Median filter

Photos taken in low light conditions can have a lot of digital noise. Noise can also show up in highly compressed JPEG images in form of compression artifacts. This noise can mislead OCR algorithms and prevent other preprocessing filters from working properly.

Aspose.OCR provides an alternative function for removing noise from an image at the cost of some detail, called the median filter. This makes the image a little blurry while preserving the edges of high-contrast objects such as letters. The results can be further improved with the auto-contrast or binarization preprocessing filters.

The median filter automatically converts the image to grayscale.

Applying the median filter

To smooth out noise in an image, run the image through OCR_IMG_PREPROCESS_MEDIAN preprocessing filter.

std::string image_path = "source.png";
custom_preprocessing_filters filters_;
filters_.filter_1 = OCR_IMG_PREPROCESS_MEDIAN;
asposeocr_preprocess_page_and_save(image_path.c_str(), "result.png", filters_);

Usage scenarios

Median filter is recommended for the following images:

Photos that were taken in low light conditions.
Poor quality printouts.
Highly compressed / low quality JPEG’s.

Improvements in recognition accuracy will be highly dependent on the original image and should be empirically tested. Applying a median filter can sometimes result in the loss of important details such as very thin characters and punctuation.

Dilation