{"id":54788,"date":"2020-01-04T00:00:00","date_gmt":"2020-01-03T21:00:00","guid":{"rendered":"https:\/\/prohoster.info\/blog\/blog_prohoster\/jpeg-algoritm-szhatiya"},"modified":"2020-02-18T14:02:50","modified_gmt":"2020-02-18T11:02:50","slug":"jpeg-algoritm-szhatiya","status":"publish","type":"post","link":"https:\/\/prohoster.info\/hu\/blog\/administrirovanie\/jpeg-algoritm-szhatiya","title":{"rendered":"JPEG. T\u00f6m\u00f6r\u00edt\u00e9si algoritmus","gt_translate_keys":[{"key":"rendered","format":"text"}]},"content":{"rendered":"

\u00dcdv \u00fajra! Tal\u00e1ltam egy cikket, amelyet 2019 m\u00e1jus\u00e1ban \u00edrtak. Ez a WAVE \u00e9s JPEG sorozat cikkjeinek folytat\u00e1sa. az els\u0151<\/a><\/noindex>. Ez a publik\u00e1ci\u00f3 inform\u00e1ci\u00f3t tartalmaz a k\u00e9pk\u00f3dol\u00e1si algoritmusr\u00f3l \u00e9s mag\u00e1r\u00f3l a form\u00e1tumr\u00f3l egyar\u00e1nt.<\/p>\n

<\/p>\n

Egy csipet t\u00f6rt\u00e9nelem<\/h3>\n

<\/p>\n

Egy ev\u0151kan\u00e1lnyi cikk a Wikip\u00e9di\u00e1b\u00f3l:<\/p>\n

<\/p>\n

A JPEG (Joint Photographic Experts Group) az egyik legn\u00e9pszer\u0171bb raszteres grafikai form\u00e1tum, amelyet f\u00e9nyk\u00e9pek \u00e9s hasonl\u00f3k t\u00e1rol\u00e1s\u00e1ra haszn\u00e1lnak.<\/p><\/blockquote>\n

Ezt a szabv\u00e1nyt a f\u00e9nyk\u00e9pez\u00e9shez kapcsol\u00f3d\u00f3 szak\u00e9rt\u0151csoport 1991-ben alkotta meg a k\u00e9pek hat\u00e9kony t\u00f6m\u00f6r\u00edt\u00e9se \u00e9rdek\u00e9ben.<\/p>\n

<\/a><\/noindex><\/p>\n

Milyen utat j\u00e1rnak be a k\u00e9pek a nyers form\u00e1tumt\u00f3l a JPEG-ig<\/h3>\n

<\/p>\n

N\u00e9h\u00e1nyan azt hiszik, hogy a JPEG-k\u00e9pek a Huffman-k\u00f3dol\u00e1s \u00e1ltal t\u00f6m\u00f6r\u00edtett nyers adatok, de ez nem igaz. A v\u00e9gs\u0151 t\u00f6m\u00f6r\u00edt\u00e9s el\u0151tt az adatok hossz\u00fa utat j\u00e1rnak be.<\/p>\n

<\/p>\n

El\u0151sz\u00f6r a sz\u00ednmodell \u00e1talakul RGB-b\u0151l YCbCr-re. Ehhez m\u00e9g egy k\u00fcl\u00f6n algoritmus is rendelkez\u00e9sre \u00e1ll \u2014 itt<\/a><\/noindex>. Az Y \u00e9rt\u00e9ket nem v\u00e1ltoztatjuk meg, mivel az felel\u0151s a f\u00e9nyer\u0151\u00e9rt, \u00e9s a m\u00f3dos\u00edt\u00e1sa \u00e9szlelhet\u0151 lenne.<\/p>\n

<\/p>\n

Az els\u0151 l\u00e9p\u00e9s, ami a k\u00e9ppel t\u00f6rt\u00e9nik \u2014 \u201emintav\u00e9tel\u201c<\/strong> (subsampling). Szerezd ezt meg k\u00f6nnyen: egy 2x2 pixel t\u00f6mb\u00f6t vesz\u00fcnk, majd a Cb \u00e9s Cr \u00e9rt\u00e9keket \u2014 az YCbCr \u00f6sszetev\u0151k \u00e1tlagos \u00e9rt\u00e9keit a n\u00e9gy pixelb\u0151l. \u00cdgy 6 b\u00e1jt nyert\u00fcnk<\/em>, a 4 Y, 4 Cb, 4 Cr helyett 4 Y-t \u00e9s mindegyikhez azonos Cb-t \u00e9s Cr-t kaptunk (4 + 4 + 4 = 12; 4 + 1 + 1 = 6; 12 \u2014 6 = 6). M\u00e9g 2x2-es m\u00e9retekben is a 2:1 ar\u00e1ny\u00fa vesztes\u00e9ges t\u00f6m\u00f6r\u00edt\u00e9s meglep\u0151en hangzik. Ez az eg\u00e9sz k\u00e9pre vonatkozik. \u00cdgy feleakkora m\u00e9retet \u00e9r\u00fcnk el. Erre a sz\u00edn\u00e9rz\u00e9kel\u00e9s\u00fcnkkel tudunk t\u00e1maszkodni. Az ember k\u00f6nnyed\u00e9n \u00e9szleli a f\u00e9nyess\u00e9gk\u00fcl\u00f6nbs\u00e9get, de nem a sz\u00ednelt\u00e9r\u00e9st, ha az egy kis mintapontban \u00e1tlagos. A mintav\u00e9tel egy vonalban is v\u00e9gezhet\u0151, 4 pixel v\u00edzszintesen \u00e9s f\u00fcgg\u0151legesen. Az els\u0151 v\u00e1ltozat a gyakoribb. Ha a k\u00e9p min\u0151s\u00e9ge fontos, akkor a mintav\u00e9telt egy\u00e1ltal\u00e1n nem alkalmazz\u00e1k.
\nA minta cs\u00f6kkent\u00e9st szeml\u00e9ltet\u0151 \u00e1bra (a Habr nem engedte a gif beilleszt\u00e9s\u00e9t) \u2014 https:\/\/i.ibb.co\/Rg5Th9H\/150953010617579181.gif<\/a><\/noindex><\/p>\n

<\/p>\n

A felk\u00e9sz\u00fcl\u00e9s f\u0151 r\u00e9sze<\/h5>\n

\nDCT<\/p>\n

Most k\u00f6vetkezik a legbonyolultabb \u00e9s legfontosabb r\u00e9sz. Az eg\u00e9sz k\u00e9p 8x8-as blokkokra van felosztva (t\u00f6lt\u00e9st haszn\u00e1lnak, ha a felbont\u00e1s nem oszthat\u00f3 a blokk oldal\u00e1val).<\/p>\n

<\/p>\n

Most minden blokkra alkalmazz\u00e1k a DCT-t (Diszkr\u00e9t Koszinusz Transzform\u00e1ci\u00f3)<\/strong>. Ebben a r\u00e9szben a k\u00e9p minden felesleges r\u00e9sz\u00e9t elt\u00e1vol\u00edtj\u00e1k. A DCT (diszkr\u00e9t koszinusz transzform\u00e1ci\u00f3) haszn\u00e1lat\u00e1val meg kell \u00e1llap\u00edtani, hogy az adott blokk (8x8) le\u00edr-e valamilyen monot\u00f3n r\u00e9szt a k\u00e9pb\u0151l: az \u00e9g, fal; vagy bonyolult strukt\u00far\u00e1t tartalmaz (haj, szimb\u00f3lumok stb.). Logikusan, 64 hasonl\u00f3 sz\u00edn\u0171 pixel le\u00edrhat\u00f3 egyetlen pixellel, mivel a blokk m\u00e9rete m\u00e1r ismert. \u00cdme a t\u00f6m\u00f6r\u00edt\u00e9s: 64-hez 1.<\/p>\n

<\/p>\n

A DCT egy blokkot spektrumra alak\u00edt \u00e1t, \u00e9s ahol az \u00e9rt\u00e9kek \u00e9les v\u00e1lt\u00e1sokon mennek kereszt\u00fcl, a koefficiens pozit\u00edvv\u00e1 v\u00e1lik, \u00e9s min\u00e9l \u00e9lesebb a v\u00e1lt\u00e1s, ann\u00e1l magasabb lesz az eredm\u00e9ny. Ahol a koefficiens magasabb, ott a k\u00e9pen \u00e9les sz\u00edn- \u00e9s f\u00e9nyer\u0151-\u00e1tmenetek l\u00e1that\u00f3k, ahol alacsonyabb, ott enyhe (l\u00e1gy) v\u00e1ltoz\u00e1sok vannak a YCbCr komponensek m\u00e9rt\u00e9keiben a blokkban.<\/p>\n

\nKvantis\u00e1l\u00e1s<\/p>\n

Itt m\u00e1r a t\u00f6m\u00f6r\u00edt\u00e9si be\u00e1ll\u00edt\u00e1sok alkalmaz\u00e1sra ker\u00fclnek. Minden egyes 8x8-as m\u00e1trixban minden egyes egy\u00fctthat\u00f3t felosztanak egy adott sz\u00e1mra. Ha a k\u00e9p min\u0151s\u00e9g\u00e9t a m\u00f3dos\u00edt\u00e1sok ut\u00e1n m\u00e1r nem k\u00edv\u00e1nja cs\u00f6kkenteni, akkor a oszt\u00f3 egyes kell hogy legyen. Ha fontosabb a f\u00e9nyk\u00e9p \u00e1ltal elfoglalt mem\u00f3ria, akkor az oszt\u00f3 nagyobb lesz, mint 1, \u00e9s a h\u00e1nyados kerek\u00edtve lesz. \u00cdgy, a kerek\u00edt\u00e9s ut\u00e1n gyakran sok null\u00e1t kapunk.<\/p>\n

<\/p>\n

A kvantis\u00e1l\u00e1st az\u00e9rt v\u00e9gzik, hogy lehet\u0151v\u00e9 tegy\u00e9k a tov\u00e1bbi t\u00f6m\u00f6r\u00edt\u00e9st. \u00cdme, hogy n\u00e9z ki a kvantis\u00e1l\u00e1s p\u00e9ld\u00e1ja a y = sin(x) grafikonon:<\/p>\n

<\/p>\n

\"JPEG.<\/p>\n

\nT\u00f6m\u00f6r\u00edt\u00e9s<\/p>\n

El\u0151sz\u00f6r v\u00e9gigj\u00e1rjuk a m\u00e1trixot zig-zag ir\u00e1nyban:<\/p>\n

<\/p>\n

\"JPEG.<\/p>\n

<\/p>\n

Kapunk egy egy dimenzi\u00f3s t\u00f6mb\u00f6t sz\u00e1mokkal. L\u00e1tjuk, hogy sok nulla van benne, amelyeket el lehet t\u00e1vol\u00edtani. Ehhez a sok nulla helyett 1 null\u00e1t \u00edrunk, majd ut\u00e1na egy sz\u00e1mot, amely a sorozatban tal\u00e1lhat\u00f3 null\u00e1k sz\u00e1m\u00e1t jel\u00f6li. \u00cdgy a teljes t\u00f6mb m\u00e9rete ak\u00e1r 1\/3-ra is cs\u00f6kkenhet. Azt\u00e1n ezt a t\u00f6mb\u00f6t egyszer\u0171en a Huffman-m\u00f3dszerrel t\u00f6m\u00f6r\u00edtj\u00fck, \u00e9s m\u00e1ris be\u00edrjuk a f\u00e1jlba.<\/p>\n

<\/p>\n

Hol haszn\u00e1lj\u00e1k<\/h3>\n

<\/p>\n

Mindenhol. Mint a PNG, a JPEG-t is f\u00e9nyk\u00e9pez\u0151g\u00e9pekben, oper\u00e1ci\u00f3s rendszerekben (v\u00e1llalati log\u00f3k, alkalmaz\u00e1sikonok, thumbnail-\u00f6k form\u00e1j\u00e1ban) \u00e9s minden olyan ter\u00fcleten haszn\u00e1lj\u00e1k, ahol hat\u00e9konyan kell t\u00e1rolni a k\u00e9peket.<\/p>\n

<\/p>\n

K\u00f6vetkeztet\u00e9s<\/h3>\n

<\/p>\n

Jelenleg a JPEG ismerete csak oktat\u00e1si c\u00e9lokra \u00e9rt\u00e9kes, mivel m\u00e1r mindenhol be van \u00e9p\u00edtve \u00e9s nagy csoportok optimaliz\u00e1lt\u00e1k, de a tudom\u00e1ny szikl\u00e1i m\u00e9gis \u00edzletesek.<\/p>\n

\nForr\u00e1sok<\/p>\n

YCbCr Wikipedia cikk<\/a><\/noindex>
\nJPEG Wikipedia cikk<\/a><\/noindex>
\nEgy kis inform\u00e1ci\u00f3 a DCT-r\u0151l a Pikabu posztja alapj\u00e1n<\/a><\/noindex>
\nA DCT Wikipedia cikk<\/a><\/noindex><\/p>\n

Forr\u00e1s: habr.com<\/a><\/p>","protected":false,"gt_translate_keys":[{"key":"rendered","format":"html"}]},"excerpt":{"rendered":"

\u0418 \u0441\u043d\u043e\u0432\u0430 \u0437\u0434\u0440\u0430\u0432\u0441\u0442\u0432\u0443\u0439\u0442\u0435! \u042f \u043d\u0430\u0448\u0435\u043b \u044d\u0442\u0443 \u0441\u0442\u0430\u0442\u044c\u044e, \u043d\u0430\u043f\u0438\u0441\u0430\u043d\u043d\u0443\u044e \u0435\u0449\u0435 \u043c\u0430\u0435 2019-\u043e\u0433\u043e \u0433\u043e\u0434\u0430. \u042d\u0442\u043e \u2014 \u043f\u0440\u043e\u0434\u043e\u043b\u0436\u0435\u043d\u0438\u0435 \u0441\u0435\u0440\u0438\u0438 \u0441\u0442\u0430\u0442\u0435\u0439 \u043e WAVE \u0438 JPEG, \u0412\u043e\u0442 \u043f\u0435\u0440\u0432\u0430\u044f. \u042d\u0442\u0430 \u043f\u0443\u0431\u043b\u0438\u043a\u0430\u0446\u0438\u044f \u0432\u043a\u043b\u044e\u0447\u0438\u0442 \u0432 \u0441\u0435\u0431\u0435 \u0438\u043d\u0444\u043e\u0440\u043c\u0430\u0446\u0438\u044e \u043e\u0431 \u0430\u043b\u0433\u043e\u0440\u0438\u0442\u043c\u0435 \u043a\u043e\u0434\u0438\u0440\u043e\u0432\u0430\u043d\u0438\u044f \u0438\u0437\u043e\u0431\u0440\u0430\u0436\u0435\u043d\u0438\u0439 \u0438 \u043e \u0441\u0430\u043c\u043e\u043c \u0444\u043e\u0440\u043c\u0430\u0442\u0435 \u0432 \u0446\u0435\u043b\u043e\u043c. \u0429\u0435\u043f\u043e\u0442\u043a\u0443 \u0438\u0441\u0442\u043e\u0440\u0438\u0438 \u0421\u0442\u043e\u043b\u043e\u0432\u0443\u044e \u043b\u043e\u0436\u043a\u0443 \u0441\u0442\u0430\u0442\u044c\u0438 \u0438\u0437 \u0412\u0438\u043a\u0438\u043f\u0435\u0434\u0438\u0438: JPEG (Joint Photographic Experts Group) \u2014 \u043e\u0434\u0438\u043d \u0438\u0437 \u043f\u043e\u043f\u0443\u043b\u044f\u0440\u043d\u044b\u0445 […]<\/p>\n","protected":false,"gt_translate_keys":[{"key":"rendered","format":"html"}]},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[688],"tags":[],"class_list":["post-54788","post","type-post","status-publish","format-standard","hentry","category-administrirovanie"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\n","aioseo_head_json":{"title":"\ud83e\udd47JPEG. 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