Bao_Block
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Transcript of Bao_Block
Hnh 2. nh x khi ca mt macro block c cnh bn l 16 pixel v tham s tm kim p c kch c l 7 pixels.II. Thut ton nh x khiGi thit nn mng cho d on chuyn ng l cc m hnh tng ng vi cc i tng v nn trong mt khung ca chui video di chuyn trong khung to thnh cc i tng tng ng trn khung tip theo. tng ng sau nh x khi l phn chia khung hnh hin thi thnh mt ma trn macro block, sau so snh vi block tng ng v cc khi ln cn trong khung trc to ra mt vector quy nh s chuyn ng mt macro block t im ny n im khc trong khung trc . Chuyn ng ny c tnh cho tt c cc macro block bao gm mt khung, vic thit lp d on chuyn ng trong khung hin thi. Vng tm kim tt cho mt macro block ph hp c quy nh ln n p im nh trn bn pha ca macro block tng ng trong khung nh trc . p c gi l tham s tm kim. Chuyn ng cng ln th yu cu p cng phi ln, v ln ca tham s tm kim c tnh ton k qu trnh ny tr thnh d on chuyn ng. Thng thng macro block c cho nh mt vung vi cnh l 16 pixel vi tham s p l 7 pixels (hnh 2). Mt macro block c nh x thnh mt block khc da trn gi tr u ra ca mt hm. Gi tr l kt qu thp nht thu c t vic nh x block hin ti gn nht thu c mt macro block mi. Mt hm th c nhiu gi tr khc nhau, trong ph bin nht v c tnh ton k nht l trung bnh sai khc tuyt i (Mean Absolute Difference - MAD) cho bi phng trnh (i). Mt gi tr khc ca hm l trung bnh sai s bnh phng (Mean Squared Error - MSE) cho bi phng trnh (ii).
Trong : N l cnh bn ca macro block. Cij v Rij l im nh ca macro block hin ti v macro block qui chun tng ng.
Hnh 3. Qu trnh tm kim 3 bc. Vector chuyn ng l (5,-3)T s tn hiu nh trn nhiu (PSNR) cho bi phng trnh (iii) m t c im ca nh b chuyn ng c to ra bng cch s dng vector chuyn ng v macro block t m hnh khung chun
A. Tm kim ton b (Exhaustive Search ES)Thut ton ny, cng c gi l tm kim y , l thut ton c s tnh ton k cng nht trong tt c cc thut ton ca nh x khi. Thut ton ny s tnh ton gi tr ca hm ti mi v tr c th c trong ca s tm kim. Kt qu l n tm v tr ph hp nht c th v cho PSNR cao nht trong s bt k thut ton nh x khi no. Th nghim thut ton nh x khi nhanh ta c cng mt PSNR nh vic c th tnh ton mt vi php tnh khng ng k. Nhc im ca ES l ca s tm kim ln i hi s tnh ton cng phi ln ph hp vi n.B. Tm kim ba bc (TSS)y l mt th nghim sm nht v thut ton nh x khi nhanh v c a ra vo gia nhng nm 1980. tng chung c th hin trong hnh 3. N bt u tm kim ti v tr trung tm v c thit lp step size S = 4, cho mt tham s tm kim thng thng c gi tr l 7. Sau , n tm kim 8 im +/- S pixel xung quanh im (0,0). T 8 v tr tm kim c n chn mt im c gi tr thp nht v bin n thnh v tr gc tm kim mi. Sau tip tc thit lp kch thc bc mi S = S/2, ri tip tc tm kim tng t nh trn, lp i lp li khong 2 ln cho n khi S = 1. Lc v tr tm kim c gi tr hm thp nht v macro block ti l ph hp nht. Tnh ton vector chuyn ng ri lu li truyn. Gip gim cc php ton khi h s tnh ton l 9. V vy vi p=7, ES s tnh ton gi tr cho 225 macro block trong khi TSS ch tnh ton gi tr cho 25 macro block. tng ng sau TSS l cc b mt li do chuyn ng trong mi macro block tr thnh unimodal. Mt unimodal surface l l mt bowl shaped surface nh b to trng s cho bi cc gi tr ca hm tng u t gi tr nh nht.
Hnh 4. nh x khi tm kim 4 bc: Phm vi kim tra ca TSS trong bc u tin l kh ln v the quares l vic m rng thm 8 im c thm vo trong bc u tin ca NTSS. Hnh tam gic v diamond l bc th hai ca NTSS biu din 3, 5 im c kim tra khi trng s thp nht trong bc u tin l mt trong nhng 8 im ca ca s trung tm.C. Tm kim ba bc mi (NTSS).NTSS [4] l ci tin nhng kt qu ca TSS bng cch cung cp mt chng tnh tm kim center biased v quy nh cch ngn chn gim gi tr tnh ton. N l mt trong cc thut ton nhanh u tin c cng nhn rng ri v thng c s dng cho vic thc hin cc tiu chun trc nh MPEG 1 v H.261.TSS s dng mt m hnh kim tra phn b u pht hin chuyn ng v cc chuyn ng nh b khuyt. Qu trnh NTSS c minh ha bng hnh 4. Trong bc u tin 16 im c kim tra thm vo tm ra trng s thp nht bng cch s dng mt gi tr ca hm. Trong nhng v tr tm kim b sung, khong cch ca S=4 l 8 (tng t nh TSS) v khc 8 nu S=1 t v tr tm kim ban u. Nu gi tr ti v tr ban u l thp nht th vic tm kim s dng li ti v vector chuyn ng c mc nh l (0, 0). Nu trng s thp nht bt k v tr no trong 8 v tr ti S=1, th chng ta thay i v tr tm kim ban u v kim tra cc trng s xng quanh n. Ty thuc vo cc im m chng ta ch kim 5 hoc 3 im (hnh 7(b) & (c)). V tr cho trng s thp nht ph hp nht v vector chuyn ng s c thit t ti y. Mt khc nu trng s tm c l thp nht sau bc u tin l mt trong 8 v tr ti S = 4, th chng ta s tin hnh TSS theo cch thng thng. Do , mc d qu trnh ny c th cn ti thiu 17 im kim tra mi macro block, nhng n cng c trng hp xu nht ca 33 v tr c kim tra.
Hnh 5. Tm kim mu tng ng vi mi gc phn t: (a) Biu din cc gc phn t (b) Gc phn t th I c chn (c) Gc phn t th II c chn (d) Gc phn t th III c chn (e) Gc phn t th IV c chn.
Hnh 6. Qu trnh SES. Trong v d ny vector chuyn ng l (3,7)D. Tm kim bn bc (4SS)Tng t nh NTSS, 4SS [6] cng s dng tm kim center biased v qui nh cch chn na chng. 4SS thit t mt kch thc mu c nh ca S = 2 trong bc u tin, khng cn bit gi tr tham s tm kim p l g. V vy, n trng nh 9 v tr trong mt ca s c 5x5. Nu trng s tm c ti trung tm ca ca s tm kim th vic tm kim c nhy sang bc bn. Nu trng s l mt trong 8 v tr ngoi tr trung tm, th ta c v tr tm kim ban u v chuyn n bc th hai. Ca s tm kim vn c duy tr nh l mt im nh c 5x5. Ty thuc vo ni c trng s thp nht, chng ta ch kim tra trng s ti 3 hoc 5 v tr. M hnh mu c biu din trong hnh 7. Lp li ln na nu v tr c trng s thp nht trung tm ca ca s tm kim 5 x 5 chng ta nhy n bc bn hoc chuyn sang bc ba. Bc th 3 ging y nh bc th 2. TRONG bc th t kch thc ca s gim xung cn 3 x 3, tc l S = 1. V tr vi trng s thp nht c nh x macro block tt nht v vector chuyn ng c thit lp ch a im . Qu trnh ly mu c th hin trong hnh 8. y l thut ton tm kim tt nht trong trng hp 17 im kim tra v xu nht trong trng hp 27 im kim tra.
Hnh 7. Qu trnh tm kim 4 bc. Vecto chuyn ng l (3, -7)