{"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\/sq\/blog\/administrirovanie\/jpeg-algoritm-szhatiya","title":{"rendered":"JPEG. Algoritmi i kompresimit","gt_translate_keys":[{"key":"rendered","format":"text"}]},"content":{"rendered":"

P\u00ebrsh\u00ebndetje p\u00ebrs\u00ebri! Gjeta k\u00ebt\u00eb artikull, i shkruar n\u00eb maj t\u00eb vitit 2019. Ky \u00ebsht\u00eb nj\u00eb vazhdim i seris\u00eb s\u00eb artikujve mbi WAVE dhe JPEG. Ja e para<\/a><\/noindex>. Ky publikim do t\u00eb p\u00ebrfshij\u00eb informacion mbi algoritmin e kodimit t\u00eb imazheve dhe p\u00ebr formatin n\u00eb p\u00ebrgjith\u00ebsi.<\/p>\n

<\/p>\n

Nj\u00eb grimc\u00eb historie<\/h3>\n

<\/p>\n

Nj\u00eb lug\u00eb t\u00eb madhe artikulli nga Wikipedia:<\/p>\n

<\/p>\n

JPEG (Grupi Ekspert\u00ebve t\u00eb Fotografive t\u00eb Bashkuar) \u00ebsht\u00eb nj\u00eb nga formatet m\u00eb t\u00eb njohura t\u00eb grafikut raster, i p\u00ebrdorur p\u00ebr ruajtjen e fotografive dhe imazheve t\u00eb ngjashme.<\/p><\/blockquote>\n

Ky standart u zhvillua nga Grupi i Ekspert\u00ebve mbi Fotografit\u00eb q\u00eb n\u00eb vitin 1991 p\u00ebr kompresimin efikas t\u00eb imazheve.<\/p>\n

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

Cila \u00ebsht\u00eb rruga q\u00eb ndjekin imazhet nga forma e kriptuar deri n\u00eb JPEG<\/h3>\n

<\/p>\n

Disa besojn\u00eb se imazhet JPEG jan\u00eb t\u00eb dh\u00ebna t\u00eb pap\u00ebrpunuara t\u00eb kompresuara me metod\u00ebn Huffman, por kjo nuk \u00ebsht\u00eb e v\u00ebrtet\u00eb. Para kompresimit p\u00ebrfundimtar, t\u00eb dh\u00ebnat kalojn\u00eb p\u00ebrmes nj\u00eb rrug\u00eb t\u00eb gjat\u00eb.<\/p>\n

<\/p>\n

S\u00eb pari ndryshohet modeli i ngjyrave nga RGB n\u00eb YCbCr. P\u00ebr k\u00ebt\u00eb ka edhe nj\u00eb algorit\u00ebm t\u00eb ve\u00e7ant\u00eb \u2014 k\u00ebtu<\/a><\/noindex>. Y nuk preket, sepse ai \u00ebsht\u00eb p\u00ebrgjegj\u00ebs p\u00ebr ndri\u00e7imin dhe nj\u00eb ndryshim do t\u00eb ishte i duksh\u00ebm.<\/p>\n

<\/p>\n

E para q\u00eb b\u00ebhet me imazhin \u00ebsht\u00eb \"rregullimi\"<\/strong> (subsampling). E kupton thjesht: merret nj\u00eb grumbull 2x2 piksel\u00ebsh, pastaj merret Cb dhe Cr \u2014 mesatarja e secil\u00ebs nga komponent\u00ebt YCbCr t\u00eb k\u00ebtyre 4 pikseve. Dhe k\u00ebshtu, ne fituam 6 byte<\/em>, n\u00eb vend t\u00eb 4 Y, 4 Cb, 4 Cr, mor\u00ebm 4 Y dhe t\u00eb nj\u00ebjtat p\u00ebr secilin prej tyre Cb dhe Cr (4 + 4 + 4 = 12; 4 + 1 + 1 = 6; 12 \u2014 6 = 6). N\u00eb shkall\u00eb edhe 2\u00d72, kompresimi me humbje me nj\u00eb koeficient kompresimi 2:1 duket i r\u00ebnd\u00ebsish\u00ebm. Kjo aplikohet n\u00eb t\u00eb gjith\u00eb imazhin. Dhe k\u00ebshtu \u2014 e shkurtuam gjysm\u00ebn e madh\u00ebsis\u00eb. Dhe nj\u00eb teknik\u00eb t\u00eb till\u00eb mund ta p\u00ebrdorim p\u00ebr shkak t\u00eb perceptimit ton\u00eb t\u00eb ngjyrave. Nj\u00eb njeri do t\u00eb v\u00ebrej\u00eb leht\u00ebsisht ndryshimin n\u00eb ndritshm\u00ebri, por jo n\u00eb ngjyr\u00eb, n\u00ebse ajo \u00ebsht\u00eb e mesatareshme n\u00eb nj\u00eb bllok t\u00eb vog\u00ebl pikselash. Gjithashtu, rregullimi mund t\u00eb kryhet n\u00eb linj\u00eb, 4 piksel\u00eb horizontalisht dhe verticalisht. Varianti i par\u00eb p\u00ebrdoret m\u00eb shpesh. N\u00ebse \u00ebsht\u00eb e r\u00ebnd\u00ebsishme cil\u00ebsia e imazhit, at\u00ebher\u00eb rregullimi nuk kryhet fare.
\nIlustrimi i qart\u00eb i nd\u00ebrrimit (Habr nuk e lejoi t\u00eb vendosja gifin) \u2014 https:\/\/i.ibb.co\/Rg5Th9H\/150953010617579181.gif<\/a><\/noindex><\/p>\n

<\/p>\n

Pjesa kryesore e p\u00ebrgatitjes<\/h5>\n

\nDCT<\/p>\n

Tani, pjesa m\u00eb e v\u00ebshtir\u00eb dhe e nevojshme. Imazhi ndahet n\u00eb blloqe 8\u00d78 (p\u00ebrdoret mbushja n\u00eb rast se rezolucioni nuk \u00ebsht\u00eb nj\u00eb shum\u00ebfish i an\u00ebs s\u00eb bllokut).<\/p>\n

<\/p>\n

Tani \u00e7do bllokut i aplikohet DCT (Transformimi Diskret i Kosin\u00ebve)<\/strong>. N\u00eb k\u00ebt\u00eb pjes\u00eb, t\u00eb gjitha gj\u00ebrat e tep\u00ebrta hiqen nga imazhi. Duke p\u00ebrdorur DCP, duhet t\u00eb kuptojm\u00eb n\u00ebse ky bllok (8\u00d78) p\u00ebrshkruan ndonj\u00eb pjes\u00eb monotone t\u00eb imazhit: qiell, mure; apo ai p\u00ebrmban nj\u00eb struktur\u00eb t\u00eb nd\u00ebrlikuar (flok\u00eb, simbole etj.). \u00cbsht\u00eb logjike q\u00eb 64 piksel\u00eb t\u00eb ngjyrave t\u00eb ngjashme mund t\u00eb p\u00ebrshkruhen me vet\u00ebm 1, pasi madh\u00ebsia e bllokut tashm\u00eb \u00ebsht\u00eb e njohur. K\u00ebtu e keni dhe kompresimin: 64 deri n\u00eb 1.<\/p>\n

<\/p>\n

DCT shnd\u00ebrron bllokun n\u00eb spekt\u00ebr, dhe aty ku leximet ndodhin papritmas, koeficienti b\u00ebhet pozitiv, dhe sa m\u00eb e fort\u00eb t\u00eb jet\u00eb kalimi, aq m\u00eb i lart\u00eb do t\u00eb jet\u00eb rezultati. Aty ku koeficienti \u00ebsht\u00eb m\u00eb i lart\u00eb, n\u00eb imazh jan\u00eb t\u00eb p\u00ebrfaq\u00ebsuara kalimet e qarta n\u00eb ngjyr\u00eb dhe ndri\u00e7im, nd\u00ebrsa aty ku \u00ebsht\u00eb m\u00eb e ul\u00ebt \u2014 kalime t\u00eb dob\u00ebta (t\u00eb buta) t\u00eb p\u00ebrb\u00ebr\u00ebsve YCbCr n\u00eb bllok.<\/p>\n

\nKvantizimi<\/p>\n

K\u00ebtu tashm\u00eb aplikohen cil\u00ebsimet e kompresimit. \u00c7do coeficient n\u00eb secil\u00ebn prej matricave 8\u00d78 ndahet me nj\u00eb num\u00ebr t\u00eb caktuar. N\u00ebse cil\u00ebsia e imazhit pas t\u00eb gjitha modifikimeve nuk do t\u00eb ulet m\u00eb, at\u00ebher\u00eb ndah\u00ebsi duhet t\u00eb jet\u00eb nj\u00eb. N\u00ebse ju intereson m\u00eb shum\u00eb kujtesa q\u00eb z\u00eb kjo foto, at\u00ebher\u00eb ndah\u00ebsi do t\u00eb jet\u00eb m\u00eb i madh se 1, dhe mbetja rrethore. K\u00ebshtu del q\u00eb pas rrethimit shpesh rezulton shum\u00eb zero.<\/p>\n

<\/p>\n

Kvantizimi b\u00ebhet p\u00ebr t\u00eb krijuar mund\u00ebsin\u00eb e nj\u00eb kompresioni edhe m\u00eb t\u00eb madh. Ja si duket me shembullin e grafikut y = sin(x):<\/p>\n

<\/p>\n

\"JPEG.<\/p>\n

\nKompresimi<\/p>\n

Fillimisht kalojm\u00eb p\u00ebrmes matric\u00ebs n\u00eb zig-zag:<\/p>\n

<\/p>\n

\"JPEG.<\/p>\n

<\/p>\n

Marrim nj\u00eb varg t\u00eb dh\u00ebnash nj\u00eb-dimensional. Ne shohim q\u00eb ka shum\u00eb zero, mund t'i heqim ato. P\u00ebr k\u00ebt\u00eb, n\u00eb vend t\u00eb nj\u00eb sekuence me shum\u00eb zero, shkruajm\u00eb 1 zero dhe pas tij nj\u00eb num\u00ebr q\u00eb tregon sasin\u00eb e tyre n\u00eb sekuenc\u00eb. K\u00ebshtu mund t\u00eb reduktojm\u00eb deri n\u00eb 1\/3 t\u00eb madh\u00ebsis\u00eb s\u00eb gjith\u00eb vargut. M\u00eb pas thjesht e kompresojm\u00eb k\u00ebt\u00eb varg me metod\u00ebn e Huffmanit dhe e shkruajm\u00eb n\u00eb dosjen vet\u00eb.<\/p>\n

<\/p>\n

Ku p\u00ebrdoret<\/h3>\n

<\/p>\n

K\u00ebshtu, kudo. Si PNG, JPEG p\u00ebrdoret n\u00eb aparatet fotografik\u00eb, OS 't\u00eb (si logot e kompanive, ikonat e aplikacioneve, miniature) dhe n\u00eb t\u00eb gjitha fushat ku \u00ebsht\u00eb e nevojshme t\u00eb ruhet efektivisht imazhet.<\/p>\n

<\/p>\n

P\u00ebrfundimi<\/h3>\n

<\/p>\n

Aktualisht, njohurit\u00eb rreth JPEG jan\u00eb t\u00eb vlefshme vet\u00ebm p\u00ebr q\u00ebllime arsimore, sepse ai tashm\u00eb \u00ebsht\u00eb i integruar n\u00eb \u00e7do vend dhe \u00ebsht\u00eb optimizuar nga grupe t\u00eb m\u00ebdha njer\u00ebzish, por guri i shkenc\u00ebs megjith\u00ebse \u00ebsht\u00eb i shijsh\u00ebm.<\/p>\n

\nBurimet<\/p>\n

Artikulli mbi YCbCr n\u00eb Wikipedia<\/a><\/noindex>
\nArtikulli mbi JPEG n\u00eb Wikipedia<\/a><\/noindex>
\nPak p\u00ebr DCT nga postimi n\u00eb Pikabu<\/a><\/noindex>
\nArtikulli mbi DCT n\u00eb Wikipedia<\/a><\/noindex><\/p>\n

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

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