MPEG hareketli görüntü sıkıştırma standardı

Başlık çevirisi mevcut değil.

Tez No: 46284
Yazar: ERHAN TELLİOĞLU
Danışmanlar: DOÇ.DR. MELİH PAZARCI
Tez Türü: Yüksek Lisans
Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
Anahtar Kelimeler: Görüntü sıkıştırma, Image compression
Yıl: 1995
Dil: Türkçe
Üniversite: İstanbul Teknik Üniversitesi
Enstitü: Fen Bilimleri Enstitüsü
Ana Bilim Dalı: Belirtilmemiş.
Bilim Dalı: Belirtilmemiş.
Sayfa Sayısı: Belirtilmemiş.

Özet

ÖZET Sayısal görüntü tekniğindeki gelişmeler sonucu görüntü sıkıştırma tekniğini sayısal yayın kodlama, telekonferans, görüntülü telefon gibi çeşitli haberleşme tiplerine uygulamak mümkün olmuştur. Bu nedenle görüntü sıkıştırma tekniğindeki standartlaşma, farklı üreticilerin ürettiği ürünler arası uyumluluk problemi ve görüntü kodlamanın maliyeti nedeni ile önem kazanmıştır. İlk defa 1988 de ISO( the International Organisation for Standardisation)' da standart için çalışmalara başlanmış ve MPEG ( Moving Pictures Expert Group) ortaya çıkmıştır. MPEG-1 ile hareketli görüntü 1.5 Mbit/s civarında sıkıştırılmış sayısal veri olarak işlenebilir, varolan haberleşme şebekeleri aracılığı ile sayısal olarak taşınabilir. 1.5 Mbit/s civarında iyi bir resim kalitesi elde edebilmek için çok yüksek sıkıştırma oranlarına ihtiyaç vardır. Bu nedenle MPEG de değişik sıkıştırma teknikleri kullanılmıştır. Öncelikle kodlanacak görüntü işareti için uygun bir çözünürlük seçilmelidir. Burada insan gözünün fizyolojik özelliklerine göre bir çözünürlük azaltma işlemi yapılmaktadır. MPEG algoritması daha sonra arka arkaya gelen resimler arası benzerlik ilişkisini kullanarak ardışıl resimler arasında bulunan fazla veriyi, hareket kompanzasyonu (Motion compensation) aracılığı ile azaltmaktadır. Hareket kompanzasyonu, o andaki resmin bir önceki resimden nedensel olarak tahmini, bir sonraki resimden ( Future picture ) nedensel olmayan tahmini veya bir önceki ( Past picture ) ve bir sonraki resimlerden interpolasyon ile tahmini için kullanılır. Hareket vektörleri ( Motion Vectors ) resimdeki her 16x16 nokta matrisi için bulunur. Daha sonra tahmin edilen resimle referans resim arasındaki farka - tahmin hatası- ( prediction error ) uzaysal ( spatial ) korelasyonların atılması için ayrık kosinüs dönüşümü (Discrete Cosine Transform) uygulanır. Kuantalama işlemi yapılarak daha az önemli görüntü bilgileri atıldıktan sonra hareket vektörleri DCT' si alınmış ve kuantalanmış işaretle birleştirilip veri oranım bir kez daha azaltmak için değişken kelime uzunluklu kodlama ( Variable Length Coding ) uygulanarak sıkıştırılmış görüntü işareti elde edilir. MPEG algoritması kullanılarak yapılan bilgisayar simülasyonunda elde edilen veriler sonuç bölümünde incelenmiş, simülasyon programı sonucu elde edilen veriler bu bölümde verilmiştir.

Özet (Çeviri)

SUMMARY The development of digital video technology in the 1980s has made it possible to use digital video compression for a variety of telecommunication applications: teleconferencing, digital broadcast codec and video telephony. Standardisation of video compression techniques has become a high priority because only a standard can reduce the high cost of video compression codecs and resolve the critical problem of interoperability of equipment from different manufacturers. Digital transmission is of prime importance for telecommunication, particularly in the telephone network, but there is a lot more to digital video than teleconferencing and visual telephony. The computer industry, the telecommunication industry and the consumer electronics industry are increasingly sharing the same technology. In the view of shared technology between different segments of the information processing industry, the International Organisation for Standardisation (ISO) has undertaken an effort to develop a standard for video and associated audio on digital storage media, where the concept of digital storage medium includes conventional storage devices CD-ROM, DAT, tape drives, Winchesters disks, writable optical drives, as well as telecommunication channels such as ISDNs and local area networks. The effort is known by the name of the expert group that started it: MPEG ( Moving Picture Expert Group). The MPEG activities cover more than video compression, since the compression of the associated audio and the issue of audio -visual synchronisation cannot be worked independently of the video compression: MPEG-1 Video is addressing the compression of video signals at about 1.5 Mbits, MPEG -Audio is addressing the compression of the digital audio signal at the rates of 64, 128 and 192 kbits/s per channel, MPEG system is addressing the issue of synchronisation and multiplexing of multiple compressed audio and video bit streams. Two very important consequence follow: Full motion video becomes a form of computer data, i.e., a data type to be integrated with text and graphics; Motion video and its associated audio can be delivered over existing telecommunication and computer networks. The difficult challenge in the design of the MPEG algorithm is the following: on one hand the quality requirements demand very high compression not achievable with intraframe coding alone; on the other hand, the random access requirement is best satisfied with pure intraframe coding. The algorithm can satisfy all the requirements only insofar as it achieves the high compression associated with interframe coding, while not compromising random access for those applications that demand it. This requires a delicate balance between intra- and interframe coding, and between recursive and non-recursive temporal redundancy reduction. Inorder to answer this challenge, the members of MPEG have resorted to using two interframe coding techniques: predictive and interpolative. The MPEG video compression algorithm relies on two basic techniques: block-based motion compensation for the reduction of the temporal redundancy and transform domain -(DCT) based compression for the reduction of spatial redundancy.Motion compensation techniques are applied with both causal (pure predictive coding) and non-causal predictors ( Interpolative coding). The difference signal (prediction error) is further compressed with spatial redundancy reduction (DCT). The information relative to motion is based on 16 x 16 blocks and is transmitted together with the spatial information. The motion information is compressed using variable-length codes to achieve maximum efficiency. Because of the importance of random access for stored video and significant bit-rate reduction afforded by motion-compensated interpolation, three types of pictures are considered in MPEG. Intrapictures ( I ), Predicted pictures ( P ) and Interpolated pictures ( B - for bi-directional prediction). Intrapictures provide an access point for random access but only with moderate compression; predicted pictures are coded with reference to a past picture ( Intra or Predicted ) and will in general be used as a reference for future predicted pictures; bi-directional pictures provide the highest amount of compression but require both a past and a future reference for prediction; in addition bi-directional pictures are never used as reference. In all cases when a picture is coded with respect to reference, motion compensation is used to improve the coding efficiency. The organisation of the pictures in MPEG is quite flexible and will depend on application specific parameters such as random accessibility and coding delay. Among the techniques that exploit the temporal redundancy of video signals, the most widely used is motion compensated prediction. It is the basis of most compression algorithms for visual telephony such as CCITT standard H.261. Motion compensated prediction assumes that 'locally' the current picture can be modelled as a translation of the picture at some previous time. Locally means that the amplitude and the direction of the displacement need not be the same everywhere in the picture. The motion information is part of the necessary information to recover the picture and has to be coded appropriately. Motion compensated interpolation is a key feature of MPEG. It is a technique that helps satisfy some of the application dependent requirements since it reduces the effect of errors while at the same time contributing significantly to the image quality. In the temporal dimension, motion compensated interpolation is a multiresolution technique; a subsignal with low temporal resolution (typically 1/2 or 1/3 of the frame rate ) is coded and full resolution signal is obtained by interpolation of the low resolution signal and addition of a correction term to a combination of a past and future reference. Motion compensated interpolation (also called bi-directional prediction in MPEG terminology ) presents a series of advantages, not the least of which is thatthe compression obtained by interpolative coding is very high. The other advantages of bi-directional prediction ( Temporal interpolation ) are:. It deals properly with uncovered areas, since an area just uncovered is not predictable from the past reference, but can be properly predicted from the future reference.. It has better statistical properties since more information is available; in particular, the effect of the noise can be decreased by averaging between the past and the future reference pictures.. It allows decoupling between prediction and coding (no error propagation). The trade off associated with the frequency of bi-directional pictures is the following: increasing the number of B-pictures between references decreases the correlation of B-pictures with the references as well as the correlation between the references themselves. Although this trade off varies with nature of the video scene, for a large class of scenes it appears reasonable to space reference at about 1/1 Oth second interval resulting in a combination of the type IBBPBBPBB IBBPBB. There is a trade-off between the coding gain provided by the motion information and cost associated with coding the motion information. The choice of 16 x 16 blocks for the motion-compensation unit is the result of such a trade-off, such motion compensation units are called Macroblocks. In the more general case of a bidirectionally coded picture, each 16 x 16 macroblock can be of type intra, Forward predicted, Backward predicted or Average. As expressed below, the expression for the predictor for a given macroblock depends on reference picture (past and future) as well as motion vectors: x is the coordinate of the picture element, mvoi the motion vector relative to the reference picture I», mva the motion vector relative to the reference picture I,. Macroblock Type INTRA FORWARD PREDICTED BACKWARD PREDICTED AVERAGE Predictor Iioo =128 Ii(X) =Io(x+mvoi) IiGO = Io (x+mv2l) Prediction Error Ii<x) - Ii(X) I kx) - Ii(X) I i(X) - I«x) Iioo - l/2(Io (x+mvoi) +I2(x+mv2i) I 1(X) I Kx) The motion information consists of one vector for forward predicted macroblocks and backward predicted macroblocks, and of two vectors for bidirectionally predicted macroblocks. The motion information associated with each 16x16 block is coded differentially with respect to the motion information present in the previous adjacent block. The range of the differential motion vector can be selected on a picture by picture basis, to match the spatial resolution, the temporal resolution and the nature of the motion in a particular sequence the maximal allowable range has been chosen large enough to accommodate even the most demanding situation. The differential motion information is further coded by means of a variable length code to provide greater efficiency by taking advantage of thestrong spatial correlation of the motion vector field ( the differential motion vector is likely to be very small except at object boundaries). Motion estimation covers a set of techniques used to extract the motion information from a video sequence. The MPEG syntax specifies how to represent the motion information : one or two motion vectors per 16 x 16 sub-block of the picture depending on the type of the motion compensation: forward-predicted, backward-predicted, average. The MPEG draft does not specify how such vectors are to be computed however. Because of the block based motion representation however, block- matching techniques are likely to be used; in a block matching techniques, the motion vector is obtained by minimising a cost function measuring the mismatch between a block and each predicted candidate. Let Mi be a macroblock in the current picture Ic, v the displacement with respect to the reference picture Ir, then the optimal displacement ( motion vector ) is obtained by the formula: v;* = min Z D[ Ic (X) -Ir(x+v) ] xeV Where the search range V of the possible motion vectors and selection of the cost function D are left entirely to the implementation. Exhaustive searches where all the possible motion vectors are considered are known to give good results, but at the expense of a very large complexity for large ranges : the decision of trade-off quality of the motion vector field versus complexity of the motion estimation process is for the implementer to make. Both still image and prediction error signals have a very high spatial redundancy. The redundancy reduction techniques usable to this effect are many, but because of the block based nature of the motion compensation process, block -based techniques are preferred. In the field of the block based spatial redundancy techniques, transform coding techniques and vector quantization coding are the two likely candidates. Transform coding techniques with a combination of visually weighted scalar quantization and run-length coding have been preferred because the DCT presents a certain number of definite advantages and has a relatively straightforward implementation. The Discrete Cosine Transform has inputs in the range -255, 255 (for P and B pictures) and output signals in the range -2048, 2047 providing enough accuracy even for the finest quantizer ( for I pictures these values are 0, 255 and 0,2047 respectively). Quantization of the DCT coefficients is a key operation, because the combination of quantization and run-length coding contributes to most of the compression; it is also through quantization that the encoder can match its output to given bit rate. Finally, adaptive quantization is one of the key tools to achieve visual quality. Subjective perception of quantization error greatly varies with the frequency and it is advantageous to use coarser quantization for the higher frequencies. The exact“quantization matrix”depends on many external parameters such as the characteristics of the intended display, the viewing distance and the amount of noise in the source. It is therefore possible to design a particularquantization matrix for an application or even for an individual sequence. A customised matrix can be stored as a context together with the compressed video. The signal from intracoded blocks should be quantized differently from the signal resulting from prediction on interpolation. Intracoded blocks contain energy in all frequencies and are very likely to produce ' Blocking effects' if too coarsely quantized.; on the other hand, prediction error type blocks contain predominantly high frequencies and can be subjected to much coarser quantization. It is assumed that coding process is capable of accurately predicting low frequencies, so that low frequencies content of the prediction error signal is minimum; if it is not the case, the intracoded block type should be preferred at encoding. This difference between intracoded blocks and differentially coded blocks results in the use of two different quantizer structures: while both quantizer are near uniform (have constant stepsize), this behaviour around zero is different. Quantizer for intracoded blocks have no dead zone( i.e., the region that gets quantized to the level zero smaller than a step size while quantizers for non intra blocks has a large dead zone). In order to further increase the compression inherent in the DCT and to reduce the impact of the motion information on the total bit rate, variable-length coding is used. A huffrnan-like table for DCT coefficients is used to code run-length coded AC coefficent ( zero run & amplitude). Only those with relatively high probability of occurrence are coded with a variable length code. The less likely events are coded with an escape symbol followed by fixed length codes, to avoid extremely long code words and reduce the cost of the implementation. The MPEG standard defines the decoding process not the decoder. There are many ways to implement a decoder and the standard does not recommend a particular way. The quality of video compressed with the MPEG-1 algorithm at rates öf about 1.2 Mbit/s has often been compared to VHS recording. The qualiftative VHS-like and better-than-VHS have been used. The spatial resolution is limited to 360x288 (SIF-Source Input Format) and video signal at the input of the source coder is limited to (24,25,30) frames/s non-interlaced. In MPEG-1 there is a considerable loss of quality in going from CCIR-601 to SIF (Source Input Format). If we want to code all or almost all the CCIR-601 data we must use more bits. As we see later that MPEG-1 syntax permits 4095x4095x60 frames per second. But main diffuculty is the interlacing. The main objective of MPEG-2 is to define a bit stream optimized for CCIR 601 resolution and bit rates. With MPEG-2 the standard has been widened to incorporate a wide range of formats including interlaced picture and higher chrominance resolution. In order to reduce the amount of technical and financial overhead, these resolution have been grouped into a hierarchy which allows one to be MPEG-2 compatible to a certain resolution and bit rate, or as classified in the proposed standard, by level and profile. In chapter 4 the MPEG-2 and its differencies from MPEG-1 briefly described. In this thesis the MPEG-1 is detailed and coding algorithm is simulated with a computer software which is written in Borland C version 3.5. The MPEG-1 simulation program was given in appendix-1 and explained chapter 5. Thecompression efficiency of the simulation program is shown with different types of pictures/ pictures sequences in chapter 5. XI

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