在渲染到位图之前缩放WPF内容

Question

背景： 我正在开发一个使用Silverlight（1.0）构建的应用程序，动态地在特定位置叠加图标和文本，形成美国地图。这个地图在浏览器中运行良好，现在我需要获取一个静态的（可打印并可以插入到文档/幻灯片中的）已显示的地图副本。

目标： 为了获得可打印的地图副本，该副本也可用于PowerPoint幻灯片、Word等，我选择创建一个ASP.NET HttpHandler，在WPF中从服务器端重新创建xaml，然后将WPF渲染为位图图像，并作为png文件返回，生成的分辨率为300dpi，以获得更好的打印质量。

问题： 这个方法非常好，但有一个问题，就是我无法将图像缩放到指定的大小。我尝试了几种不同的方法，其中一些可以在注释掉的行中看到。我需要能够指定图像的高度和宽度，无论是英寸还是像素，我并不在意，只要xaml能够按照指定的大小进行缩放生成位图即可。目前，如果我将大小设置得比根画布大，那么画布会以其原始大小呈现在生成的图像的左上角，并且大小与指定的大小相同。下面是我的httphandler的重要部分。存储为"MyImage"的根画布高度为600，宽度为800。我错过了什么，以便使内容按比例缩放以适应指定的大小？

我不完全理解传递到Arrange()和Measure()中的维度所做的工作，因为其中一些代码是从在线示例中获取的。我也不完全理解RenderTargetBitmap的东西。任何指导都将不胜感激。

Public Sub Capture(ByVal MyImage As Canvas)
    ' Determine the constraining scale to maintain the aspect ratio and the bounds of the image size
    Dim scale As Double = Math.Min(Width / MyImage.Width, Height / MyImage.Height)

    'Dim vbox As New Viewbox()
    'vbox.Stretch = Stretch.Uniform
    'vbox.StretchDirection = StretchDirection.Both
    'vbox.Height = Height * scale * 300 / 96.0
    'vbox.Width = Width * scale * 300 / 96.0
    'vbox.Child = MyImage

    Dim bounds As Rect = New Rect(0, 0, MyImage.Width * scale, MyImage.Height * scale)
    MyImage.Measure(New Size(Width * scale, Height * scale))
    MyImage.Arrange(bounds)
    'MyImage.UpdateLayout()

    ' Create the target bitmap
    Dim rtb As RenderTargetBitmap = New RenderTargetBitmap(CInt(Width * scale * 300 / 96.0), CInt(Height * scale * 300 / 96.0), 300, 300, PixelFormats.Pbgra32)

    ' Render the image to the target bitmap
    Dim dv As DrawingVisual = New DrawingVisual()
    Using ctx As DrawingContext = dv.RenderOpen()
        Dim vb As New VisualBrush(MyImage)
        'Dim vb As New VisualBrush(vbox)
        ctx.DrawRectangle(vb, Nothing, New Rect(New System.Windows.Point(), bounds.Size))
    End Using
    rtb.Render(dv)

    ' Encode the image in the format selected
    Dim encoder As System.Windows.Media.Imaging.BitmapEncoder
    Select Case Encoding.ToLower
        Case "jpg"
            encoder = New System.Windows.Media.Imaging.JpegBitmapEncoder()
        Case "png"
            encoder = New System.Windows.Media.Imaging.PngBitmapEncoder()
        Case "gif"
            encoder = New System.Windows.Media.Imaging.GifBitmapEncoder()
        Case "bmp"
            encoder = New System.Windows.Media.Imaging.BmpBitmapEncoder()
        Case "tif"
            encoder = New System.Windows.Media.Imaging.TiffBitmapEncoder()
        Case "wmp"
            encoder = New System.Windows.Media.Imaging.WmpBitmapEncoder()
    End Select
    encoder.Frames.Add(System.Windows.Media.Imaging.BitmapFrame.Create(rtb))

    ' Create the memory stream to save the encoded image.
    retImageStream = New System.IO.MemoryStream()
    encoder.Save(retImageStream)
    retImageStream.Flush()
    retImageStream.Seek(0, System.IO.SeekOrigin.Begin)
    MyImage = Nothing
End Sub

- Tom

Answer 1

这应该足以让您入门：

private void ExportCanvas(int width, int height)
{
    string path = @"c:\temp\Test.tif";
    FileStream fs = new FileStream(path, FileMode.Create);


    RenderTargetBitmap renderBitmap = new RenderTargetBitmap(width,
                                                             height, 1/300, 1/300, PixelFormats.Pbgra32);

    DrawingVisual visual = new DrawingVisual();
    using (DrawingContext context = visual.RenderOpen())
    {
        VisualBrush brush = new VisualBrush(MyCanvas);
        context.DrawRectangle(brush,
                              null,
                              new Rect(new Point(), new Size(MyCanvas.Width, MyCanvas.Height)));
    }

    visual.Transform = new ScaleTransform(width / MyCanvas.ActualWidth, height / MyCanvas.ActualHeight);

    renderBitmap.Render(visual);

    BitmapEncoder encoder = new TiffBitmapEncoder();
    encoder.Frames.Add(BitmapFrame.Create(renderBitmap));
    encoder.Save(fs);
    fs.Close();
}

- Donnelle

---

这最终激发了我找到了正确的解决方案，所以我将其标记为答案，但以下是真正适用于我的代码。 - Jon Galloway

Answer 2

这是最终对我起作用的代码：

Public Sub Capture(ByVal MyImage As Canvas)
        ' Normally we would obtain a user's configured DPI setting to account for the possibilty of a high DPI setting. 
        ' However, this code is running server side so the client's DPI is not obtainable.
        Const SCREEN_DPI As Double = 96.0   ' Screen DPI
        Const TARGET_DPI As Double = 300.0  ' Print Quality DPI

        ' Determine the constraining scale to maintain the aspect ratio and the bounds of the image size
        Dim scale As Double = Math.Min(Width * SCREEN_DPI / MyImage.Width, Height * SCREEN_DPI / MyImage.Height)

        ' Setup the bounds of the image
        Dim bounds As Rect = New Rect(0, 0, MyImage.Width * scale, MyImage.Height * scale)
        MyImage.Measure(New Size(MyImage.Width * scale, MyImage.Height * scale))
        MyImage.Arrange(bounds)

        ' Create the target bitmap
        Dim rtb As RenderTargetBitmap = New RenderTargetBitmap(CDbl(MyImage.Width * scale / SCREEN_DPI * TARGET_DPI), CDbl(MyImage.Height * scale / SCREEN_DPI * TARGET_DPI), TARGET_DPI, TARGET_DPI, PixelFormats.Pbgra32)

        ' Render the image to the target bitmap
        Dim dv As DrawingVisual = New DrawingVisual()
        Using ctx As DrawingContext = dv.RenderOpen()
            Dim vb As New VisualBrush(MyImage)
            ctx.DrawRectangle(vb, Nothing, New Rect(New System.Windows.Point(), bounds.Size))
        End Using
        ' Transform the visual to scale the image to our desired size.
        'dv.Transform = New ScaleTransform(scale, scale)

        ' Render the visual to the bitmap.
        rtb.Render(dv)

        ' Encode the image in the format selected. If no valid format was selected, default to png.
        Dim encoder As System.Windows.Media.Imaging.BitmapEncoder
        Select Case Encoding.ToLower
            Case "jpg"
                encoder = New System.Windows.Media.Imaging.JpegBitmapEncoder()
            Case "png"
                encoder = New System.Windows.Media.Imaging.PngBitmapEncoder()
            Case "gif"
                encoder = New System.Windows.Media.Imaging.GifBitmapEncoder()
            Case "bmp"
                encoder = New System.Windows.Media.Imaging.BmpBitmapEncoder()
            Case "tif"
                encoder = New System.Windows.Media.Imaging.TiffBitmapEncoder()
            Case "wmp"
                encoder = New System.Windows.Media.Imaging.WmpBitmapEncoder()
            Case Else
                encoder = New System.Windows.Media.Imaging.PngBitmapEncoder()
        End Select
        encoder.Frames.Add(System.Windows.Media.Imaging.BitmapFrame.Create(rtb))

        ' Create the memory stream to save the encoded image.
        retImageStream = New System.IO.MemoryStream()
        encoder.Save(retImageStream)
        retImageStream.Flush()
        retImageStream.Seek(0, System.IO.SeekOrigin.Begin)
        MyImage = Nothing
    End Sub

- Tom

---

我认为您可能错过了将帧插入编码器的那一行代码。 - bryanbcook