OCR Processor Features

27 Sep 202324 minutes to read

Performing OCR for an entire document

To perform OCR for an entire PDF document using PerformOCR method of the OCRProcessor class. Refer to the following code example.

C# [Cross-platform]

//Initialize the OCR processor.
using (OCRProcessor processor = new OCRProcessor())
{
    //Load an existing PDF document.
    FileStream stream = new FileStream("Input.pdf", FileMode.Open);
    PdfLoadedDocument document = new PdfLoadedDocument(stream);
    //Set OCR language.
    processor.Settings.Language = Languages.English;
    //Perform OCR with input document and tessdata (Language packs).
    processor.PerformOCR(document);
    //Create file stream.
    using (FileStream outputFileStream = new FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite))
    {
        //Save the PDF document to file stream.
        document.Save(outputFileStream);
    }
    //Close the document.
    document.Close(true);
}

VB.NET [Windows-specific]

'Initialize the OCR processor.
Using processor As OCRProcessor = New OCRProcessor()
    'Load an existing PDF document. 
    Dim stream As FileStream = New FileStream("Input.pdf", FileMode.Open)
    Dim document As PdfLoadedDocument = New PdfLoadedDocument(stream)
    'Set OCR language. 
    processor.Settings.Language = Languages.English
    'Perform OCR with input document and tessdata (Language packs). 
    processor.PerformOCR(document)
    'Create file stream.
    Using outputFileStream As FileStream = New FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite)
        'Save the PDF document to file stream.
        document.Save(outputFileStream)
    End Using
    'Close the document
    document.Close(True)
End Using

NOTE

The PerformOCR method returns only the text OCRed by OCRProcessor. Other existing text in the PDF page will not be returned in this method. Please check text extraction feature for this.

You can downloaded a complete working sample from GitHub.

Performing OCR for a region of the document

To perform OCR on a particular region or several regions of a PDF page with the help of PageRegion class in OCRSettings, refer to the following code sample.

C# [Cross-platform]

//Initialize the OCR processor.
using (OCRProcessor processor = new OCRProcessor())
{
    //Load an existing PDF document.
    FileStream stream = new FileStream("Input.pdf", FileMode.Open);
    PdfLoadedDocument document = new PdfLoadedDocument(stream);

    //Set OCR language.
    processor.Settings.Language = Languages.English;
    //Assign rectangles to the page.
    RectangleF rect = new RectangleF(0, 100, 950, 150);
    List<PageRegion> pageRegions = new List<PageRegion>();
    //Create page region.
    PageRegion region = new PageRegion();
    //Set page index.
    region.PageIndex = 0;
    //Set page region.
    region.PageRegions = new RectangleF[] { rect };
    //Add region to page region.
    pageRegions.Add(region);
    //Set page regions.
    processor.Settings.Regions = pageRegions;

    //Perform OCR with input document and tessdata (Language packs).
    processor.PerformOCR(document);

    //Create file stream.
    using (FileStream outputFileStream = new FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite))
    {
        //Save the PDF document to file stream.
        document.Save(outputFileStream);
    }
    //Close the document.
    document.Close(true);
}

VB.NET [Windows-specific]

'Initialize the OCR processor. 
Using processor As OCRProcessor = New OCRProcessor()
    'Load an existing PDF document. 
    Dim stream As FileStream = New FileStream("Input.pdf", FileMode.Open)
    Dim document As PdfLoadedDocument = New PdfLoadedDocument(stream)

    'Set OCR language. 
    processor.Settings.Language = Languages.English
    'Assign rectangles to the page.
    Dim rect As RectangleF = New RectangleF(0, 100, 950, 150)
    Dim pageRegions As List(Of PageRegion) = New List(Of PageRegion)()
    'Create page region. 
    Dim region As PageRegion = New PageRegion()
    'Set page index.
    region.PageIndex = 0
    'Set page region.
    region.PageRegions = New RectangleF() {rect}
    'Add region to page region.
    pageRegions.Add(region)
    'Set page regions.
    processor.Settings.Regions = pageRegions

    'Perform OCR with input document and tessdata (Language packs).
    processor.PerformOCR(document)

    'Create file stream.
    Using outputFileStream As FileStream = New FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite)
        'Save the PDF document to file stream.
        document.Save(outputFileStream)
    End Using
    'Close the document. 
    document.Close(True)
End Using

You can downloaded a complete working sample from GitHub.

Performing OCR with tesseract version 3.05

The TesseractVersion property is used to switch the tesseract version between 3.02 and 3.05. By default, OCR works with tesseract version 3.02.

NOTE

The starting supported version of tesseract in ASP.NET Core is 4.0. So the lower tesseract versions 3.02 and 3.05 are not supported and we don’t have the property called TesseractVersion in ASP.NET Core platform.

The following code sample demonstrates the OCR processor with Tesseract version 3.05 for PDF documents.

C# [Cross-platform]

//Initialize the OCR processor.
using (OCRProcessor processor = new OCRProcessor())
{
    //Load an existing PDF document.
    FileStream stream = new FileStream("Input.pdf", FileMode.Open);
    PdfLoadedDocument document = new PdfLoadedDocument(stream);

    //Set OCR language.
    processor.Settings.Language = Languages.English;
    //Set tesseract OCR Engine.
    processor.Settings.TesseractVersion = TesseractVersion.Version3_05;
    //Perform OCR with input document, tessdata (Language packs) and enabling isMemoryOptimized property.
    processor.PerformOCR(document);

    //Create file stream.
    using (FileStream outputFileStream = new FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite))
    {
        //Save the PDF document to file stream.
        document.Save(outputFileStream);
    }
    //Close the document.
    document.Close(true);
}

VB.NET [Windows-specific]

'Initialize the OCR processor with tesseract binaries folder path. 
Using processor As OCRProcessor = New OCRProcessor("TesseractBinaries/3.05/")
    'Load an existing PDF document. 
    Dim stream As FileStream = New FileStream("Input.pdf", FileMode.Open)
    Dim document As PdfLoadedDocument = New PdfLoadedDocument(stream)

    'Set OCR language. 
    processor.Settings.Language = Languages.English
    'Set tesseract OCR Engine. 
    processor.Settings.TesseractVersion = TesseractVersion.Version3_05
    'Perform OCR with input document, tessdata (Language packs) and enabling isMemoryOptimized property.
    processor.PerformOCR(document)

    'Create file stream. 
    Using outputFileStream As FileStream = New FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite)
        'Save the PDF document to file stream.
        document.Save(outputFileStream)
    End Using
    'Close the document. 
    document.Close(True)
End Using

Performing OCR with Tesseract Version 4.0

The TesseractVersion property is used to switch the tesseract version to 4.0. By default, OCR will be performed with tesseract version 3.02.

NOTE

In ASP.NET Core platform, the default and starting supported version of tesseract is 4.0. So we did not have the property called TesseractVersion in ASP.NET Core platform.

The following code sample explains the OCR processor with Tesseract version 4.0 for PDF documents.

C# [Cross-platform]

//Initialize the OCR processor.
using (OCRProcessor processor = new OCRProcessor())
{
    //Load an existing PDF document.
    FileStream stream = new FileStream("Input.pdf", FileMode.Open);
    PdfLoadedDocument document = new PdfLoadedDocument(stream);

    //Set OCR language.
    processor.Settings.Language = Languages.English;
    //Set tesseract OCR Engine.
    processor.Settings.TesseractVersion = TesseractVersion.Version4_0;
    //Perform OCR with input document, tessdata (Language packs) and enabling isMemoryOptimized property.
    processor.PerformOCR(document);

    //Create file stream.
    using (FileStream outputFileStream = new FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite))
    {
        //Save the PDF document to file stream.
        document.Save(outputFileStream);
    }
    //Close the document.
    document.Close(true);
}

VB.NET [Windows-specific]

'Initialize the OCR processor with tesseract binaries folder path. 
Using processor As OCRProcessor = New OCRProcessor("TesseractBinaries/4.0/")
    'Load an existing PDF document. 
    Dim stream As FileStream = New FileStream("Input.pdf", FileMode.Open)
    Dim document As PdfLoadedDocument = New PdfLoadedDocument(stream)

    'Set OCR language. 
    processor.Settings.Language = Languages.English
    'Set tesseract OCR Engine. 
    processor.Settings.TesseractVersion = TesseractVersion.Version4_0
    'Perform OCR with input document, tessdata (Language packs) and enabling isMemoryOptimized property.
    processor.PerformOCR(document)

    'Create file stream. 
    Using outputFileStream As FileStream = New FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite)
        'Save the PDF document to file stream.
        document.Save(outputFileStream)
    End Using
    'Close the document. 
    document.Close(True)
End Using

Performing OCR on image

The below code example illustrates how to perform OCR on image file using PerformOCR method in OCRProcessor class.

C# [Cross-platform]

//Initialize the OCR processor.
using (OCRProcessor processor = new OCRProcessor())
{
    //Load the input image. 
    FileStream imageStream = new FileStream("Input.jpg", FileMode.Open);
    //Set OCR language.
    processor.Settings.Language = Languages.English;
    //Perform OCR with input document, tessdata (Language packs) and enabling isMemoryOptimized property.
    String OCRText = processor.PerformOCR(imageStream);
}

VB.NET [Windows-specific]

'Initialize the OCR processor. 
Using processor As OCRProcessor = New OCRProcessor()
    'Load the input image. 
    Dim imageStream As FileStream = New FileStream("Input.jpg", FileMode.Open)
    'Set OCR language.
    processor.Settings.Language = Languages.English
    'Perform OCR with input document, tessdata (Language packs) and enabling isMemoryOptimized property.
    Dim ocrText As String = processor.PerformOCR(imageStream)
End Using

You can downloaded a complete working sample from GitHub.

You can get the OCRed Unicode text from an image file by using the UnicodeFont property in OCRProcessor. For more information, refer to the following code sample.

ASP.NET Core

//Initialize the OCR processor.
using (OCRProcessor processor = new OCRProcessor())
{
    //Load the input image. 
    FileStream imageStream = new FileStream("Input.jpg", FileMode.Open);

    //Set OCR language.
    processor.Settings.Language = Languages.English;

    //Get stream from the font file. 
    FileStream fontStream = new FileStream(@"ARIALUNI.ttf", FileMode.Open);
    //Sets Unicode font to preserve the Unicode characters in a PDF document.
    processor.UnicodeFont = new PdfTrueTypeFont(fontStream, 8);

    //Perform OCR with input document, tessdata (Language packs) and enabling isMemoryOptimized property.
    String OCRText = processor.PerformOCR(imageStream);
}

C# [Cross-platform]

//By default, Unicode characters can be extracted from image file in .NET Framework applications like WF, WPF, ASP.NET and ASP.NET MVC.

VB.NET [Windows-specific]

//By default, Unicode characters can be extracted from image file in .NET Framework applications like WF, WPF, ASP.NET and ASP.NET MVC.

Performing OCR for large PDF documents

To optimize memory to performing OCR on large PDF documents, enable the isMemoryOptimized property in PerformOCR method of OCRProcessor class. Optimization will be effective only with Multithreading environment or PDF document with more images. For more details, refer to the following code examples.

NOTE

Memory optimization for performing OCR on large PDF documents is not supported in ASP.NET Core platform.

C# [Cross-platform]

//Initialize the OCR processor.
using (OCRProcessor processor = new OCRProcessor())
{
    //Load an existing PDF document.
    FileStream stream = new FileStream("Input.pdf", FileMode.Open);
    PdfLoadedDocument document = new PdfLoadedDocument(stream);
    //Set OCR language.
    processor.Settings.Language = Languages.English;
    //Perform OCR with input document, tessdata (Language packs) and enable isMemoryOptimized property.
    processor.PerformOCR(document);

    //Create file stream.
    using (FileStream outputFileStream = new FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite))
    {
        //Save the PDF document to file stream.
        document.Save(outputFileStream);
    }
    //Close the document.
    document.Close(true);
}

VB.NET [Windows-specific]

'Initialize the OCR processor. 
Using processor As OCRProcessor = New OCRProcessor()

    'Load an existing PDF document. 
    Dim stream As FileStream = New FileStream("Input.pdf", FileMode.Open)
    Dim document As PdfLoadedDocument = New PdfLoadedDocument(stream)
    'Set OCR language. 
    processor.Settings.Language = Languages.English
    'Perform OCR with input document, tessdata (Language packs) and enable isMemoryOptimized property. 
    processor.PerformOCR(document)

    'Create file stream. 
    Using outputFileStream As FileStream = New FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite)
        'Save the PDF document to file stream.
        document.Save(outputFileStream)
    End Using

    'Close the document.  
    document.Close(True)
End Using

Performing OCR on rotated page of PDF document

You can get the OCRed text from the rotated page of PDF document using the PageSegment property by specifying AutoOsd through PageSegmentMode Enum. For more details, refer to the following code sample.

C# [Cross-platform]

//Initialize the OCR processor.
using (OCRProcessor processor = new OCRProcessor())
{
    //Load an existing PDF document.
    FileStream stream = new FileStream("Input.pdf", FileMode.Open);
    PdfLoadedDocument document = new PdfLoadedDocument(stream);
    //Set OCR language.
    processor.Settings.Language = Languages.English;
    //Set OCR page auto detection rotation.
    processor.Settings.PageSegment = PageSegMode.AutoOsd;
    //Perform OCR with input document and tessdata (Language packs).
    processor.PerformOCR(document);

    //Create file stream.
    using (FileStream outputFileStream = new FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite))
    {
        //Save the PDF document to file stream.
        document.Save(outputFileStream);
    }
    //Close the document.
    document.Close(true);
}

VB.NET [Windows-specific]

'Initialize the OCR processor. 
Using processor As OCRProcessor = New OCRProcessor()

    'Load an existing PDF document. 
    Dim stream As FileStream = New FileStream("Input.pdf", FileMode.Open)
    Dim document As PdfLoadedDocument = New PdfLoadedDocument(stream)
    'Set OCR language. 
    processor.Settings.Language = Languages.English
    'Set OCR page auto detection rotation. 
    processor.Settings.PageSegment = PageSegMode.AutoOsd
    'Perform OCR with input document and tessdata (Language packs). 
    processor.PerformOCR(document)

    'Create file stream. 
    Using outputFileStream As FileStream = New FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite)
        'Save the PDF document to file stream. 
        document.Save(outputFileStream)
    End Using

    'Close the document.
    document.Close(True)
End Using

You can downloaded a complete working sample from GitHub.

Layout result from OCR

You can get the OCRed text and its bounds from a scanned PDF document by using the OCRLayoutResult Class. Refer to the following code sample.

C# [Cross-platform]

//Initialize the OCR processor.
using (OCRProcessor processor = new OCRProcessor())
{
    //Load an existing PDF document.
    FileStream stream = new FileStream("Input.pdf", FileMode.Open);
    PdfLoadedDocument document = new PdfLoadedDocument(stream);

    //Set OCR language.
    processor.Settings.Language = Languages.English;
    //Create the layout result. 
    OCRLayoutResult layoutResult = new OCRLayoutResult();
    //Perform OCR with input document and tessdata (Language packs).
    processor.PerformOCR(document, @"Tessdata/", out layoutResult);
    //Get OCRed line collection from first page.
    OCRLineCollection lines = layoutResult.Pages[0].Lines;
    //Get each OCR'ed line and its bounds.
    foreach (Line line in lines)
    {
        string text = line.Text;
        RectangleF bounds = line.Rectangle;
    }

    //Create file stream.
    using (FileStream outputFileStream = new FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite))
    {
        //Save the PDF document to file stream.
        document.Save(outputFileStream);
    }
    //Close the document.
    document.Close(true);
}

VB.NET [Windows-specific]

'Initialize the OCR processor. 
Using processor As OCRProcessor = New OCRProcessor()

    'Load an existing PDF document. 
    Dim stream As FileStream = New FileStream("Input.pdf", FileMode.Open)
    Dim document As PdfLoadedDocument = New PdfLoadedDocument(stream)
    'Set OCR language.
    processor.Settings.Language = Languages.English
    'Create the layout result. 
    Dim layoutResult As OCRLayoutResult = New OCRLayoutResult()
    'Perform OCR with input document and tessdata (Language packs).
    processor.PerformOCR(document, "Tessdata/", layoutResult)
    'Get OCR'ed line collection from first page. 
    Dim lines As OCRLineCollection = layoutResult.Pages(0).Lines
    'Get each OCR'ed line and its bounds. 
    For Each line As Line In lines
        Dim text As String = line.Text
        Dim bounds As RectangleF = line.Rectangle
    Next

    'Create file stream. 
    Using outputFileStream As FileStream = New FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite)
        'Save the PDF document to file stream.
        document.Save(outputFileStream)
    End Using
    'Close the document.
    document.Close(True)
End Using

You can downloaded a complete working sample from GitHub.

Native call

Enabling native calls will not launch any temporary process for OCR processing; instead, it will invoke the native calls.

NOTE

The starting supported version of tesseract in ASP.NET Core is 4.0. So, the lower tesseract versions 3.02 and 3.05 are not supported and we don’t have the property called TesseractVersion and EnableNativeCall in ASP.NET Core platform.

Tesseract 3.02

Tesseract 3.02 supports only 32-bit version. By default, OCR works with this tesseract version 3.02.

NOTE

Enabling native calls will not work in 64-bit tesseract 3.02 version. Instead, a temporary process will be launched for OCR processing.

The following code sample demonstrates the OCR processor with native call support of tesseract 3.02 by setting TesseractVersion as 3.02.

C# [Cross-platform]

//Initialize the OCR processor.
using (OCRProcessor processor = new OCRProcessor())
{
    //Load an existing PDF document.
    FileStream stream = new FileStream("Input.pdf", FileMode.Open);
    PdfLoadedDocument document = new PdfLoadedDocument(stream);
    //Set OCR language.
    processor.Settings.Language = Languages.English;
    //Set tesseract OCR Engine.
    processor.Settings.TesseractVersion = TesseractVersion.Version3_02;
    //Perform OCR with input document, tessdata (Language packs) and enabling isMemoryOptimized property.
    processor.PerformOCR(document);

    //Create file stream.
    using (FileStream outputFileStream = new FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite))
    {
        //Save the PDF document to file stream.
        document.Save(outputFileStream);
    }
    //Close the document.
    document.Close(true);
}

VB.NET [Windows-specific]

'Initialize the OCR processor. 
Using processor As OCRProcessor = New OCRProcessor()

    'Load an existing PDF document. 
    Dim stream As FileStream = New FileStream("Input.pdf", FileMode.Open)
    Dim document As PdfLoadedDocument = New PdfLoadedDocument(stream)
    'Set OCR language. 
    processor.Settings.Language = Languages.English
    'Set tesseract OCR Engine. 
    processor.Settings.TesseractVersion = TesseractVersion.Version3_02
    'Perform OCR with input document, tessdata (Language packs) and enabling isMemoryOptimized property.
    processor.PerformOCR(document)

    'Create file stream. 
    Using outputFileStream As FileStream = New FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite)
        'Save the PDF document to file stream.
        document.Save(outputFileStream)
    End Using
    'Close the document. 
    document.Close(True)
End Using

Tesseract 3.05

Tesseract 3.05 supports the native call for both x86 and x64 architectures. By default, the x86 tesseract binaries are available with Syncfusion NuGet package or the tesseract installer.

You can download the x64 supporting tesseract binaries from the following link.
Tesseract 64-bit binaries

NOTE

This 64-bit binaries are required only when the native call property is enabled.
Make sure to provide the 64-bit binaries path while using the 64-bit environment.

The following code sample demonstrates the OCR processor with native call support of tesseract 3.05 by setting TesseractVersion as 3.05.

C# [Cross-platform]

//Initialize the OCR processor.
using (OCRProcessor processor = new OCRProcessor())
{
    //Load an existing PDF document.
    FileStream stream = new FileStream("Input.pdf", FileMode.Open);
    PdfLoadedDocument document = new PdfLoadedDocument(stream);
    //Set OCR language.
    processor.Settings.Language = Languages.English;
    //Set tesseract OCR Engine.
    processor.Settings.TesseractVersion = TesseractVersion.Version3_05;
    //Set enable native call.
    processor.Settings.EnableNativeCall = true;
    //Perform OCR with input document, tessdata (Language packs) and enabling isMemoryOptimized property.
    processor.PerformOCR(document);

    //Create file stream.
    using (FileStream outputFileStream = new FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite))
    {
        //Save the PDF document to file stream.
        document.Save(outputFileStream);
    }
    //Close the document.
    document.Close(true);
}

VB.NET [Windows-specific]

'Initialize the OCR processor with tesseract binaries folder path. 
Using processor As OCRProcessor = New OCRProcessor()

    'Load an existing PDF document. 
    Dim stream As FileStream = New FileStream("Input.pdf", FileMode.Open)
    Dim document As PdfLoadedDocument = New PdfLoadedDocument(stream)
    'Set OCR language. 
    processor.Settings.Language = Languages.English
    'Set tesseract OCR Engine. 
    processor.Settings.TesseractVersion = TesseractVersion.Version3_05
    'Set enable native call
    processor.Settings.EnableNativeCall = True
    'Perform OCR with input document, tessdata (Language packs) and enabling isMemoryOptimized property.
    processor.PerformOCR(document)

    'Create file stream. 
    Using outputFileStream As FileStream = New FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite)
        'Save the PDF document to file stream.
        document.Save(outputFileStream)
    End Using
    'Close the document. 
    document.Close(True)
End Using

Advantages of Native Call over Normal API

Enabling this property will process OCR with native calls (PInvoke) instead of surrogate process.
For surrogate process, it requires permission for creating and executing a process, whereas the native calls (PInvoke) does not required. And also, performance will be better in PInvoke instead of surrogate process.

Customizing temp folder

While performing OCR on an existing scanned PDF document, the OCR Processor will create temporary files (.temp, .tiff, .txt) and the files are deleted after the process is completed. You can change this temporary files folder location using the TempFolder property available in the OCRSettings Instance. Refer to the following code sample to change the path of temp folder when performing the OCR.

C# [Cross-platform]

//Initialize the OCR processor.
using (OCRProcessor processor = new OCRProcessor())
{
    //Load an existing PDF document.
    FileStream stream = new FileStream("Input.pdf", FileMode.Open);
    PdfLoadedDocument document = new PdfLoadedDocument(stream);
    //Set OCR language.
    processor.Settings.Language = Languages.English;
    //Set custom temp file path location.
    processor.Settings.TempFolder = "D:/Temp/";
    //Perform OCR with input document and tessdata (Language packs).
    processor.PerformOCR(document);
    //Create file stream.
    using (FileStream outputFileStream = new FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite))
    {
        //Save the PDF document to file stream.
        document.Save(outputFileStream);
    }
    //Close the document.
    document.Close(true);
}

VB.NET [Windows-specific]

'Initialize the OCR processor. 
Using processor As OCRProcessor = New OCRProcessor()

    'Load an existing PDF document. 
    Dim stream As FileStream = New FileStream("Input.pdf", FileMode.Open)
    Dim document As PdfLoadedDocument = New PdfLoadedDocument(stream)
    'Set OCR language. 
    processor.Settings.Language = Languages.English
    'Set custom temp file path location.
    processor.Settings.TempFolder = "D:/Temp/"
    'Perform OCR with input document and tessdata (Language packs).
    processor.PerformOCR(document)

    'Create file stream. 
    Using outputFileStream As FileStream = New FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite)
        'Save the PDF document to file stream.
        document.Save(outputFileStream)
    End Using
    'Close the document. 
    document.Close(True)
End Using

You can downloaded a complete working sample from GitHub.

Performing OCR with different Page Segmentation Mode

The PageSegment property is used to set the page segmentation mode. By default, OCR works with the “Auto” page segmentation mode. Kindly refer to the following code example to perform OCR with different page segmentation mode.

NOTE

The page segmentation mode is supported only in the Tesseract version 4.0 and above.

C# [Cross-platform]

//Initialize the OCR processor.
using (OCRProcessor processor = new OCRProcessor())
{
    //Load an existing PDF document.
    FileStream stream = new FileStream("Input.pdf", FileMode.Open);
    PdfLoadedDocument document = new PdfLoadedDocument(stream);
    //Set OCR language.
    processor.Settings.Language = Languages.English;
    //** For .NET Framework only **.
    //processor.Settings.TesseractVersion = TesseractVersion.Version4_0;
    //Set OCR Page segment mode to process.
    processor.Settings.PageSegment = PageSegMode.AutoOsd;
    //Perform OCR with input document and tessdata (Language packs).
    processor.PerformOCR(document);

    //Create file stream.
    using (FileStream outputFileStream = new FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite))
    {
        //Save the PDF document to file stream.
        document.Save(outputFileStream);
    }
    //Close the document.
    document.Close(true);
}

VB.NET [Windows-specific]

'Initialize the OCR processor. 
Using processor As OCRProcessor = New OCRProcessor()

    'Load an existing PDF document. 
    Dim stream As FileStream = New FileStream("Input.pdf", FileMode.Open)
    Dim document As PdfLoadedDocument = New PdfLoadedDocument(stream)
    'Set OCR language. 
    processor.Settings.Language = Languages.English
    ' ** For .NET Framework only **.
    'processor.Settings.TesseractVersion = TesseractVersion.Version4_0
    'Set OCR Page segment mode to process.
    processor.Settings.PageSegment = PageSegMode.AutoOsd
    'Perform OCR with input document and tessdata (Language packs).
    processor.PerformOCR(document)

    'Create file stream. 
    Using outputFileStream As FileStream = New FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite)
        'Save the PDF document to file stream.
        document.Save(outputFileStream)
    End Using
    'Close the document. 
    document.Close(True)
End Using

Performing OCR with different OCR Engine Mode

The OCREngineMode property is used to set the OCR Engine modes. By default, OCR works with OCR Engine mode “Default”. Refer to the following code example to perform OCR with different OCR engine mode.

C# [Cross-platform]

//Initialize the OCR processor.
using (OCRProcessor processor = new OCRProcessor())
{
    //Load an existing PDF document.
    FileStream stream = new FileStream("Input.pdf", FileMode.Open);
    PdfLoadedDocument document = new PdfLoadedDocument(stream);
    //Set OCR language.
    processor.Settings.Language = Languages.English;
    //Set tesseract version. ** For .NET Framework only. **
    //processor.Settings.TesseractVersion = TesseractVersion.Version4_0;
    //Set OCR engine mode to process.
    processor.Settings.OCREngineMode = OCREngineMode.LSTMOnly;
    //Perform OCR with input document and tessdata (Language packs).
    processor.PerformOCR(document);

    //Create file stream.
    using (FileStream outputFileStream = new FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite))
    {
        //Save the PDF document to file stream.
        document.Save(outputFileStream);
    }
    //Close the document.
    document.Close(true);
}

VB.NET [Windows-specific]

'Initialize the OCR processor. 
Using processor As OCRProcessor = New OCRProcessor()

    'Load an existing PDF document. 
    Dim stream As FileStream = New FileStream("Input.pdf", FileMode.Open)
    Dim document As PdfLoadedDocument = New PdfLoadedDocument(stream)
    'Set OCR language. 
    processor.Settings.Language = Languages.English
    'Set tesseract version. ** For .NET Framework only. **
    'processor.Settings.TesseractVersion = TesseractVersion.Version4_0
    'Set OCR engine mode to process.
    processor.Settings.OCREngineMode = OCREngineMode.LSTMOnly
    'Perform OCR with input document and tessdata (Language packs).
    processor.PerformOCR(document)

    'Create file stream. 
    Using outputFileStream As FileStream = New FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite)
        'Save the PDF document to file stream.
        document.Save(outputFileStream)
    End Using
    'Close the document. 
    document.Close(True)
End Using

NOTE

The OCR Engine Mode is supported only in the Tesseract version 4.0 and above.

White List

The WhiteList property specifies a list of characters that the OCR engine is only allowed to recognize. If a character is not on the white list, it will not be included in the output OCR results. For more information, refer to the following code sample.

C# [Cross-platform]

//Initialize the OCR processor.
using (OCRProcessor processor = new OCRProcessor())
{
    //Load an existing PDF document.
    FileStream stream = new FileStream("Input.pdf", FileMode.Open);
    PdfLoadedDocument document = new PdfLoadedDocument(stream);
    //Set OCR language.
    processor.Settings.Language = Languages.English;
    //Set tesseract version. ** For .NET Framework only. **
    //processor.Settings.TesseractVersion = TesseractVersion.Version4_0;
    //Set OCR engine mode to process.
    processor.Settings.OCREngineMode = OCREngineMode.LSTMOnly;
    //Set WhiteList Property.
    processor.Settings.WhiteList = "PDF";
    //Perform OCR with input document and tessdata (Language packs).
    processor.PerformOCR(document);

    //Create file stream.
    using (FileStream outputFileStream = new FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite))
    {
        //Save the PDF document to file stream.
        document.Save(outputFileStream);
    }
    //Close the document.
    document.Close(true);
}

VB.NET [Windows-specific]

'Initialize the OCR processor. 
Using processor As OCRProcessor = New OCRProcessor()

    'Load an existing PDF document. 
    Dim stream As FileStream = New FileStream("Input.pdf", FileMode.Open)
    Dim document As PdfLoadedDocument = New PdfLoadedDocument(stream)
    'Set OCR language. 
    processor.Settings.Language = Languages.English
    'Set tesseract version. ** For .NET Framework only. **
    'processor.Settings.TesseractVersion = TesseractVersion.Version4_0
    'Set OCR engine mode to process.
    processor.Settings.OCREngineMode = OCREngineMode.LSTMOnly
    'Set WhiteList Property.
    processor.Settings.WhiteList = "PDF"
    'Perform OCR with input document and tessdata (Language packs).
    processor.PerformOCR(document)

    'Create file stream. 
    Using outputFileStream As FileStream = New FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite)
        'Save the PDF document to file stream.
        document.Save(outputFileStream)
    End Using
    'Close the document. 
    document.Close(True)
End Using

Black List

The BlackList property specifies the characters that exclude from the character set used for recognition and the OCR will not return any of the characters you are specified in the list. For more information, refer to the following code sample.

C# [Cross-platform]

//Initialize the OCR processor.
using (OCRProcessor processor = new OCRProcessor())
{
    //Load an existing PDF document.
    FileStream stream = new FileStream("Input.pdf", FileMode.Open);
    PdfLoadedDocument document = new PdfLoadedDocument(stream);
    //Set OCR language.
    processor.Settings.Language = Languages.English;
    //Set tesseract version. ** For .NET Framework only. **
    //processor.Settings.TesseractVersion = TesseractVersion.Version4_0;
    //Set OCR engine mode to process.
    processor.Settings.OCREngineMode = OCREngineMode.LSTMOnly;
    //Set BlackList Property.
    processor.Settings.BlackList = "PDF";
    //Perform OCR with input document and tessdata (Language packs).
    processor.PerformOCR(document);

    //Create file stream.
    using (FileStream outputFileStream = new FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite))
    {
        //Save the PDF document to file stream.
        document.Save(outputFileStream);
    }
    //Close the document.
    document.Close(true);
}

VB.NET [Windows-specific]

'Initialize the OCR processor. 
Using processor As OCRProcessor = New OCRProcessor()

    'Load an existing PDF document. 
    Dim stream As FileStream = New FileStream("Input.pdf", FileMode.Open)
    Dim document As PdfLoadedDocument = New PdfLoadedDocument(stream)
    'Set OCR language. 
    processor.Settings.Language = Languages.English
    'Set tesseract version. ** For .NET Framework only. **
    'processor.Settings.TesseractVersion = TesseractVersion.Version4_0
    'Set OCR engine mode to process.
    processor.Settings.OCREngineMode = OCREngineMode.LSTMOnly
    'Set BlackList Property.
    processor.Settings.BlackList = "PDF"
    'Perform OCR with input document and tessdata (Language packs).
    processor.PerformOCR(document)

    'Create file stream. 
    Using outputFileStream As FileStream = New FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite)
        'Save the PDF document to file stream.
        document.Save(outputFileStream)
    End Using
    'Close the document. 
    document.Close(True)
End Using

OCR an Image to PDF

You can perform OCR on an image and convert it to a searchable PDF document. It is also possible to specify the conformance level through PdfConformanceLevel Enum to the output PDF document using OCR processor settings.

NOTE

This PDF conformance option only applies for image OCR to PDF documents.

The following code sample illustrates how to OCR an image to a PDF document:

C# [Cross-platform]

//Initialize the OCR processor.
using (OCRProcessor processor = new OCRProcessor())
{
    //Get stream from an image file. 
    FileStream imageStream = new FileStream(@"Input.jpg", FileMode.Open);    
    //Set OCR language to process.
    processor.Settings.Language = Languages.English;
    //Sets Unicode font to preserve the Unicode characters in a PDF document.
    FileStream fontStream = new FileStream(@"ARIALUNI.ttf", FileMode.Open);
    //Set the unicode font. 
    processor.UnicodeFont = new PdfTrueTypeFont(fontStream, true, PdfFontStyle.Regular, 10);
    //Set the PDF conformance level.
    processor.Settings.Conformance = PdfConformanceLevel.Pdf_A1B;
    //Process OCR by providing the bitmap image.  
    PdfDocument document = processor.PerformOCR(imageStream);

    //Create file stream.
    using (FileStream outputFileStream = new FileStream(@"Output.pdf", FileMode.Create, FileAccess.ReadWrite))
    {
        //Save the PDF document to file stream.
        document.Save(outputFileStream);
    }
    //Close the document.
    document.Close(true);
}

VB.NET [Windows-specific]

'Initialize the OCR processor. 
Using processor As OCRProcessor = New OCRProcessor()

    'Get stream from an image file.  
    Dim imageStream As FileStream = New FileStream("Input.jpg", FileMode.Open)
    'Set OCR language to process. 
    processor.Settings.Language = Languages.English
    'Sets Unicode font to preserve the Unicode characters in a PDF document. 
    Dim fontStream As FileStream = New FileStream("ARIALUNI.ttf", FileMode.Open)
    'Set the unicode font.  
    processor.UnicodeFont = New PdfTrueTypeFont(fontStream, True, PdfFontStyle.Regular, 10)
    'Set the PDF conformance level. 
    processor.Settings.Conformance = PdfConformanceLevel.Pdf_A1B
    'Process OCR by providing the bitmap image.   
    Dim document As PdfDocument = processor.PerformOCR(imageStream)

    'Create file stream.
    Using outputFileStream As FileStream = New FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite)
        'Save the PDF document to file stream.
        document.Save(outputFileStream)
    End Using
    'Close the document.
    document.Close(True)
End Using

You can downloaded a complete working sample from GitHub.

Performing OCR with Unicode characters

You can perform OCR on Images with Unicode characters. To preserve the Unicode characters in the PDF document, use the UnicodeFont property. For more information, refer to the following code sample.

ASP.NET Core

//Initialize the OCR processor.
using (OCRProcessor processor = new OCRProcessor())
{
    //Load an existing PDF document.
    FileStream stream = new FileStream("Input.pdf", FileMode.Open);
    PdfLoadedDocument document = new PdfLoadedDocument(stream);
    //Set OCR language.
    processor.Settings.Language = Languages.English;
    //Sets Unicode font to preserve the Unicode characters in a PDF document.
    FileStream fontStream = new FileStream(@"ARIALUNI.ttf", FileMode.Open);
    processor.UnicodeFont = new PdfTrueTypeFont(fontStream, 8);
    //Perform OCR with input document and tessdata (Language packs).
    processor.PerformOCR(document);

    //Create file stream.
    using (FileStream outputFileStream = new FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite))
    {
        //Save the PDF document to file stream.
        document.Save(outputFileStream);
    }
    //Close the document.
    document.Close(true);
}

C# [Cross-platform]

//By default unicode characters can be extracted from image file in .NET Framework applications like WF, WPF, ASP.NET and ASP.NET MVC.

VB.NET [Windows-specific]

//By default unicode characters can be extracted from image file in .NET Framework applications like WF, WPF, ASP.NET and ASP.NET MVC.

You can downloaded a complete working sample from GitHub.

Best Practices

You can improve the accuracy of the OCR process by choosing the correct compression method when converting the scanned paper to a TIFF image and then to a PDF document.

• Use (zip) lossless compression for color or gray-scale images.
• Use CCITT Group 4 or JBIG2 (lossless) compression for monochrome images. This ensures that optical character recognition works on the highest-quality image, thereby improving the OCR accuracy. This is especially useful in low-resolution scans.
• In addition, rotated images and skewed images can also affect the accuracy and readability of the OCR process.

Tesseract works best with text when at least 300 dots per inch (DPI) are used, so it is beneficial to resize images.

For more details regarding quality improvement, refer to the following link.

https://github.com/tesseract-ocr/tesseract/wiki/ImproveQuality

You can set the different performance level to the OCRProcessor using Performance enumeration.

• Rapid - high speed OCR performance and provide normal OCR accuracy.
• Fast - provides moderate OCR processing speed and accuracy.
• Slow - Slow OCR performance and provide best OCR accuracy.

Refer to the following code sample to set the performance of the OCR.

C# [Windows-specific]

//Initialize the OCR processor
OCRProcessor processor = new OCRProcessor();

//set the OCR performance
processor.Settings.Performance = Performance.Fast;

VB.NET [Windows-specific]

'Initialize the OCR processor
Dim processor As New OCRProcessor()

'Set the OCR performance
processor.Settings.Performance = Performance.Fast

TesseractBinaries Paths and Tesseract Language Data

Starting with v21.1.x, TesseractBinaries, and Tesseract language data folder paths are added by default. So, there is no need to provide these paths explicitly. However, you can refer to TesseractBinaries and Tessdata paths manually in your application as per the requirement.

NOTE

You can get the TessseractBinaries or TessData files from the NuGet package runtimes folder or bin folder of the application.

C# [Cross-platform]

//Initialize the OCR processor by providing the path of the tesseract binaries (SyncfusionTesseract.dll and liblept168.dll)
using (OCRProcessor processor = new OCRProcessor(@"TesseractBinaries\"))
{
    //Load an existing PDF document.
    FileStream stream = new FileStream("Input.pdf", FileMode.Open);
    PdfLoadedDocument document = new PdfLoadedDocument(stream);
    //Set OCR language to process.
    processor.Settings.Language = Languages.English;
    //Perform OCR with input document and tessdata (Language packs).
    processor.PerformOCR(document, @"TessData\");
    
    //Create file stream.
    using (FileStream outputFileStream = new FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite))
    {
        //Save the PDF document to file stream.
        document.Save(outputFileStream);
    }
    //Close the document.
    document.Close(true);
}

VB.NET [Windows-specific]

'Initialize the OCR processor 
Using processor As OCRProcessor = New OCRProcessor("TesseractBinaries\")
    'Load an existing PDF document.
    Dim stream As FileStream = New FileStream("Input.pdf", FileMode.Open)
    Dim document As PdfLoadedDocument = New PdfLoadedDocument(stream)
    'Set OCR language to process.
    processor.Settings.Language = Languages.English
    'Perform OCR with input document and tessdata (Language packs).
    processor.PerformOCR(document, "TessData\")

    'Create file stream.
    Using outputFileStream As FileStream = New FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite)
        'Save the PDF document to file stream.
        document.Save(outputFileStream)
    End Using
    'Close the document.
    document.Close(True)
End Using

Get image rotation angle from OCR processor

To get the Image rotation angle, you can rotate the image with 4 angles (0,90,180, and 360) from the OCR Processor. This feature works in multiple Images and multiple pages. The following code sample illustrates support for the Image Rotation angle from the OCR Processor.

C# [Cross-platform]

//Initialize the OCR processor.@
using (OCRProcessor processor = new OCRProcessor())
{
    //Get the stream from an image file. 
    FileStream stream = new FileStream(@"D:\Input.pdf", FileMode.Open);
    //Set the OCR language to process.
    PdfLoadedDocument document = new PdfLoadedDocument(stream);
    //Set the OCR language.
    processor.Settings.Language = Languages.English;
    //Set the Unicode font to preserve the Unicode characters in a PDF document.
    processor.TesseractPath = @"D:\Tesseractbinaries_core\Windows\x64";
    processor.PerformOCR(document, 0, 0, @"D:\tessdata", out OCRLayoutResult result);
    float angle = 0;
    if (result != null)
    {
        foreach (var page in result.Pages)
        {
            angle = page.ImageRotation;
            if (angle == 180)
            {
                document.Pages[0].Rotation = PdfPageRotateAngle.RotateAngle180;
            }
        }
    }    
    //Create file stream.
    using (FileStream outputFileStream = new FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite))
    {
        //Save the PDF document to file stream.
        document.Save(outputFileStream);
    }
}

VB.NET [Windows-specific]

'Initialize the OCR processor
Using processor As OCRProcessor = New OCRProcessor()
    'Get the stream from an image file. 
    Dim stream As FileStream = New FileStream(@"D:\Input.pdf", FileMode.Open);
    'Set the OCR language to process.
    Dim document As PdfLoadedDocument = New PdfLoadedDocument(stream);
    'Set the OCR language.
    processor.Settings.Language = Languages.English;
    'Set the Unicode font to preserve the Unicode characters in a PDF document.
    processor.TesseractPath = @"D:\Tesseractbinaries_core\Windows\x64";
    processor.PerformOCR(document, 0, 0, @"D:\tessdata", out OCRLayoutResult result);
    float angle = 0;
    If result IsNot Nothing Then
    For Each page As var In result.Pages
        angle = page.ImageRotation
        If angle = 180 Then
            document.Pages(0).Rotation = PdfPageRotateAngle.RotateAngle180
        End If
    Next
	End If  
    'Create file stream.
     Using outputFileStream As FileStream = New FileStream("Output.pdf", FileMode.Create, FileAccess.ReadWrite)
        'Save the PDF document to file stream.
        document.Save(outputFileStream)
    End Using
	'Close the document.
    document.Close(True)
End Using

You can downloaded a complete working sample from GitHub.