▁later ▁frame ▁or ▁both ▁them . ▁( B - frames ▁can ▁also ▁be ▁less ▁efficient ▁than ▁P - frames ▁under ▁certain ▁cases , ▁eg : ▁loss less ▁encoding ) ▁ ▁Similar ▁to ▁P - frames , ▁B - frames ▁are ▁expressed ▁as ▁motion ▁vectors ▁and ▁transform ▁coefficients . ▁In ▁order ▁to ▁avoid ▁a ▁growing ▁propag ation ▁error , ▁B - frames ▁are ▁not ▁used ▁as ▁a ▁reference ▁to ▁make ▁further ▁predictions ▁in ▁most ▁encoding ▁standards . ▁However , ▁in ▁newer ▁encoding ▁methods ▁( such ▁as ▁H . 2 6 4 / MP EG - 4 ▁A VC ▁and ▁HE VC ), ▁B - frames ▁may ▁be ▁used ▁as ▁reference ▁for ▁better ▁explo itation ▁of ▁temporal ▁redund ancy . ▁ ▁Typ ical ▁Group ▁Of ▁Pictures ▁( G OP ) ▁structure ▁ ▁The ▁typical ▁Group ▁Of ▁Pictures ▁( G OP ) ▁structure ▁is ▁I BB P BB P ... ▁The ▁I - frame ▁is ▁used ▁to ▁predict ▁the ▁first ▁P - frame ▁and ▁these ▁two ▁frames ▁are ▁also ▁used ▁to ▁predict ▁the ▁first ▁and ▁the ▁second ▁B - frames . ▁The ▁second ▁P - frame ▁is ▁predicted ▁also ▁using ▁the ▁first ▁I - frame . ▁Both ▁P - frames ▁join ▁together ▁to ▁predict ▁the ▁third ▁and ▁fourth ▁B - frames . ▁The ▁scheme ▁is ▁shown ▁in ▁the ▁next ▁picture : ▁ ▁This ▁structure ▁suggests ▁a ▁problem ▁because ▁the ▁fourth ▁frame ▁( a ▁P - frame ) ▁is ▁needed ▁in ▁order ▁to ▁predict ▁the ▁second ▁and ▁the ▁third ▁( B - frames ). ▁So ▁we ▁need