Unfortunately I had to abandon this project. For the final project I had to generate halftones with stipples either with a fixed radius or with a radius within a very limited range (5 mm max, 3 mm min) and the stipples should not overlap.
I found out that using a stipple pattern like that I could never get the gradients that would be required to get a good representation of the subject image. The dark parts would remain too light and the highlights (white stipples) would still look dull.
About the changes to the code to get StippleGen working under processing 2.0b7: i was a bit too enthusiast in my former post. It did not generate an error anymore in processing, but the load and safe files functionalities were not working. I managed to make a work around. I had to click the load button a second time before the the loadpath would be properly loaded. Not a neat solution but ok for me at that time.
Thanks for your support anyway! I have to say its an interesting program that I will keep in mind for some other projects that I am thinking about!!