The target of the image processing step is to create a binary image where the text can be extracted easily. The optimum would be if

• all objects that are no letters or words were removed
• every letter is one object

In my implementation the image processing is done by objects of the type ImageProcessor.

Creating the binary image

The first part of the image processing is to create a binary image out of the original, as most of the following processing steps require a binary image as input.

Approach 1: color value validation

In this approach the image is converted to a binary image by setting a certain range of color values to white and everything else to black.

Evaluation

This approach needs a lot of additional processing to remove undesired objects from the binary image. So there are a lot of parameters to adjust for each map.

Implementation

• BinaryProcessor (based on the grayscale image)
• BinaryColorProcessor (based on the color image)

Approach 2: removing the image background

In this approach the color difference of the image background and the image itself is calculated and converted to a binary image (a certain range is set white, the rest black).

Evaluation

This approach delivers quite good results without the need for much additional processing. As a result there are only a view parameters needed to configure the processing step.

The performance however is worse than approach 1 in the two test maps "Mooskirchen" and "British Isles".

Implementation

• BackgroundProcessor

Improving the image

There are several processors that can be used to improve the image.

Defining a value range for certain attributes

• ChromaticityProcessor
• DensityProcessor
• FirstDerivativeProcessor
• PerimeterProcessor
• SecondDerivativeProcessor
• ThicknessProcessor (SkelettonProcessor also has the ability to restrict the thickness)

Transforming the image

• LineSegmentsProcessor (should be used in combination with EraseProcessor to remove lines in the image)
• MedianProcessor
• SkelettonProcessor

Morphological operations

• CloseProcessor
• DilateProcessor
• ErodeProcessor
• OpenProcessor

Combining images

• EraseProcessor
• MaskProcessor