WinRT從視頻流中提取幀

Question

我正在使用內置攝像頭解碼條形碼，我使用capElement.Source.CapturePhotoToStreamAsync來捕獲預覽中的照片。 它有效，但凍結了應用程序一小段時間，感覺非常笨拙和馬車。

因此，我希望在后台進行此操作，同時至少在處理照片時留下響應式用戶界面。

到目前為止，我想出了這個來捕獲視頻流：

 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()); 
        }

以及從流中獲取字節的方法：

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;
        }

從ScanInBackground的評論中，您可以看到我不知道如何將流分割為照片/幀。

Answer 1

Microsoft github頁面上有一個相關的示例，盡管它們的目標是Windows 10.您可能有興趣遷移項目以獲得此功能。

GetPreviewFrame ：此示例將捕獲預覽幀而不是完整的照片。 一旦它有一個預覽框架，它就可以讀取和編輯它上面的像素。

以下是相關部分：

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;
                }
            }
        }
    }
}

並在課外宣布：

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

當然，您的項目必須允許所有這些的不安全代碼才能工作。

仔細查看示例，了解如何獲取所有詳細信息。 或者，要進行演練，您可以觀看最近//構建/會議中的攝像機會話 ，其中包括一些相機示例的演練。

Answer 2

我相信顯示媒體預覽和處理不同的可能異常是必要的，這里有一個簡單的例子，如何做到這一點，

假設您有以下UI，其中CaptureElement用於顯示預覽，而Image控件用於顯示捕獲的圖片，

 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>

在后面的代碼聲明一個mediaCapture字段，

private MediaCapture _mediaCapture;  

然后在頁面加載的事件處理程序中你需要

• 啟動媒體捕獲設備，
• 設置幾個可能的異常處理程序，

• 並啟動凸輪預覽

    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()); } 

最后在這里如何正確地拍攝，每一件事都是異步的，所以沒有任何延遲或者無論如何

 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();
            }
        }
    }