Fractal images can be extraordinarily compute intensive. If you zoom far enough into a fractal, you can make the world's fastest computer seem slow. Guessing is a way to try to avoid some of the time-consuming calculations. The guessing technique comes from the observation that, although some parts of fractals are infinitely complex, there are also frequently places that have very large iteration bands. Fractal eXtreme tries very aggressively to detect such areas and minimize the number of redundant calculations it makes. This aggressive guessing will cause noticeable errors on some pictures, which is why Fractal eXtreme finishes off each calculation by scanning for potentially incorrectly guessed pixels, and fixing them. Even with this correction stage at the end, guessing speeds up Fractal eXtreme's calculations by up to a factor of ten, or more, with no discernible loss of quality.