Captured photos are black when using MediaCapture

问题:

I am using MediaCapture to capture photos and store them. It works in the emulator. But when running the app on a real phone (Nokia Lumia 530) the captured photos are just black. They have a correct size and the file has a certain byte length, but when displaying the photo it's black. Please note that I do not use Silverlight and am fixed on using MediaCapture. The camera on the phone works when using the default camera app. The App's manifest includes the capabilities "Pictures Library" and "Webcam".

Does someone know what could be wrong?

Here is the test code I use:

using (var mediaCapture = new MediaCapture())
{
   await mediaCapture.InitializeAsync();

   ImageEncodingProperties imageFormat = ImageEncodingProperties.CreateJpeg();
   StorageFile photoFile = await KnownFolders.PicturesLibrary.CreateFileAsync("TestPhoto.jpg", CreationCollisionOption.GenerateUniqueName);
   await mediaCapture.CapturePhotoToStorageFileAsync(imageFormat, photoFile);

   BitmapImage bitmap = new BitmapImage();
   using (var photoStream = await photoFile.OpenReadAsync())
   {
      bitmap.SetSource(photoStream);
   }
}

Edit

I found a solution. The photo is captured correctly if we have a CaptureElement, set it's source to the MediaCapture object, invoke MediaCapture.StartPreviewAsync before taking the photo, take the photo (using CapturePhotoToStorageFileAsync) and finally invoke StopPreviewAsync. It seems that MediaCapture needs an existing (and displayed) preview to be able to capture photos. Strange that this is not documented and using CapturePhotoToStorageFileAsync without a preview does not throw an Exception.

回答1:

The photo is captured correctly if we have a CaptureElement, set it's source to the MediaCapture object, invoke MediaCapture.StartPreviewAsync before taking the photo, take the photo (using CapturePhotoToStorageFileAsync) and finally invoke StopPreviewAsync. It seems that MediaCapture needs an existing (and displayed) preview to be able to capture photos. Strange that this is not documented and using CapturePhotoToStorageFileAsync without a preview does not throw an Exception.