为什么ZXing使用前置摄像头而不是后置摄像头解码条形码？ 通用应用程序Win10

Question

I am creating a simple Barcode scanner using the ZXing libuary for Windows 10 Universal.

Using the front camera it will recognise a barcode and display the result at the bottom of the page.

Unfortunatly forcing the phone to use the rear camera will never successfully decode a barcode.

Has anyone got an explination as to why this is? Could it be a focusing issue?

My test device is a Nokia Lumia 650 (Front and Rear Cameras)

Below is an example of what I have: (VB.NET)

Public NotInheritable Class MainPage
Inherits Page

Private capture As New MediaCapture
Private result As Result
Private reader As IBarcodeReader

Public Sub New()

    ' This call is required by the designer.
    InitializeComponent()

    ' Add any initialization after the InitializeComponent() call.

End Sub

''' <summary>
''' On Navigation to this page handler.
''' This is the main sub which handles the init, saving and decoding of each frame viewed by the camera.
''' the decoding is then delegated to a different function
''' </summary>
''' <param name="e"></param>
Protected Overrides Async Sub onNavigatedTo(e As NavigationEventArgs)

    ' Check for camera existance
    Dim cameras = Await DeviceInformation.FindAllAsync(DeviceClass.VideoCapture)
    If (cameras.Count < 1) Then
        ErrorMsg.Text = "No camera found"
        Exit Sub
    End If

    ' Get camera settings
    Dim settings = New MediaCaptureInitializationSettings()

    If (cameras.Count = 1) Then
        settings.VideoDeviceId = cameras(1).Id ' Front Camera
    Else
        settings.VideoDeviceId = cameras(0).Id ' Back Camera
    End If

    Await capture.InitializeAsync(settings)

    ' Set the Camera Preview
    ' Rotate the camera to fix portrait issues
    capture.SetPreviewRotation(VideoRotation.Clockwise90Degrees)

    VideoCapture.Source = capture

    Await capture.StartPreviewAsync()

    ' While there has been no result, Keep checking
    While (result Is Nothing)
        ' Createa a JPEG of the current frame in the preview finder
        Dim photoStorageFile = Await KnownFolders.PicturesLibrary.CreateFileAsync("scan.jpg", CreationCollisionOption.GenerateUniqueName)
        Await capture.CapturePhotoToStorageFileAsync(ImageEncodingProperties.CreateJpeg(), photoStorageFile)

        ' Open the File as a bitmap
        Dim stream = Await photoStorageFile.OpenReadAsync()
        Dim writeableBmp = New WriteableBitmap(1, 1)
        writeableBmp.SetSource(stream)
        writeableBmp = New WriteableBitmap(writeableBmp.PixelWidth, writeableBmp.PixelHeight)
        stream.Seek(0)
        writeableBmp.SetSource(stream)

        ' Scan the frame for the existance of a barcode, If there is a barcode, then pass result the details
        result = ScanBitmap(writeableBmp)

        ' Delete the saved frame in order to save on memory VERY IMPORTANT!!
        Await photoStorageFile.DeleteAsync(StorageDeleteOption.PermanentDelete)
    End While

    ' Broken out of the while loop meaning there is a result!
    ' Stop the Prevew and show the barcode result
    Await capture.StopPreviewAsync()
    VideoCapture.Visibility = Visibility.Collapsed
    CaptureImage.Visibility = Visibility.Visible
    ScanResult.Text = result.Text

End Sub

''' <summary>
''' Scans the bitmap passed in with the ZXing lib in order to determine if the preview frame now has a barcode in view.
''' </summary>
''' <param name="writeableBmp">Bitmap that is intended to be decoded</param>
''' <returns>Result of the barcode</returns>
Private Function ScanBitmap(writeableBmp As WriteableBitmap) As Result
    ' Create a ZXing Barcode reader and init
    Dim barcodeReader = New BarcodeReader
    barcodeReader.Options.TryHarder = True
    barcodeReader.AutoRotate = True
    barcodeReader.TryInverted = True

    ' Pass the bitmap to the barcode reader to decode any possible barcodes
    Dim result = barcodeReader.Decode(writeableBmp)

    ' If result is something then Show this frame in the preview
    If (result IsNot Nothing) Then
        CaptureImage.Source = writeableBmp
    End If

    ' Return the result back to the main sub.
    Return result

End Function

End Class

Answer 1

I have now discovered it has something to do with the rear camera not having a sharp enough image

The Line below is what I added to get the rear camera to work. It forces the auto-focus. As this would not work in every situation (Devices of which cannot focus) other checks would need to be implemented.

capture.VideoDeviceController.FocusControl.FocusAsync()

Unfortunatly, The scanning is rather slow now but it is working.

Answer 2

Phones tend to have a much higher quality rear camera that supports autofocus. For QR code reading, you will have to manually focus the lens before getting a frame. Have a look at the CameraManualControls SDK sample to learn how to operate the focus modes.

For example, this is a snippet that shows you how to do Continuous AutoFocus:

private async void CafFocusRadioButton_Checked(object sender, RoutedEventArgs e)
{
    var focusControl = _mediaCapture.VideoDeviceController.FocusControl;

    await focusControl.UnlockAsync();

    var settings = new FocusSettings { Mode = FocusMode.Continuous, AutoFocusRange = AutoFocusRange.FullRange };
    focusControl.Configure(settings);
    await focusControl.FocusAsync();
}

Have a look at the full sample for more information.