▁of ▁inter ▁frames : ▁P - frames ▁and ▁B - frames . ▁These ▁two ▁kinds ▁of ▁frames ▁and ▁the ▁I - frames ▁( Int ra - coded ▁pictures ) ▁usually ▁join ▁in ▁a ▁G OP ▁( Group ▁Of ▁Pictures ). ▁The ▁I - frame ▁doesn ' t ▁need ▁additional ▁information ▁to ▁be ▁dec oded ▁and ▁it ▁can ▁be ▁used ▁as ▁a ▁reliable ▁reference . ▁This ▁structure ▁also ▁allows ▁to ▁achieve ▁an ▁I - frame ▁period icity , ▁which ▁is ▁needed ▁for ▁dec oder ▁synchron ization . ▁ ▁Frame ▁types ▁The ▁difference ▁between ▁P - frames ▁and ▁B - frames ▁is ▁the ▁reference ▁frame ▁they ▁are ▁allowed ▁to ▁use . ▁ ▁P - frame ▁P - frame ▁is ▁the ▁term ▁used ▁to ▁define ▁the ▁forward ▁Pred icted ▁pictures . ▁The ▁prediction ▁is ▁made ▁from ▁an ▁earlier ▁picture , ▁mainly ▁an ▁I - frame ▁or ▁P - frame , ▁so ▁that ▁require ▁less ▁coding ▁data ▁( ≈ 5 0 % ▁when ▁compared ▁to ▁I - frame ▁size ). ▁ ▁The ▁amount ▁of ▁data ▁needed ▁for ▁doing ▁this ▁prediction ▁consist ▁of ▁motion ▁vectors ▁and ▁transform ▁coefficients ▁describing ▁prediction ▁correction . ▁It ▁involves ▁the ▁use ▁of ▁motion ▁compens ation . ▁ ▁B - frame ▁B - frame ▁is ▁the ▁term ▁for ▁bid irection ally ▁predicted ▁pictures . ▁This ▁kind ▁of ▁prediction ▁method ▁occup ies ▁less ▁coding ▁data ▁than ▁P - frames ▁generally ▁( ≈ 2 5 % ▁when ▁compared ▁to ▁I - frame ▁size ) ▁because ▁the ▁prediction ▁is ▁made ▁from ▁either ▁an ▁earlier ▁frame ▁or ▁a