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EnableDoublePass Property

Gets or sets a value indicating whether to enable the double pass setting when reading QR symbols.
public bool EnableDoublePass { get; set; } 
@property (nonatomic, assign) BOOL enableDoublePass; 
public boolean getEnableDoublePass() 
public void setEnableDoublePass(boolean value) 
property bool EnableDoublePass { 
   bool get(); 
   void set (    bool ); 
EnableDoublePass # get and set (QRBarcodeReadOptions) 

Property Value

true to use double pass when reading QR symbols. Otherwise, false. The default value is false.


When reading 2D barcodes, LEADTOOLS can perform internal auto preprocessing on the image and try again if the engine cannot detect the barcode required. This may take extra processing and increase the barcode detection time. Therefore, it is best to set EnableDoublePass to true only in certain situations, as follows:

  • The application is reading barcodes of this type exclusively (for example, a dedicated QR reader is being used). In this case, reading the barcodes is always more important than recognition speed.

  • The application uses barcode detection on a server. In this case, the time spent on the actual barcode detection is most probably less than the time required to send the image to the server and get the result back due to network latency. Thus it makes sense to enable double pass in this situation.

  • The images provided have a small resolution (DPI) or are known to come from a source that does not always provide clean images (for example, a Fax machine or a Scanner). Enabling double pass here ensures that the barcodes can be read regardless of the quality of the input image.

  • The LEADTOOLS .NET C# Barcode demos leave double pass disabled by default. If the engine tries to read a QR barcode but cannot find it, it will then ask the user if DoublePass should be enabled and reading be retried. An approach like that (or an automated one: Read. If failure, enable and read again) can also be used.

using Leadtools; 
using Leadtools.Codecs; 
using Leadtools.Barcode; 
using Leadtools.ImageProcessing; 
public void QRBarcode_Example() 
   string imageFileName = Path.Combine(LEAD_VARS.ImagesDir, "MyQRBarcodes.tif"); 
   BarcodeEngine engine = new BarcodeEngine(); 
   // Create the image to write the barcodes to 
   int resolution = 300; 
   using (RasterImage image = RasterImage.Create((int)(8.5 * resolution), (int)(11.0 * resolution), 1, resolution, RasterColor.FromKnownColor(RasterKnownColor.White))) 
      // Write two QR barcodes 
      WriteBarcode(engine.Writer, image, QRBarcodeSymbolModel.Model1AutoSize, "QR Data 1", false); 
      WriteBarcode(engine.Writer, image, QRBarcodeSymbolModel.Model2Version30, "QR Data 2", true); 
      // Save the image 
      using (RasterCodecs codecs = new RasterCodecs()) 
         codecs.Save(image, imageFileName, RasterImageFormat.CcittGroup4, 1); 
   // Now read the barcodes again 
   using (RasterCodecs codecs = new RasterCodecs()) 
      using (RasterImage image = codecs.Load(imageFileName)) 
         // Read all the QR barcodes 
         ReadBarcodes(engine.Reader, image); 
private void WriteBarcode(BarcodeWriter writer, RasterImage image, QRBarcodeSymbolModel symbolModel, string value, bool topLeft) 
   // Create the barcode data 
   QRBarcodeData barcode = BarcodeData.CreateDefaultBarcodeData(BarcodeSymbology.QR) as QRBarcodeData; 
   barcode.SymbolModel = symbolModel; 
   barcode.Value = value; 
   barcode.Symbology = BarcodeSymbology.QR; 
   // We will use the alignment to position the barcodes, so use all of the image 
   barcode.Bounds = new LeadRect(0, 0, image.ImageWidth, image.ImageHeight); 
   // Set the write options 
   QRBarcodeWriteOptions options = new QRBarcodeWriteOptions(); 
   if (topLeft) 
      options.HorizontalAlignment = BarcodeAlignment.Near; 
      options.VerticalAlignment = BarcodeAlignment.Near; 
      options.HorizontalAlignment = BarcodeAlignment.Far; 
      options.VerticalAlignment = BarcodeAlignment.Far; 
   options.GroupNumber = 0; 
   options.GroupTotal = 0; 
   options.XModule = 30; 
   options.ECCLevel = QRBarcodeECCLevel.LevelL; 
   // Write it 
   Console.WriteLine("Writing barcode with symbol model: {0} and Data: {1}", symbolModel, value); 
   writer.WriteBarcode(image, barcode, options); 
private void ReadBarcodes(BarcodeReader reader, RasterImage image) 
   // Setup read options 
   QRBarcodeReadOptions options = new QRBarcodeReadOptions(); 
   options.EnableDoublePass = false; 
   options.EnableDoublePassIfSuccess = true; 
   options.EnablePreprocessing = true; 
   Console.WriteLine("Reading QR barcodes"); 
   BarcodeData[] barcodes = reader.ReadBarcodes(image, LeadRect.Empty, 0, new BarcodeSymbology[] { BarcodeSymbology.QR }, new BarcodeReadOptions[] { options }); 
   Console.WriteLine("{0} barcodes found:", barcodes.Length); 
   foreach (QRBarcodeData barcode in barcodes) 
      string value = BarcodeData.ParseECIData(barcode.GetData()); 
      if (string.IsNullOrEmpty(value)) 
         value = barcode.Value; 
      Console.WriteLine(" At {0}, symbol model: {1} data: {2} and symbology: {3}", barcode.Bounds, barcode.SymbolModel, value, barcode.Symbology); 
static class LEAD_VARS 
   public const string ImagesDir = @"C:\LEADTOOLS22\Resources\Images"; 

Target Platforms

Help Version 22.0.2023.5.9
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Leadtools.Barcode Assembly

Products | Support | Contact Us | Intellectual Property Notices
© 1991-2023 LEAD Technologies, Inc. All Rights Reserved.