How to Set DPI in OCR Using C#

To illustrate the difference, here's the result of processing the same low-resolution image without setting the TargetDPI property. This comparison demonstrates why proper DPI configuration is crucial for reliable text extraction.

Finding the correct DPI requires balance. High DPI provides more accuracy but slower processing; low DPI is faster but less reliable. When in doubt, let IronTesseract's automatic preprocessing determine optimal settings. You can disable automatic upscaling by setting the TargetDPI property to 0. For specialized applications, explore our guide on Tesseract image DPI optimization.

What Happens Without DPI Upscaling?

Without upscaling, the confidence score drops to 79%, and the extracted text is significantly less accurate. This comparison highlights how setting a TargetDPI improves OCR results on low-quality images. The difference becomes more pronounced when dealing with complex documents containing tables, forms, or multi-column layouts.

How Do I Set DPI for PDF Documents?

When processing a PDF, IronOCR upscales the entire document to the target DPI, not just the images within it. This comprehensive approach ensures consistent quality throughout the document, which is important for PDF OCR text extraction in business applications. For advanced PDF processing, such as working with searchable PDFs, proper DPI configuration becomes even more critical.

While higher DPI often leads to better OCR results, the optimal setting varies between PDFs. If you're unsure which value to use, leave the DPI at its default setting and allow IronOCR to determine the best configuration automatically. The library's algorithms analyze document characteristics such as font size, image quality, and content density to select appropriate preprocessing parameters.

// Example: Processing a PDF with custom DPI settings
using IronOcr;

var ocrTesseract = new IronTesseract();

using var ocrInput = new OcrInput();

// Configure DPI specifically for PDF processing
ocrInput.TargetDPI = 250; // Lower DPI often works well for PDFs

// Load a multi-page PDF document
ocrInput.LoadPdf(@"documents\scanned-report.pdf");

// Optional: Process only specific pages
// ocrInput.LoadPdf(@"documents\scanned-report.pdf", PageSelection: new int[] {1, 3, 5});

// Apply additional filters if needed
ocrInput.DeNoise(); // Remove digital noise
ocrInput.Sharpen(); // Enhance text edges

// Perform OCR with confidence tracking
var ocrResult = ocrTesseract.Read(ocrInput);

// Process results page by page
foreach (var page in ocrResult.Pages)
{
    Console.WriteLine($"Page {page.PageNumber}: {page.Confidence}% confidence");
    Console.WriteLine(page.Text);
}

// Example: Processing a PDF with custom DPI settings
using IronOcr;

var ocrTesseract = new IronTesseract();

using var ocrInput = new OcrInput();

// Configure DPI specifically for PDF processing
ocrInput.TargetDPI = 250; // Lower DPI often works well for PDFs

// Load a multi-page PDF document
ocrInput.LoadPdf(@"documents\scanned-report.pdf");

// Optional: Process only specific pages
// ocrInput.LoadPdf(@"documents\scanned-report.pdf", PageSelection: new int[] {1, 3, 5});

// Apply additional filters if needed
ocrInput.DeNoise(); // Remove digital noise
ocrInput.Sharpen(); // Enhance text edges

// Perform OCR with confidence tracking
var ocrResult = ocrTesseract.Read(ocrInput);

// Process results page by page
foreach (var page in ocrResult.Pages)
{
    Console.WriteLine($"Page {page.PageNumber}: {page.Confidence}% confidence");
    Console.WriteLine(page.Text);
}

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For enterprise applications processing large volumes of documents, consider implementing a dynamic DPI adjustment strategy. Start with a moderate setting (300 DPI) and adjust based on confidence scores. This approach balances processing speed with accuracy, ensuring optimal performance across diverse document types. Combining DPI optimization with other preprocessing techniques can yield better results for challenging documents.

When working with specific document types, DPI requirements may vary. For instance, reading license plates typically requires higher DPI settings due to small character size and potential image distortion from camera angles. Similarly, processing MICR cheques benefits from specific DPI configurations to accurately capture specialized MICR font characters.

For developers integrating OCR into web applications or cloud services, understanding DPI optimization becomes crucial for managing processing times and server resources. The ability to fine-tune DPI settings allows you to optimize the balance between accuracy and performance based on your specific application requirements and infrastructure constraints.