Florence Jacquey Frederic Comby Olivier Strauss Non-additive Approach for Omnidirectional Image Gradient Estimation. 1-6 2007 ICCV http://dx.doi.org/10.1109/ICCV.2007.4409193 conf/iccv/2007 db/conf/iccv/iccv2007.html#JacqueyCS07