Guide to using IronOCR Filters
IronOCR provides the tools you need to read images that may need preprocessing in the form of filters. You can choose from a wide array of filters that can manipulate your images to become processable.
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List of OCR Image Filters
The following Image filters can really improve performance:
Filters to change the Image OrientationRotate- Rotates images by a number of degrees clockwise. For anti-clockwise, use negative numbers.Deskew- Rotates an image so it is the right way up and orthogonal. This is very useful for OCR because Tesseract's tolerance for skewed scans can be as low as 5 degrees.Scale- Scales OCR input pages proportionally.
Filters to manipulate Image ColorsBinarize- This image filter turns every pixel black or white with no middle ground. This may improve OCR performance in cases of very low text-to-background contrast.ToGrayScale- This image filter turns every pixel into shades of gray. Unlikely to improve OCR accuracy but may improve speed.Invert- Inverts every color. E.g. White becomes black and vice versa.ReplaceColor- Replaces a color in an image with another color, within a certain threshold.
Filters to improve Contrast in an ImageContrast- Increases contrast automatically. This filter often improves OCR speed and accuracy in low-contrast scans.Dilate- Advanced Morphology. Dilation adds pixels to the boundaries of objects in an image. Opposite of Erode.Erode- Advanced Morphology. Erosion removes pixels from object boundaries. Opposite of Dilate.
Filters to reduce Image NoiseSharpen- Sharpens blurred OCR Documents and flattens alpha channels to white.DeNoise- Removes digital noise. This filter should only be used in scenarios where noise is expected.DeepCleanBackgroundNoise- Heavy background noise removal. Only use this filter if extreme document background noise is known, as it might reduce OCR accuracy of clean documents and is CPU intensive.EnhanceResolution- Enhances the resolution of low-quality images. This filter is not often needed becauseOcrInput.MinimumDPIandOcrInput.TargetDPIwill automatically catch and resolve low-resolution inputs.
Filter Example and Usage
In the following example, we demonstrate how to apply filters within your code.
:path=/static-assets/ocr/content-code-examples/tutorials/c-sharp-ocr-image-filters-1.csusing IronOcr;
using System;
var ocr = new IronTesseract();
using var input = new OcrInput();
input.LoadImage("my_image.png");
input.Deskew();
var result = ocr.Read(input);
Console.WriteLine(result.Text);Imports IronOcr
Imports System
Private ocr = New IronTesseract()
Private input = New OcrInput()
input.LoadImage("my_image.png")
input.Deskew()
Dim result = ocr.Read(input)
Console.WriteLine(result.Text)Debug Filter / What is the filter doing?
If you are having difficulty with reading images or barcodes within your program, there is a way to save an image of a filtered result. This way, you can debug and see exactly what each filter does and how it is manipulating your image.
:path=/static-assets/ocr/content-code-examples/tutorials/c-sharp-ocr-image-filters-2.csusing IronOcr;
using System;
var file = "skewed_image.tiff";
var ocr = new IronTesseract();
using var input = new OcrInput();
var pageindices = new int[] { 1, 2 };
input.LoadImageFrames(file, pageindices);
// Here we apply the filter: Deskew
input.Deskew();
// Save the input with filter(s) applied
input.SaveAsImages("my_deskewed");
// We read, then print the text to the console
var result = ocr.Read(input);
Console.WriteLine(result.Text);Imports IronOcr
Imports System
Private file = "skewed_image.tiff"
Private ocr = New IronTesseract()
Private input = New OcrInput()
Private pageindices = New Integer() { 1, 2 }
input.LoadImageFrames(file, pageindices)
' Here we apply the filter: Deskew
input.Deskew()
' Save the input with filter(s) applied
input.SaveAsImages("my_deskewed")
' We read, then print the text to the console
Dim result = ocr.Read(input)
Console.WriteLine(result.Text)Filter Use Cases
Rotate
Filter Explanation
Rotate is a filter used to manually set a known rotation to an image to get it closest to being straight. IronOCR has functionality to run Deskew(), however, the degree of tolerance for this is rather narrow and is best used for images that are almost perfectly straight (within 15 degrees or so). For input images that are 90 degrees off, or upside down, we should call Rotate().
Use-Case Code Example
This is an example of calling Rotate to correct an upside-down image:
:path=/static-assets/ocr/content-code-examples/tutorials/c-sharp-ocr-image-filters-3.csusing IronOcr;
using System;
var image = "screenshot.png";
var ocr = new IronTesseract();
using var input = new OcrInput();
// Load at least one image
input.LoadImage(image);
// Rotate 180 degrees because image is upside-down
input.Rotate(180);
// Read image into variable: result
var result = ocr.Read(input);
// Example print to console
Console.WriteLine(result.Text);Imports IronOcr
Imports System
Private image = "screenshot.png"
Private ocr = New IronTesseract()
Private input = New OcrInput()
' Load at least one image
input.LoadImage(image)
' Rotate 180 degrees because image is upside-down
input.Rotate(180)
' Read image into variable: result
Dim result = ocr.Read(input)
' Example print to console
Console.WriteLine(result.Text)BeforeInput.Rotate(180)`` | AfterInput.Rotate(180)`` |
|---|---|
 |  |
Deskew
Filter Explanation
Uses a Hough Transform to attempt to straighten an image within certain degrees of tolerance. This is important for images that are not completely straight because a tilted document may result in a misread.
Please note
Use-Case Code Example
This is an example of calling Deskew to correct a skewed image:
:path=/static-assets/ocr/content-code-examples/tutorials/c-sharp-ocr-image-filters-4.csusing IronOcr;
using System;
var image = @"paragraph_skewed.png";
var ocr = new IronTesseract();
using var input = new OcrInput();
// Load at least one image
input.LoadImage(image);
// Apply deskew with 15 degree snap
bool didDeskew = input.Deskew(15);
if (didDeskew)
{
// Read image into variable: result
var result = ocr.Read(input);
Console.WriteLine(result.Text);
}
else
{
Console.WriteLine("Deskew not applied because Image Orientation could not be determined.");
}Imports IronOcr
Imports System
Private image = "paragraph_skewed.png"
Private ocr = New IronTesseract()
Private input = New OcrInput()
' Load at least one image
input.LoadImage(image)
' Apply deskew with 15 degree snap
Dim didDeskew As Boolean = input.Deskew(15)
If didDeskew Then
' Read image into variable: result
Dim result = ocr.Read(input)
Console.WriteLine(result.Text)
Else
Console.WriteLine("Deskew not applied because Image Orientation could not be determined.")
End IfBeforeDeskew()`` | AfterDeskew()`` |
|---|---|
 |  |
Scale
Filter Explanation
Scale is a useful image manipulation filter that helps to resize an image using the pixels it already has. This can be used when a barcode is not being scanned because the image is only tens of pixels wide, with each bar as one pixel, or if text is too small with no anti-aliasing.
Please note
1000px x 1000px where barcodes can be read well, which should be considered if your barcode is not being found.Use-Case Code Example
This is an example of calling Scale to enlarge the gaps between bars in a barcode for scanning:
:path=/static-assets/ocr/content-code-examples/tutorials/c-sharp-ocr-image-filters-5.csusing IronOcr;
using System;
var image = @"small_barcode.png";
var ocr = new IronTesseract();
// Optional: This example uses a barcode
ocr.Configuration.ReadBarCodes = true;
using var input = new OcrInput();
// Load at least one image
input.LoadImage(image);
// Apply scale
input.Scale(400); // 400% is 4 times larger
// Read image into variable: result
var result = ocr.Read(input);
// Example print to console
Console.WriteLine(result.Text);Imports IronOcr
Imports System
Private image = "small_barcode.png"
Private ocr = New IronTesseract()
' Optional: This example uses a barcode
ocr.Configuration.ReadBarCodes = True
Dim input = New OcrInput()
' Load at least one image
input.LoadImage(image)
' Apply scale
input.Scale(400) ' 400% is 4 times larger
' Read image into variable: result
Dim result = ocr.Read(input)
' Example print to console
Console.WriteLine(result.Text)BeforeScale()`` | AfterScale()`` |
|---|---|
 |  |
Binarize
Filter Explanation
The Binarize filter classifies all pixels in an image as either black or white, depending on an adaptive algorithm. This removes all colors and separates the background into a flat white, with anything recognized as text colored a full black for easy reading.
Use-Case Code Example
This is an example of calling Binarize to align colored text and remove background colors and noise:
:path=/static-assets/ocr/content-code-examples/tutorials/c-sharp-ocr-image-filters-6.csusing IronOcr;
using System;
var image = @"no-binarize.jpg";
var ocr = new IronTesseract();
using var input = new OcrInput();
// Load at least one image
input.LoadImage(image);
// Apply Binarize
input.Binarize();
// Read image into variable: result
var result = ocr.Read(input);
// Example print to console
Console.WriteLine(result.Text);Imports IronOcr
Imports System
Private image = "no-binarize.jpg"
Private ocr = New IronTesseract()
Private input = New OcrInput()
' Load at least one image
input.LoadImage(image)
' Apply Binarize
input.Binarize()
' Read image into variable: result
Dim result = ocr.Read(input)
' Example print to console
Console.WriteLine(result.Text)BeforeBinarize()`` | AfterBinarize()`` |
|---|---|
 |  |
Invert
Filter Explanation
IronOCR reads best when the image is black text on a white background. The Invert filter is used to achieve this by inverting all colors on an image.
Use-Case Code Example
This is an example of calling Invert to turn white on black into black on white:
:path=/static-assets/ocr/content-code-examples/tutorials/c-sharp-ocr-image-filters-7.csusing IronOcr;
using System;
var image = @"before-invert.png";
var ocr = new IronTesseract();
using var input = new OcrInput();
// Load at least one image
input.LoadImage(image);
// Apply Invert
input.Invert(true);
// Read image into variable: result
var result = ocr.Read(input);
// Example print to console
Console.WriteLine(result.Text);Imports IronOcr
Imports System
Private image = "before-invert.png"
Private ocr = New IronTesseract()
Private input = New OcrInput()
' Load at least one image
input.LoadImage(image)
' Apply Invert
input.Invert(True)
' Read image into variable: result
Dim result = ocr.Read(input)
' Example print to console
Console.WriteLine(result.Text)Before | After |
|---|---|
 |  |
Frequently Asked Questions
What are OCR image filters?
OCR image filters in IronOCR are tools that allow you to preprocess images to improve their quality for Optical Character Recognition. These filters can manipulate image orientation, colors, contrast, and noise.
How can image orientation be adjusted?
Image orientation can be adjusted using the Rotate and Deskew filters in IronOCR. Rotate allows you to rotate images by a specified number of degrees, while Deskew automatically straightens images that are slightly tilted.
What is the purpose of a Binarize filter?
The Binarize filter in IronOCR converts all pixels in an image to either black or white, removing colors and flattening the background to improve OCR performance, especially in cases of low text-to-background contrast.
When should a DeepCleanBackgroundNoise filter be used?
The DeepCleanBackgroundNoise filter in IronOCR should be used in scenarios where there is extreme document background noise. It is CPU intensive and may reduce OCR accuracy on clean documents.
Can a Scale filter improve barcode scanning?
Yes, the Scale filter in IronOCR can resize an image, making it useful for enlarging barcodes that are too small to scan effectively. It helps increase the gaps between bars for better scanning accuracy.
What does an Invert filter do?
The Invert filter in IronOCR reverses the colors in an image, turning white into black and vice versa. This is useful for converting images to have black text on a white background, which is optimal for OCR.
How can you save a filtered image for debugging?
You can save a filtered image for debugging in IronOCR by using the SaveAsImages function after applying any desired filters. This allows you to visually inspect the effects of the filters on the image.
What is the function of a Contrast filter?
The Contrast filter in IronOCR automatically increases the contrast of an image, improving OCR speed and accuracy in scans with low contrast between the text and background.
What is the difference between Dilate and Erode filters?
Dilate adds pixels to the boundaries of objects in an image, whereas Erode removes pixels from object boundaries. Both are advanced morphology filters in IronOCR used to enhance the clarity of image features.
How does an EnhanceResolution filter work?
The EnhanceResolution filter in IronOCR improves the resolution of low-quality images, though it is not often needed because IronOCR's OcrInput.MinimumDPI and TargetDPI settings automatically handle low-resolution inputs.
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