▁the ▁I FF ▁file ▁format ▁consisting ▁of ▁a ▁number ▁of ▁consecutive ▁ch unks , ▁whose ▁order ▁can , ▁to ▁some ▁extent , ▁be ▁varied . ▁Each ▁chunk ▁has ▁a ▁different ▁function ▁and ▁has ▁the ▁same ▁basic ▁format . ▁This ▁means ▁that ▁a ▁program ▁does ▁not ▁have ▁to ▁read ▁or ▁decode ▁every ▁chunk ▁in ▁a ▁file , ▁only ▁the ▁ones ▁it ▁wants ▁to ▁deal ▁with ▁or ▁the ▁ones ▁it ▁can ▁understand . ▁ ▁IL B M ▁files ▁usually ▁contain ▁enough ▁information ▁to ▁allow ▁them ▁to ▁be ▁displayed ▁by ▁an ▁image ▁editing ▁program , ▁including ▁image ▁dimensions , ▁p alette ▁and ▁pixel ▁data . ▁Some ▁files ▁were ▁designed ▁to ▁act ▁as ▁pale tt es ▁for ▁paint ▁programs ▁( p ixel ▁data ▁left ▁blank ) ▁or ▁to ▁be ▁merged ▁into ▁another ▁image . ▁This ▁makes ▁them ▁much ▁more ▁flexible , ▁but ▁also ▁much ▁more ▁complex ▁than ▁other ▁formats ▁such ▁as ▁B MP . ▁ ▁For ▁IL B M s ▁the ▁B M HD ▁chunk ▁and ▁any ▁other ▁' v ital ' ▁ch unks ▁must ▁appear ▁before ▁the ▁B OD Y ▁chunk . ▁Any ▁ch unks ▁appearing ▁after ▁B OD Y ▁are ▁considered ▁' extra ' ▁and ▁many ▁programs ▁will ▁leave ▁them ▁un read ▁and ▁un changed . ▁ ▁B M HD : ▁Bitmap ▁Header ▁ ▁The ▁B M HD ▁chunk ▁spec ifies ▁how ▁the ▁image ▁is ▁to ▁be ▁displayed ▁and ▁is ▁usually ▁the ▁first ▁chunk ▁inside ▁the ▁F ORM . ▁ ▁It ▁not ▁only ▁defines ▁the ▁image ' s ▁height / width , ▁but ▁where ▁it