全新AdaBoost算法及应用.ppt

? Overview ? The AdaBoost Algorithm ? How and why AdaBoost works? ? AdaBoost for Face Detection Overview AdaBoost Adaptive A learning algorithm Building a strong classifier a lot of weaker ones Boosting 1 { 1 } ) , 1 ( h x ???... weak classifiers slightly better than random 1 ( ) ( ) Tt t Tt h x H x sign ??? ??? ?? ?? 2 { 1 } ) , 1 ( h x ???{ 1 (}) , 1 T h x ??? strong classifier 1 { 1 } ) , 1 ( h x ???... weak classifiers slightly better than random 1 ( ) ( ) Tt t Tt h x H x sign ??? ??? ?? ?? 2 { 1 } ) , 1 ( h x ???{ 1 (}) , 1 T h x ??? strong classifier 训练一个弱分类器(特征 f) 就是在当前权重分布的情况下, 确定 f 的最优阈值以及不等号的方向,使得这个弱分类器(特征 f )对所有训练样本的分类误差最低。 1 ( ) ( , , , ) 0 pf x p h x f p ????????? ???????????????????其他 1 { 1 } ) , 1 ( h x ???... weak classifiers slightly better than random 1 ( ) ( ) Tt t Tt h x H x sign ??? ??? ?? ?? 2 { 1 } ) , 1 ( h x ???{ 1 (}) , 1 T h x ??? strong classifier How good the strong one will be? How good the strong one will be? The AdaBoost Algorithm Given: 1 1 where ( , ), , ( , ) , { 1, 1} m m i i x y x y x X y ? ???? Initialization: 11 ( ) , 1, , m D i i m ? ?? For : 1, , t T ??? Find classifier which minimizes error wrt D t ,., : { 1, 1} t h X ??? 1 where argmin ( )[ ( )] jm t j j t i j i ih h D i y h x ? ??? ???:probability distribution of 's at time ( ) t i D i x t ? Weight classifier: 11 ln2 ttt?????? Update distribution: 1 ( ) exp[ ( )] , is for normalizati ( )on t t i t i t t t D i y h x D i Z Z ???? minimize weighted error for minimize exponent