windows phone 8.1显示PreviewFrame并计算每帧红色像素的平均值

Question

目前我正在使用Lumia。图像获取预览帧并显示。

我创建了新的方法"GetPreview()"去像素，找到红色像素，然后我想计算每帧红色像素的平均值。

我的问题是，当我通过像素有滞后的应用程序:(

• 在不损失性能的情况下计算每帧红色像素平均值的合适解决方案是什么?

• 当预览开始时如何打开闪光灯?

      private async Task startCameraPreview()
  {
      // Create a camera preview image source (from the Lumia Imaging SDK)
      _cameraPreviewImageSource = new CameraPreviewImageSource();
      // Checking id of back camera 
      DeviceInformationCollection devices = await Windows.Devices.Enumeration.DeviceInformation.FindAllAsync(Windows.Devices.Enumeration.DeviceClass.VideoCapture);
      String backCameraId = devices.FirstOrDefault(x => x.EnclosureLocation != null && x.EnclosureLocation.Panel == Windows.Devices.Enumeration.Panel.Back).Id;
      await _cameraPreviewImageSource.InitializeAsync(backCameraId); // use the back camera
      var previewProperties = await _cameraPreviewImageSource.StartPreviewAsync();
      fps = previewProperties.FrameRate.Numerator/previewProperties.FrameRate.Denominator;
      _cameraPreviewImageSource.PreviewFrameAvailable += drawPreview; // call the drawPreview method every time a new frame is available
      // Create a preview bitmap with the correct aspect ratio using the properties object returned when the preview started.
      var width = 640.0;
      var height = (width / previewProperties.Width) * previewProperties.Height;
      var bitmap = new WriteableBitmap((int)width, (int)height);
      _writeableBitmap = bitmap;
      // Create a BitmapRenderer to turn the preview Image Source into a bitmap we hold in the PreviewBitmap object
      _effect = new FilterEffect(_cameraPreviewImageSource);
      _effect.Filters = new IFilter[0]; // null filter for now
      _writeableBitmapRenderer = new WriteableBitmapRenderer(_effect, _writeableBitmap);
  }
  
  private async void drawPreview(IImageSize args)
  {
      // Prevent multiple rendering attempts at once
      if (_isRendering == false)
      {
          _isRendering = true;
          await _writeableBitmapRenderer.RenderAsync(); // Render the image (with no filter)
          // Draw the image onto the previewImage XAML element
          await Windows.ApplicationModel.Core.CoreApplication.MainView.CoreWindow.Dispatcher.RunAsync(CoreDispatcherPriority.High,
              () =>
              {
                  getPreview();
                  previewImage.Source = _writeableBitmap; // previewImage is an image element in MainPage.xaml
                  _writeableBitmap.Invalidate(); // force the PreviewBitmap to redraw
              });
  
          _isRendering = false;
      }
  }
  private void getPreview()
  {
      var pixelBuffer = _writeableBitmap.PixelBuffer;
      for (uint i = 0; i + 4 < pixelBuffer.Length; i += 4)
      {
          var red = pixelBuffer.GetByte(i + 2);
      }
  }
  

Answer 1

在Lumia Imaging SDK处理图像之后，但在您使位图无效之前，您可以:

• 立即使可写位图无效，然后在单独的异步任务中执行分析步骤。这意味着内容将立即显示，而您的分析将单独完成。基于示例的一些伪代码将是:

    await Windows.ApplicationModel.Core.CoreApplication.MainView.CoreWindow.Dispatcher.RunAsync(CoreDispatcherPriority.High, () => {                
    var analysisTask = Task.Run(() => getPreview());
    previewImage.Source = _writeableBitmap; // previewImage is an image element in MainPage.xaml
    _writeableBitmap.Invalidate(); // force the PreviewBitmap to redraw
    await analysisTask;
    });

这样分析图像的任务就不会阻碍屏幕上的更新。当然，如果您需要呈现链本身的分析结果，则此选项可能不可行。

• 为分析创建自定义过滤器，这样您就可以利用优化的Lumia Imaging SDK处理。

  要开始编写自定义过滤器，请参阅文档。