IronOCR on Windows: DeNoise() Amplifies Background Noise into Speckle (Colored-Paper Scans)

Overview

DeNoise() (aliased internally as Despeckle()) is built for one job: removing small, isolated dark specks on a clean white background. It doesn't handle continuous texture.

Colored-paper documents scanned as black and white lose their color contrast and turn into a uniform gray grain across the whole page. DeNoise()'s fixed threshold misreads that grain as widespread speckle — so it makes the image noisier, not cleaner.

Fix: use AdaptiveThreshold() instead of DeNoise().

Environment

• OS: Windows 11
• Language/Runtime: C# (.NET 8 WinForms)

Version Metadata

• Version found: 2026.6.1
• Version resolved: 2026.10.2

Cause

DeNoise() is built only for isolated dark specks on an otherwise clean background — it does not adapt to continuous texture. Colored-paper pages scanned as black and white lose their color contrast and become a uniform gray-toned grain across the whole page, which the fixed global threshold misreads as widespread speckle rather than isolated noise.  This is a design limitation of the default DeNoise() for this input type, not an environment-specific bug. From v2026.10.2, the adaptive mode (DeNoise(false, true)) is designed for it. 

Solution

1. From v2026.10.2: use adaptive DeNoise(). Pass adaptive: true to switch DeNoise() to local (Sauvola) thresholding, which is designed for pages with a continuous background texture such as colored paper scanned as black and white:

   input.DeNoise(use3x3: false, adaptive: true); 

   Plain DeNoise() keeps its original behavior, so this has to be enabled explicitly. On earlier versions, or if the result is still noisy, use AdaptiveThreshold() as shown below.

2. Replace DeNoise() with AdaptiveThreshold() for this document type:

   using (var input = new OcrInput())
   {
   
       foreach (var imagePath in pageImagePaths)
       {
           input.LoadImage(imagePath);
       }
   
       // Replaces input.DeNoise() for this document type
       input.AdaptiveThreshold(0.15f);
   
       var ocrResult = ocr.Read(input);
   
       ocrResult.SaveAsSearchablePdf(outputPath);
   }

   Suggested tuning range for thresholdLimit: 0.10 – 0.20.
3. If AdaptiveThreshold() alone doesn't fully clean up other documents in your batch, apply Contrast() first to widen the separation between text and background before binarizing:

   input.Contrast(1.3f); // try a range of 1.3f–1.4f
   
   input.AdaptiveThreshold(0.15f);

    

Workarounds That Don't Work

• Applying DeNoise() alone, with no Deskew or other filter in the pipeline — confirmed ineffective; it worsens the image by amplifying background noise.