompositionPreview.GetElementVisual(this).Compositor;

    // Get scrollviewer using UWP Community Toolkit extension method
    ScrollViewer myScrollViewer = ImageList.FindDescendant<ScrollViewer>();
    _scrollProperties = ElementCompositionPreview.GetScrollViewerManipulationPropertySet(myScrollViewer);

    // Setup the expression
    var scrollPropSet = _scrollProperties.GetSpecializedReference<ManipulationPropertySetReferenceNode>();
    var startOffset = ExpressionValues.Constant.CreateConstantScalar("startOffset", 0.0f);
    var parallaxValue = 0.5f;
    var parallax = (scrollPropSet.Translation.Y + startOffset);
    _parallaxExpression = parallax * parallaxValue - parallax;
}

Animate each image in the list when the container changes (NOTE: subscribe to the ContainerContentChanging event of your ListView)

private void ImageList_ContainerContentChanging(ListViewBase sender, ContainerContentChangingEventArgs args)
{
    // Find the image element to animate
    Image image = args.ItemContainer.ContentTemplateRoot.GetFirstDescendantOfType<Image>();

    Visual visual = ElementCompositionPreview.GetElementVisual(image);
    // You'll want to use the right size for your images
    visual.Size = new Vector2(960f, 960f);

    if (_parallaxExpression != null)
    {
        _parallaxExpression.SetScalarParameter("StartOffset", (float)args.ItemIndex * visual.Size.Y / 4.0f);
    visual.StartAnimation("Offset.Y", _parallaxExpression);
    }
}

This sample works with a ListView or GridView.