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第8章圖像壓縮(圖像編碼)目的是減少表示數(shù)字圖像的數(shù)據(jù)量。去除冗余。35年前,電視傳輸?shù)膸拤嚎s;60年前,C.E.Shannon等的信息理論發(fā)展成目前的國(guó)際圖像壓縮標(biāo)準(zhǔn)。目前,圖像壓縮技術(shù)被認(rèn)為是一種“使能技術(shù)”(enablingtechnology”)。提高圖像分辨率,開發(fā)新的電視標(biāo)準(zhǔn),電視會(huì)議、RS、FAX等。Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.8.1基礎(chǔ)數(shù)據(jù)壓縮——表示定量信息所需數(shù)據(jù)量的減少過程數(shù)據(jù)、信息、知識(shí)。比如,不同人講相同的事件。數(shù)據(jù)冗余:是數(shù)字圖像壓縮的中心問題。這不是抽象的概念,而是可以用數(shù)學(xué)式子度量的實(shí)體:如果表示相同信息的兩組數(shù)據(jù)所用的信息載體單元數(shù)量分別為n1和n2,則第一組數(shù)據(jù)的相對(duì)(第二組數(shù)據(jù)的)數(shù)據(jù)冗余RD可定義為:

RD=1-1/CR

∈(-∞,1)其中:CR=n1/n2,∈(0,∞)Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd. RD=1-1/CR

∈(-∞,1)

其中:CR=n1/n2,∈(0,∞)

當(dāng)n1=n2,CR=1而RD=0,表示相對(duì)于第二組數(shù)據(jù),第一種數(shù)據(jù)表達(dá)沒有包含冗余數(shù)據(jù)。當(dāng)n2<<n1,CR∞,RD1,表示顯著壓縮和高度冗余當(dāng)n2>>n1,CR0,RD-∞,表示第二組數(shù)據(jù)比原始表示包含多得多得數(shù)據(jù)。這也是通常不希望出現(xiàn)得數(shù)據(jù)膨脹。比如CR=10,RD=0.9,表示第一組數(shù)據(jù)中有90%的數(shù)據(jù)是冗余的。Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.數(shù)據(jù)冗余的類別

1.空間冗余2.時(shí)間冗余3.信息熵冗余(編碼冗余)4.結(jié)構(gòu)冗余5.知識(shí)冗余6.視覺冗余7.其它冗余在數(shù)字圖像壓縮中,3種基本的數(shù)據(jù)冗余是:編碼冗余,像素間冗余,心理視覺冗余。減少其中1到多種冗余就是數(shù)據(jù)壓縮

Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.8.1.1編碼冗余Code1的二進(jìn)制平均碼長(zhǎng)為3;Code1的二進(jìn)制平均碼長(zhǎng)(∑l2pr)為2.7;Huffman碼壓縮比CR=3/2.7=1.11Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.上例壓縮的原理:l2

(rk)與pr(rk)成反比變長(zhǎng)編碼Code1不能使其平均碼長(zhǎng)(∑l2pr)最小,有編碼冗余。數(shù)字圖像一般都有編碼冗余Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.8.1.2

像元間的冗余c,d直方圖分別對(duì)應(yīng)a,b圖,相同的三峰,兩邊是火柴:等長(zhǎng)碼有編碼冗余,可用變長(zhǎng)碼壓縮。但是,變長(zhǎng)碼還是不能改變像元間的相關(guān)性。計(jì)算像元間的相關(guān)性γ可用P414中的6式。沿某水平線e,f分別反映a,b圖像像元在不同間隔后灰度的相關(guān)程度利用像元值的空間相關(guān)可預(yù)測(cè)Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.減少像元間冗余的一種方法:行程編碼(RLC)Run-LengthCodeabcd,nonvisualp.417,CR的計(jì)算Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.

8.1.3視覺冗余256128643216842在8個(gè)比特的灰度值中,每次去掉一個(gè)bit面,從左/右?Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.8.1.3Psycho-visualRedundancy眼睛對(duì)所有視覺信息的響應(yīng)具有不同的靈敏度。消除心理視覺冗余會(huì)導(dǎo)致數(shù)量信息的損失,又稱量化。原圖256級(jí)16級(jí),2:1IGC16級(jí),2:1Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.量化,不可逆,信息損失IGS,ImprovedGray-ScalequantizationIGS意識(shí)到眼睛對(duì)邊緣特有的敏感性,通過加入偽隨機(jī)數(shù)來破壞邊緣:偽隨機(jī)數(shù)從鄰近像元的低4位獲得,加入當(dāng)前像元灰度值后才量化。(圖像的低bit面很象隨機(jī)數(shù),見p.89)低4位的初值為0000,對(duì)高4位為1111的像素不加隨機(jī)數(shù)。0110110000000110

110010001011

11001001

011110000111

01111000

111011110100

111011110100Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.8.1.4保真度準(zhǔn)則Objective/SubjectiveFidelityCriteriaObjective:SNR,p.420Subjective:judgedbyagroupofpeopleObjective:Subjective=3:7Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.8.2ImageCompressionModels圖8.5通用壓縮系統(tǒng)模型信源編碼:消除輸入冗余。信道編碼:對(duì)源編碼的輸出加強(qiáng)噪聲免疫。Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.abMapper:減少空間冗余Quantizer:減少視覺冗余。不可逆,不失真(熵)編碼要省去。Symbolencoder:減少編碼冗余Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientProfile.Copyright2004-2011AsposePtyLtd.Chapter8ImageCompressionEvaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientPro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