Image being saved from canvas is not rendered properly

问题

I'm trying to use the following code to save the contents of myCanvas as an image file in the gallery (for Windows Phone 8.1, not Silverlight). When I run the App, the image is saved but it is distorted. What am I doing wrong? I have uploaded the resultant image and the expected result.

public async void SaveFileToPhone()
  {
      var file = await KnownFolders.PicturesLibrary.CreateFileAsync("bug.png", CreationCollisionOption.GenerateUniqueName);

     await SaveVisualElementToFile(myCanvas, file);          
  }

async Task SaveVisualElementToFile(FrameworkElement element, StorageFile file)
{
    var renderTargetBitmap = new RenderTargetBitmap();
    await renderTargetBitmap.RenderAsync(element, (int)element.Width, (int)element.Height);
    var pixels = await renderTargetBitmap.GetPixelsAsync();
    txt_bug.Text = "Width: " + (int)element.Width + " Height:" + (int)element.Height;

    using (IRandomAccessStream stream = await file.OpenAsync(FileAccessMode.ReadWrite))
    {
        var encoder = await
            BitmapEncoder.CreateAsync(BitmapEncoder.PngEncoderId, stream);
        byte[] bytes = pixels.ToArray();

        encoder.SetPixelData(BitmapPixelFormat.Bgra8,
                             BitmapAlphaMode.Ignore,
                             (uint)element.Width, (uint)element.Height,
                             96, 96, bytes);

        await encoder.FlushAsync();
    }
}

The XAML code for canvas is as follows:

<Canvas x:Name="myCanvas" Background="#FF33FFE3" Margin="25,75,26,10" 
ManipulationStarted="myCanvas_ManipulationStarted" 
ManipulationCompleted="myCanvas_ManipulationCompleted" 
ManipulationDelta="myCanvas_ManipulationDelta" ManipulationMode="All" 
Tapped="myCanvas_Tapped" MinHeight="555" MinWidth="350" Width="350" Height="555">
    <Canvas.Clip>
        <RectangleGeometry Rect="0 0 350 555"/>
    </Canvas.Clip>
</Canvas>

回答1:

The params to the SetPixelData method are incorrect.

encoder.SetPixelData(BitmapPixelFormat.Bgra8,
                         BitmapAlphaMode.Ignore,
                         (uint)element.Width, (uint)element.Height,
                         96, 96, bytes);

Change them to the follow can make it work.

encoder.SetPixelData(BitmapPixelFormat.Bgra8,
                BitmapAlphaMode.Ignore,
                (uint)renderTargetBitmap.PixelWidth,
                (uint)renderTargetBitmap.PixelHeight,
                DisplayInformation.GetForCurrentView().LogicalDpi,
                DisplayInformation.GetForCurrentView().LogicalDpi,
                bytes);

You can download the XAML render to bitmap sample from MSDN for reference.

来源：https://stackoverflow.com/questions/29574481/image-being-saved-from-canvas-is-not-rendered-properly