Ua ntej tham txog cov yees duab, cia peb tham txog cov duab ua ntej. Ib daim duab yog tsim los ntawm ntau cov xim dots. Qhov taw tes no yog hu ua pixel, uas yog lub hauv paus ntawm cov duab tso saib. Feem ntau, qhov loj ntawm daim duab yog 1920 x 1080, uas yog 1920 pixels ntev thiab 1080 pixels dav. Cov khoom yog 2073600, uas txhais tau tias cov duab no yog ob lab pixels. 1920 x 1080, tseem hu ua kev daws teeb meem ntawm daim duab no.
Yog li, dab tsi feem ntau hu ua PPI? PPI yog tus naj npawb ntawm pixels ib nti. Qhov ntawd yog hais tias, pes tsawg "pixels" tuaj yeem tso rau ntawm lub vijtsam ntawm lub xov tooj ntawm tes (lossis saib) ib nti. Qhov siab dua PPI, qhov pom tseeb thiab muag ntau dua cov duab yuav yog.
Hauv cov tshuab computer, cov xim tau qhia siv cov lej. Hauv computers, R, G, thiab B kuj raug hu ua "thawj xim yam". Lawv qhov tseem ceeb ntawm 0 mus rau 255, nrog rau tag nrho ntawm 256 qib (256 yog lub zog thib 8 ntawm 2). Yog li, txhua xim tuaj yeem sawv cev los ntawm kev sib xyaw ntawm R, G, thiab B.
Los ntawm txoj kev no, tag nrho ntawm 256 x 256 x 256=16777216 xim tuaj yeem nthuav tawm, tseem hu ua 16 lab xim. RGB peb xim, txhua tus nrog 8 khoom, qhia cov xim hauv txoj kev no, tseem hu ua 24-ntsis xim (nyob 24 khoom). Cov xim no tau dhau tag nrho cov xim pom rau tib neeg lub qhov muag, yog li nws tseem hu ua xim tseeb. Txawm hais tias nws siab npaum li cas, nws tsis muaj nuj nqis rau tib neeg lub qhov muag thiab tsis paub meej.
Tom qab tham txog cov duab, tam sim no cia peb pib tham txog cov yeeb yaj kiab.
Ntau cov duab txuas ua ke ua ib qho video. Qhov tseem ceeb tshaj plaws hauv kev ntsuas qhov yees duab yog nws tus ncej tus nqi. Hauv ib daim vis dis aus, lub thav duab hais txog ib daim duab tseem. Ncej tus nqi yog hais txog tus naj npawb ntawm cov thav ntawv suav nrog hauv video ib ob. Qhov siab dua tus ncej tus nqi, ntau qhov tseeb thiab du cov yeeb yaj kiab ua. Tom qab muaj cov vis dis aus, muaj ob qhov teeb meem cuam tshuam: kev cia thiab kev sib kis. Tus yuam sij rau video encoding nyob rau hauv no: ib tug yees duab, yog tsis encoded, muaj ib tug loj heev ntim.
Coj ib daim vis dis aus nrog kev daws teeb meem ntawm 1920 x 1280 thiab tus ncej tus nqi ntawm 30 ua piv txwv. 1920 x 1280=2073600 pixels ib pixel, uas txhais tau tias 2073600 x 24=49766400 khoom rau ib daim duab. 8 bits =1 byte. Yog li ntawd, 49766400 ntsis=6220800 bytes ≈ 6.22MB. Qhov no yog thawj qhov loj me ntawm 1920 x 1280 duab, muab faib los ntawm tus ncej tus nqi ntawm 30, uas txhais tau hais tias qhov video loj ib ob yog 186.6MB, uas yog kwv yees li 11GB ib feeb. Ib zaj yeeb yaj kiab 90 feeb yog kwv yees li 1000GB.
Dab tsi yog encoding?
Encoding yog tus txheej txheem ntawm kev hloov cov ntaub ntawv los ntawm ib daim ntawv (hom ntawv) mus rau lwm qhov siv ib txoj kev teev tseg. Video encoding yog tus txheej txheem ntawm converting ib video hom ntawv rau lwm tus. Lub hom phiaj kawg ntawm coding yog compress. Ntau txoj kev yees duab encoding yog siv los ua cov yeeb yaj kiab me me, uas yog qhov zoo rau kev khaws cia thiab kev sib kis. Cia peb saib tag nrho cov txheej txheem ntawm kev kaw thiab ua si cov yeeb yaj kiab ua ntej
Firstly, muaj video capture. Feem ntau, cov koob yees duab yog siv rau kev ntes video. Tom qab sau cov ntaub ntawv video, kev hloov pauv ntawm analog-rau-digital yuav tsum hloov pauv cov teeb liab analog rau hauv cov teeb liab digital. Qhov tseeb, niaj hnub no ntau lub koob yees duab ncaj qha tso tawm cov cim digital. Tom qab lub teeb liab tso zis, preprocessing tseem yuav tsum tau hloov RGB teeb liab rau hauv YUV teeb liab.
Yav dhau los, RGB teeb liab tau qhia, tab sis YUV teeb liab yog dab tsi?
Yooj yim muab tso, YUV yog lwm tus sawv cev digital ntawm cov xim. Yog vim li cas vim li cas cov kev sib txuas lus video siv YUV es tsis txhob RGB yog vim RGB cov teeb liab tsis haum rau compression. Nyob rau hauv YUV, lub tswvyim ntawm brightness yog ntxiv.
Nyob rau hauv kaum xyoo dhau los, video engineers tau pom tias ob lub qhov muag muaj kev nkag siab ntau dua rau qhov ci thiab qhov tsaus ntuj dua li xim, txhais tau hais tias tib neeg lub qhov muag tsis tshua muaj siab rau chromaticity dua li qhov ci. Yog li, engineers ntseeg hais tias nyob rau hauv video cia, nws tsis yog tsim nyog los khaws tag nrho cov xim teeb liab. Peb tuaj yeem faib cov bandwidth ntau dua rau cov teeb liab dawb dub (hu ua "brightness") thiab me ntsis bandwidth rau xim xim (hu ua "chromaticity"). Yog li ntawd, muaj YUV.
Lub "Y" hauv YUV sawv cev qhov ci ntsa iab (Luma), thaum "U" thiab "V" sawv cev chromaticity (Chroma).
Qee lub sij hawm pom Y'CbCr, tseem hu ua YUV, yog qhov compressed version ntawm YUV. Qhov txawv yog qhov tseeb tias Y'CbCr siv nyob rau hauv cov duab digital, YUV siv nyob rau hauv cov teeb liab analog, thiab feem ntau siv YUV hauv MPEG, DVD, thiab cov koob yees duab yog tiag tiag Y'CbCr.