▁is ▁most ▁commonly ▁used ▁in ▁image ▁processing ▁for ▁object ▁detection ▁and ▁tracking , ▁primarily ▁fa cial ▁detection ▁and ▁recognition . ▁ ▁The ▁first ▁cas c ading ▁class ifier ▁was ▁the ▁face ▁det ector ▁of ▁Vi ola ▁and ▁Jones ▁( 2 0 0 1 ). ▁The ▁requirement ▁for ▁this ▁class ifier ▁was ▁to ▁be ▁fast ▁in ▁order ▁to ▁be ▁implemented ▁on ▁low - power ▁CPU s , ▁such ▁as ▁cam eras ▁and ▁ph ones . ▁ ▁Algorithm ▁properties ▁ ▁S cal ing ▁and ▁rot ations ▁ ▁It ▁can ▁be ▁seen ▁from ▁this ▁description ▁that ▁the ▁class ifier ▁will ▁not ▁accept ▁faces ▁that ▁are ▁ups ide ▁down ▁( the ▁e y eb rows ▁are ▁not ▁in ▁a ▁correct ▁position ) ▁or ▁the ▁side ▁of ▁the ▁face ▁( the ▁nose ▁is ▁no ▁longer ▁in ▁the ▁center , ▁and ▁sh adows ▁on ▁the ▁side ▁of ▁the ▁nose ▁might ▁be ▁missing ). ▁Se par ate ▁cas cade ▁class ifiers ▁have ▁to ▁be ▁trained ▁for ▁every ▁rotation ▁that ▁is ▁not ▁in ▁the ▁image ▁plane ▁( side ▁of ▁face ) ▁and ▁will ▁have ▁to ▁be ▁retra ined ▁or ▁run ▁on ▁rot ated ▁features ▁for ▁every ▁rotation ▁that ▁is ▁in ▁the ▁image ▁plane ▁( face ▁ups ide ▁down ▁or ▁t ilt ed ▁to ▁the ▁side ). ▁S cal ing ▁is ▁not ▁a ▁problem , ▁since ▁the ▁features ▁can ▁be ▁scale d ▁( center p ixel , ▁left p ix els ▁and ▁right p ix els ▁have ▁a ▁dimension ▁only ▁relative ▁to ▁the ▁rectangle ▁exam ined ). ▁In ▁recent ▁cas c ades