Postprocessing

Adaptive Threshold

This filter converts gray scale images into lineart images. For each pixel of the input image, it calculates the average value of the pixels in a square area around this pixel. The threshold value for the center pixel is this average minus an offset value. This allows reliable thresholding even for documents with considerable changes of the background color.

Adaptive Threshold tab

A drawback of the filter: If you have dark areas that are larger than the averaging square, it can happen that only the edges of the area will stay dark, and that the "inner part" of such an area will become white. This occurs, if the offset value is too small. In this case you can either increase the averaging square or increase the threshold value.

The runtime of the algorithm is proportional to O(n), where n is the length of the averaging square, i.e., it is much faster for larger squares than a universal convolution filter, whose runtime is often O(n*n).

Clean

This filter works only on lineart images.

For a "noisy" image, thresholding can lead to many small speckles in the image. This can occur both for a simple constant threshold, as well as for the adaptive threshold. The clean filter counts the number of black pixels in a square area around each pixel; if this number is below a selectable threshold, the center pixel becomes a white pixel.

Adaptive Clean tab

ImageMagick

This plugin simply calls the program "convert" from ImageMagick. It is mostly intended to be an example, that it is quite simple to include an external program as a filter for Eikazo. Including the lengthy copyright notice, the source code consists of only 126 lines. Its "real" usefulness for daily scan tasks is at least in its current setup - only "boolean" options are supported - questionable.

Please note that convert may need considerable amounts of memory and may run for a long time for some filter options.

ImageMagick tab