▁point . ▁High er ▁cell ▁values ▁are ▁rendered ▁br ighter . ▁The ▁two ▁distinct ly ▁bright ▁sp ots ▁are ▁the ▁H ough ▁parameters ▁of ▁the ▁two ▁lines . ▁From ▁these ▁sp ots ' ▁positions , ▁angle ▁and ▁distance ▁from ▁image ▁center ▁of ▁the ▁two ▁lines ▁in ▁the ▁input ▁image ▁can ▁be ▁determined . ▁ ▁Vari ations ▁and ▁extensions ▁ ▁Using ▁the ▁gradient ▁direction ▁to ▁reduce ▁the ▁number ▁of ▁votes ▁An ▁improvement ▁suggested ▁by ▁O ' G orm an ▁and ▁C low es ▁can ▁be ▁used ▁to ▁detect ▁lines ▁if ▁one ▁takes ▁into ▁account ▁that ▁the ▁local ▁gradient ▁of ▁the ▁image ▁intensity ▁will ▁necessarily ▁be ▁orthogonal ▁to ▁the ▁edge . ▁Since ▁edge ▁detection ▁generally ▁involves ▁computing ▁the ▁intensity ▁gradient ▁magnitude , ▁the ▁gradient ▁direction ▁is ▁often ▁found ▁as ▁a ▁side ▁effect . ▁If ▁a ▁given ▁point ▁of ▁coordinates ▁( x , y ) ▁happens ▁to ▁indeed ▁be ▁on ▁a ▁line , ▁then ▁the ▁local ▁direction ▁of ▁the ▁gradient ▁gives ▁the ▁ θ ▁parameter ▁corresponding ▁to ▁said ▁line , ▁and ▁the ▁r ▁parameter ▁is ▁then ▁immediately ▁obtained . ▁( Sh api ro ▁and ▁Stock man , ▁ 3 0 5 ) ▁The ▁gradient ▁direction ▁can ▁be ▁estimated ▁to ▁within ▁ 2 0 ° , ▁which ▁short ens ▁the ▁sin us oid ▁trace ▁from ▁the ▁full ▁ 1 8 0 ° ▁to ▁roughly ▁ 4 5 ° . ▁This ▁reduces ▁the ▁computation ▁time ▁and ▁has ▁the ▁interesting ▁effect ▁of ▁reducing ▁the ▁number ▁of ▁useless ▁votes , ▁thus ▁en han cing ▁the ▁visibility ▁of ▁the ▁sp ikes