The main issues with digital images and image processing are spatial resolution and grayscale depth. Good spatial resolution is needed for statistical methods to be efficient, and sheer enlargening has proven to be very helpful to the human eye. This is because the human eye can not distinguish small gray scale differences of small objects. The smaller the grey scale difference the larger areas are needed. Good amount of different grayscales again makes it possible to analyze the dynamical properties - for example, noise and edges.
Good dynamics is essential in finding the best contrast area from a picture.
Without good enough contrast we simply don't have readable text. The methods themselves are not directly comparable. They all have their own benefits and weaknesses, which we have tried to find out in our tests. Before judging any method one must be familiar enough with it. For example, for taking best possible photographs one must spend lots of time in experimenting, as the amount of variables to control is quite large.