Color inversion

When working with white (or light) text on a black (or other dark) background, recognition accuracy may suffer. It can be greatly improved by reversing the colors of the image so that light areas appear dark and dark areas appear light. In addition, other colors are also swapped: red becomes cyan, green becomes magenta, blue becomes yellow, and so on.

Aspose.OCR provides the automated processing filter that inverts colors in the image before proceeding to recognition.

Inverting image colors

To automatically invert colors in an image before recognition, run the image through Invert processing filter.

Aspose.OCR.AsposeOcr recognitionEngine = new Aspose.OCR.AsposeOcr();
// Invert colors
Aspose.OCR.Models.PreprocessingFilters.PreprocessingFilter filters = new Aspose.OCR.Models.PreprocessingFilters.PreprocessingFilter();
filters.Add(Aspose.OCR.Models.PreprocessingFilters.PreprocessingFilter.Invert());
// Add an image to OcrInput object and apply processing filters
Aspose.OCR.OcrInput input = new Aspose.OCR.OcrInput(Aspose.OCR.InputType.SingleImage, filters);
input.Add("source.png");
// Save processed image to the folder
Aspose.OCR.ImageProcessing.Save(input, @"C:\result");
// Recognize image
List<Aspose.OCR.RecognitionResult> results = recognitionEngine.Recognize(input);
foreach(Aspose.OCR.RecognitionResult result in results)
{
	Console.WriteLine(result.RecognitionText);
}
White text on dark background Inverted image

Image regions processing

You can invert certain areas of the image, such as black on a white headers, while leaving the rest of the content unchanged.

To apply a filter to an area, specify its top left corner along with width and height as Aspose.Drawing.Rectangle object. If the region is omitted, the filter is applied to the entire image.

Aspose.Drawing.Rectangle rectangle = new Aspose.Drawing.Rectangle(5, 161, 340, 113);
Aspose.OCR.Models.PreprocessingFilters.PreprocessingFilter filters = new Aspose.OCR.Models.PreprocessingFilters.PreprocessingFilter();
filters.Add(Aspose.OCR.Models.PreprocessingFilters.PreprocessingFilter.Invert(rectangle));

Usage scenarios

Automatic color inversion is recommended for the following images:

  • White text on black background.
  • Advertisements.
  • Business cards.
  • Screenshots.
Conversion to grayscale Dilation