Most of the adaptive optics experimented for astronomy uses a Shack-Hartmann wavefront sensor. By their principle these sensors require a large amount of real time calculations. The time spent to perform these calculations may become a limiting factor of the bandwidth. It should be probably the case for the mirrors with high numbers of actuators which will be built for the very large telescope projects. Recently, F. Roddier has proposed an analog curvature sensing method which could theoritically avoid any calculations. We will demonstrate that optical filterings which transform wavefront distorsions in intensity modulations can be used with a reasonable amount of calculations to perform the control of an adaptive optical system. Some of these filtering which permits differential measurements offers the possibility to eliminate the inexpected speckle intensity modulations.