, ▁pixel ▁value ▁from ▁some ▁part ▁of ▁a ▁rectangle ▁compared ▁to ▁another ▁have ▁been ▁replaced ▁with ▁H aar ▁wave lets . ▁ ▁Stage ▁properties ▁▁ ▁To ▁have ▁good ▁overall ▁performance , ▁the ▁following ▁criteria ▁must ▁be ▁met : ▁ ▁Each ▁stage ▁must ▁validate ▁all ▁faces , ▁and ▁can ▁produce ▁many ▁false ▁posit ives . ▁For ▁example , ▁if ▁stage ▁ 1 ▁were ▁to ▁mark ▁as ▁' does ▁not ▁contain ▁a ▁face ' ▁ 2 0 % ▁of ▁rect angles ▁containing ▁a ▁face ▁( false ▁negative ▁rate = 2 0 % ), ▁then ▁the ▁total ▁performance ▁of ▁the ▁chain ▁cannot ▁be ▁higher ▁than ▁ 8 0 % ▁true ▁positive , ▁whatever ▁the ▁next ▁stages ▁are , ▁since ▁ 2 0 % ▁of ▁faces ▁have ▁already ▁been ▁rejected . ▁ ▁This ▁suggests ▁that ▁a ▁good ▁stage ▁must ▁have ▁ 1 0 0 % ▁true ▁positive ▁and ▁for ▁example ▁ 4 0 % ▁false ▁positive , ▁that ▁is ▁accept ▁all ▁rect angles ▁containing ▁faces ▁and ▁err one ously ▁mark ▁many ▁rect angles ▁as ▁potentially ▁containing ▁a ▁face , ▁to ▁be ▁elimin ated ▁by ▁later ▁stages . ▁For ▁a ▁first ▁stage , ▁ 1 0 0 % ▁true ▁positive ▁and ▁ 4 0 % ▁false ▁positive ▁still ▁gives ▁a ▁lot ▁of ▁false ▁negative , ▁if ▁only ▁ 1 ▁in ▁a ▁ 1 0 0 0 ▁rect angles ▁in ▁an ▁image ▁contain ▁a ▁face , ▁there ▁will ▁still ▁be ▁ 4 0 0 ▁to ▁ 1 ▁false ▁possible ▁faces ▁after ▁the ▁first ▁stage . ▁ ▁If ▁the ▁first