c# - UWP Composition - 将不透明蒙版应用于 ListView 的顶部 30px

Question

如何将效果应用于顶部 30px 从完全透明变为完全不透明的 ListView？这个想法是，当您向下滚动时，顶部的项目逐渐消失。

我正在构建一个 UWP 应用程序，其中设计要求 ListView 的顶部 30px 从不透明度 0 开始并过渡到不透明度 1。从概念上讲，我正在想象一个将应用于 SpriteVisual 顶部的不透明蒙版，但是我不知道如何实现这一目标。

我正在尝试使用 Windows 10 周年纪念版、Composition 和 Win2D。

编辑:一张图可能画出1000字:

如果您看这张图片，在左下角和右下角有两个内容元素。虽然背景看起来是黑色，但实际上是渐变色。如果您检查这两个元素的顶部，它们在顶部变得更加透明，透过背景显示出来。这就是我想要达到的效果。

编辑 2:为了显示我正在寻找的效果的结果，这里有一个 GIF，它显示了我使用叠加位图时的效果:

页眉背景图片为:

下方的 30 像素具有 alpha 渐变并出现在 gridview 上方，给出了 grid view 项目淡出并在背景下滑动的明显效果。

XAML 布局如下所示:

<Page
x:Class="App14.MainPage"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:App14"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d">
<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="150" />
        <RowDefinition Height="*" />
    </Grid.RowDefinitions>

    <Image Source="/Assets/background.png"
           Grid.Row="0"
           Grid.RowSpan="2"
           VerticalAlignment="Top"
           Stretch="None" />

    <GridView Grid.Row="1"
              Margin="96,-30,96,96">
        <GridView.Resources>
            <Style TargetType="Image">
                <Setter Property="Height" Value="400" />
                <Setter Property="Width" Value="300" />
                <Setter Property="Margin" Value="30" />
            </Style>
        </GridView.Resources>
        <Image Source="Assets/1.jpg" />
        <Image Source="Assets/2.jpg" />
        <Image Source="Assets/3.jpg" />
        <Image Source="Assets/4.jpg" />
        <Image Source="Assets/5.jpg" />
        <Image Source="Assets/6.jpg" />
        <Image Source="Assets/7.jpg" />
        <Image Source="Assets/8.jpg" />
        <Image Source="Assets/9.jpg" />
        <Image Source="Assets/10.jpg" />
        <Image Source="Assets/11.jpg" />
        <Image Source="Assets/12.jpg" />
    </GridView>

    <!-- Header above content -->

    <Image Grid.Row="0" Source="/Assets/header_background.png"
           Stretch="None" />

    <TextBlock x:Name="Title"
               Grid.Row="0"
               FontSize="48"
               Text="This Is A Title"
               HorizontalAlignment="Center"
               VerticalAlignment="Center"
               Foreground="White" />


</Grid>

Answer 1

所以在 @sohcatt 的帮助下在 Windows UI 开发实验室问题列表中，我构建了一个有效的解决方案。

这是 XAML:

    <Grid x:Name="LayoutRoot">

    <Image x:Name="BackgroundImage"
           ImageOpened="ImageBrush_OnImageOpened"
           Source="../Assets/blue-star-background-wallpaper-3.jpg"
           Stretch="UniformToFill" />

    <GridView x:Name="Posters" Margin="200,48">
        <GridView.Resources>
            <Style TargetType="ListViewItem" />
            <Style TargetType="Image">
                <Setter Property="Stretch" Value="UniformToFill" />
                <Setter Property="Width" Value="300" />
                <Setter Property="Margin" Value="12" />
            </Style>
        </GridView.Resources>
        <GridViewItem>
            <Image Source="Assets/Posters/1.jpg" />
        </GridViewItem>
        <GridViewItem>
            <Image Source="Assets/Posters/2.jpg" />
        </GridViewItem>
        <GridViewItem>
            <Image Source="Assets/Posters/3.jpg" />
        </GridViewItem>
        <GridViewItem>
            <Image Source="Assets/Posters/4.jpg" />
        </GridViewItem>
        <GridViewItem>
            <Image Source="Assets/Posters/5.jpg" />
        </GridViewItem>
        <GridViewItem>
            <Image Source="Assets/Posters/6.jpg" />
        </GridViewItem>
        <GridViewItem>
            <Image Source="Assets/Posters/7.jpg" />
        </GridViewItem>
        <GridViewItem>
            <Image Source="Assets/Posters/8.jpg" />
        </GridViewItem>
        <GridViewItem>
            <Image Source="Assets/Posters/9.jpg" />
        </GridViewItem>
        <GridViewItem>
            <Image Source="Assets/Posters/10.jpg" />
        </GridViewItem>
        <GridViewItem>
            <Image Source="Assets/Posters/11.jpg" />
        </GridViewItem>
        <GridViewItem>
            <Image Source="Assets/Posters/12.jpg" />
        </GridViewItem>
    </GridView>
</Grid>

代码如下:

        private bool _imageLoaded;

    // this is an initial way of handling resize 
    // I will investigate expressions
    private async void OnSizeChanged(object sender, SizeChangedEventArgs args)
    {
        if (!_imageLoaded)
        {
            return;
        }
        await RenderOverlayAsync();
    }

    private async void ImageBrush_OnImageOpened(object sender, RoutedEventArgs e)
    {
        _imageLoaded = true;
        await RenderOverlayAsync();
    }

    // this method must be called after the background image is opened, otherwise
    // the render target bitmap is empty
    private async Task RenderOverlayAsync()
    {
        // setup composition
        // (in line here for readability - will be member variables moving forwards)
        var compositor = ElementCompositionPreview.GetElementVisual(this).Compositor;
        var canvasDevice = new CanvasDevice();
        var compositionDevice = CanvasComposition.CreateCompositionGraphicsDevice(compositor, canvasDevice);

        // determine what region of the background we need to "cut out" for the overlay
        GeneralTransform gt = Posters.TransformToVisual(LayoutRoot);
        Point elementPosition = gt.TransformPoint(new Point(0, 0));

        // our overlay height is as wide as our poster control and is 30 px high
        var overlayHeight = 30;
        var areaToRender = new Rect(elementPosition.X, elementPosition.Y, Posters.ActualWidth, overlayHeight);

        // Capture the image from our background.
        //
        // Note: this is just the <Image/> element, not the Grid. If we took the <Grid/>, 
        // we would also have all of the child elements, such as the <GridView/> rendered as well -
        // which defeats the purpose!
        // 
        // Note 2: this method must be called after the background image is opened, otherwise
        // the render target bitmap is empty
        var bitmap = new RenderTargetBitmap();
        await bitmap.RenderAsync(BackgroundImage);
        var pixels = await bitmap.GetPixelsAsync();

        // we need the display DPI so we know how to handle the bitmap correctly when we render it
        var dpi = DisplayInformation.GetForCurrentView().LogicalDpi;

        // load the pixels from RenderTargetBitmap onto a CompositionDrawingSurface
        CompositionDrawingSurface uiElementBitmapSurface;
        using (
            // this is the entire background image
            // Note we are using the display DPI here.
            var canvasBitmap = CanvasBitmap.CreateFromBytes(
                canvasDevice, pixels.ToArray(),
                bitmap.PixelWidth,
                bitmap.PixelHeight,
                DirectXPixelFormat.B8G8R8A8UIntNormalized,
                dpi)
        )
        {
            // we create a surface we can draw on in memory.
            // note we are using the desired size of our overlay
            uiElementBitmapSurface =
                compositionDevice.CreateDrawingSurface(
                    new Size(areaToRender.Width, areaToRender.Height),
                    DirectXPixelFormat.B8G8R8A8UIntNormalized, DirectXAlphaMode.Premultiplied);
            using (var session = CanvasComposition.CreateDrawingSession(uiElementBitmapSurface))
            {
                // here we draw just the part of the background image we wish to use to overlay
                session.DrawImage(canvasBitmap, 0, 0, areaToRender);
            }
        }

        // assign CompositionDrawingSurface to the CompositionSurfacebrush with which I want to paint the relevant SpriteVisual
        var backgroundImageBrush = _compositor.CreateSurfaceBrush(uiElementBitmapSurface);

        // load in our opacity mask image.
        // this is created in a graphic tool such as paint.net
        var opacityMaskSurface = await SurfaceLoader.LoadFromUri(new Uri("ms-appx:///Assets/OpacityMask.Png"));

        // create surfacebrush with ICompositionSurface that contains the background image to be masked
        backgroundImageBrush.Stretch = CompositionStretch.UniformToFill;

        // create surfacebrush with ICompositionSurface that contains the gradient opacity mask asset
        CompositionSurfaceBrush opacityBrush = _compositor.CreateSurfaceBrush(opacityMaskSurface);
        opacityBrush.Stretch = CompositionStretch.UniformToFill;

        // create maskbrush
        CompositionMaskBrush maskbrush = _compositor.CreateMaskBrush();
        maskbrush.Mask = opacityBrush; // surfacebrush with gradient opacity mask asset
        maskbrush.Source = backgroundImageBrush; // surfacebrush with background image that is to be masked

        // create spritevisual of the approproate size, offset, etc.
        SpriteVisual maskSprite = _compositor.CreateSpriteVisual();
        maskSprite.Size = new Vector2((float)Posters.ActualWidth, overlayHeight);
        maskSprite.Brush = maskbrush; // paint it with the maskbrush

        // set the sprite visual as a child of the XAML element it needs to be drawn on top of
        ElementCompositionPreview.SetElementChildVisual(Posters, maskSprite);
    }