Question

我正在使用内置摄像头解码条形码，我使用df_new执行此操作以从预览中捕获照片。它有效，但冻结了应用程序一小段时间，感觉非常笨拙和马车。

因此，我希望在后台使用此功能，同时至少在处理照片时留下响应式用户界面。

到目前为止，我想出了这个来捕获视频流：

capElement.Source.CapturePhotoToStreamAsync

这个方法从流中获取字节：

 private async void ScanInBackground()
        {
            bool failedScan = true;

            var stream = new InMemoryRandomAccessStream();

            await  capElement.Source.StartRecordToStreamAsync(MediaEncodingProfile.CreateWmv(VideoEncodingQuality.HD1080p), stream);

            while(failedScan)
            {
                Byte[] bytes = await GetBytesFromStream(stream);
                //How to split the bytes into frames?

                Task.Delay(50);
            }

            Dispatcher.RunAsync(CoreDispatcherPriority.Low,() => StopCap()); 
        }

从public static async Task<byte[]> GetBytesFromStream(IRandomAccessStream randomStream) { var reader = new DataReader(randomStream.GetInputStreamAt(0)); var bytes = new byte[randomStream.Size]; try { await reader.LoadAsync((uint)randomStream.Size); reader.ReadBytes(bytes); } catch(Exception ex) { Logger.LogExceptionAsync(ex, "GetBytesFromStream"); } return bytes; }的评论中，您可以看到我不知道如何将流分割为照片/帧。

Answer 1

Microsoft github页面上有一个相关的示例，尽管它们的目标是Windows 10.您可能有兴趣迁移项目以获得此功能。

GetPreviewFrame：此示例将捕获预览帧而不是完整的照片。一旦它有预览框架，它就可以读取和编辑它上面的像素。

以下是相关部分：

private async Task GetPreviewFrameAsSoftwareBitmapAsync()
{
    // Get information about the preview
    var previewProperties = _mediaCapture.VideoDeviceController.GetMediaStreamProperties(MediaStreamType.VideoPreview) as VideoEncodingProperties;

    // Create the video frame to request a SoftwareBitmap preview frame
    var videoFrame = new VideoFrame(BitmapPixelFormat.Bgra8, (int)previewProperties.Width, (int)previewProperties.Height);

    // Capture the preview frame
    using (var currentFrame = await _mediaCapture.GetPreviewFrameAsync(videoFrame))
    {
        // Collect the resulting frame
        SoftwareBitmap previewFrame = currentFrame.SoftwareBitmap;

        // Add a simple green filter effect to the SoftwareBitmap
        EditPixels(previewFrame);
    }
}

private unsafe void EditPixels(SoftwareBitmap bitmap)
{
    // Effect is hard-coded to operate on BGRA8 format only
    if (bitmap.BitmapPixelFormat == BitmapPixelFormat.Bgra8)
    {
        // In BGRA8 format, each pixel is defined by 4 bytes
        const int BYTES_PER_PIXEL = 4;

        using (var buffer = bitmap.LockBuffer(BitmapBufferAccessMode.ReadWrite))
        using (var reference = buffer.CreateReference())
        {
            // Get a pointer to the pixel buffer
            byte* data;
            uint capacity;
            ((IMemoryBufferByteAccess)reference).GetBuffer(out data, out capacity);

            // Get information about the BitmapBuffer
            var desc = buffer.GetPlaneDescription(0);

            // Iterate over all pixels
            for (uint row = 0; row < desc.Height; row++)
            {
                for (uint col = 0; col < desc.Width; col++)
                {
                    // Index of the current pixel in the buffer (defined by the next 4 bytes, BGRA8)
                    var currPixel = desc.StartIndex + desc.Stride * row + BYTES_PER_PIXEL * col;

                    // Read the current pixel information into b,g,r channels (leave out alpha channel)
                    var b = data[currPixel + 0]; // Blue
                    var g = data[currPixel + 1]; // Green
                    var r = data[currPixel + 2]; // Red

                    // Boost the green channel, leave the other two untouched
                    data[currPixel + 0] = b;
                    data[currPixel + 1] = (byte)Math.Min(g + 80, 255);
                    data[currPixel + 2] = r;
                }
            }
        }
    }
}

并在课堂外宣布：

[ComImport]
[Guid("5b0d3235-4dba-4d44-865e-8f1d0e4fd04d")]
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
unsafe interface IMemoryBufferByteAccess
{
    void GetBuffer(out byte* buffer, out uint capacity);
}

当然，您的项目必须允许所有这些的不安全代码才能正常工作。

仔细查看示例，了解如何获取所有详细信息。或者，要进行演练，您可以观看最近//版本/会议中的camera session，其中包括一些相机示例的演练。

Answer 2

我认为展示媒体预览和处理不同的可能异常是必要的，这里有一个如何做到这一点的简单示例，

假设您有以下用户界面，其中CaptureElement显示预览，Image控件显示已捕获的图片，

 mc:Ignorable="d" Loaded="MainPage_OnLoaded">

<Grid Background="{StaticResource ApplicationPageBackgroundThemeBrush}" VerticalAlignment="Stretch" HorizontalAlignment="Stretch">
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="auto"/>
        <ColumnDefinition Width="auto"/>
    </Grid.ColumnDefinitions>
    <Grid.RowDefinitions>
        <RowDefinition Height="auto"/>
        <RowDefinition Height="auto"/>
    </Grid.RowDefinitions>
    <CaptureElement x:Name="PreviewElement" Width="400" Height="400" Grid.Column="0" Grid.Row="0"/>
    <Image x:Name="ImageElement"  Width="400"   Height="400" Grid.Column="1" Grid.Row="0"/>
    <Button Click="TakePhoto_Click"  Content="Take Photo" Grid.Column="0" Grid.Row="1" Grid.ColumnSpan="2" HorizontalAlignment="Stretch" />                                    
</Grid>

在后面的代码声明了一个mediaCapture字段，

private MediaCapture _mediaCapture;

然后在页面加载的事件处理程序中你需要

启动媒体捕获设备，
设置几个可能的异常处理程序，

并启动凸轮预览

 private async void MainPage_OnLoaded(object sender, RoutedEventArgs e)
{
    //Start the device 
    try
    {
        _mediaCapture = new MediaCapture();
        _mediaCapture.RecordLimitationExceeded += MediaCapture_RecordLimitationExceeded;
        _mediaCapture.Failed += MediaCapture_Failed;
        await _mediaCapture.InitializeAsync();
    }
    catch (UnauthorizedAccessException ex)
    {
        (new MessageDialog("Set the permission to use the webcam")).ShowAsync();                
    }
    catch (Exception ex)
    {
        (new MessageDialog("Can't initialize the webcam !")).ShowAsync();                
    }

    //Start the preview 
    if (_mediaCapture != null)
    {
        try
        {
            PreviewElement.Source = _mediaCapture;
            await _mediaCapture.StartPreviewAsync();
        }
        catch (Exception ex)
        {
            (new MessageDialog("Something went wrong !")).ShowAsync();
        }
    }
}
private async void MediaCapture_Failed(MediaCapture sender, MediaCaptureFailedEventArgs errorEventArgs)
{
    await Dispatcher.RunAsync(Windows.UI.Core.CoreDispatcherPriority.Normal, () => (new MessageDialog("Media capture failed")).ShowAsync());
}
private async void MediaCapture_RecordLimitationExceeded(MediaCapture sender)
{
    await _mediaCapture.StopRecordAsync();
    await Dispatcher.RunAsync(Windows.UI.Core.CoreDispatcherPriority.Normal, () => (new MessageDialog("Record limitation exceeded")).ShowAsync());
}

最后在这里如何正确地拍摄，每一件事都是异步的，所以没有任何延迟或者无论如何

 private async void TakePhoto_Click(object sender, RoutedEventArgs e)
    {
        if (_mediaCapture != null)
        {
            try
            {
                ImageEncodingProperties encodingProperties = ImageEncodingProperties.CreateJpeg();
                WriteableBitmap bitmap = new WriteableBitmap((int)ImageElement.Width, (int)ImageElement.Height);
                using (var imageStream = new InMemoryRandomAccessStream())
                {
                    await this._mediaCapture.CapturePhotoToStreamAsync(encodingProperties, imageStream);
                    await imageStream.FlushAsync();
                    imageStream.Seek(0);
                    bitmap.SetSource(imageStream);
                    await Dispatcher.RunAsync(Windows.UI.Core.CoreDispatcherPriority.Normal,
                                    () =>
                                    {
                                        ImageElement.Source = bitmap;
                                    });
                }
            }
            catch (Exception ex)
            {
                (new MessageDialog("Something went wrong !")).ShowAsync();
            }
        }
    }

WinRT从视频流中提取帧

2 个答案: