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1、Unit 17 Images and Televisions,Passage A Fundamental Concepts Passage B Compression/Decompression Techniques Passage C Television,Passage A Digital Image Fundamentals 1. Digital Image Resolution A digital image is made up of many rows and columns of pixels. For gray scale images, each pixel is assig

2、ned a number that represents the gray shade assigned to that pixel. The larger the number of pixels in an image, and the larger the number of available gray scale levels, the better the resolution of the image. Figure 17.1 is an 17-bit images, with 217256 possible gray scale levels. The number of ro

3、w and column are 303228.,Figure 17.1 An 30322817 Digital Image,2. Histograms The gray scales present in a digital image can be summarized by its histogram (see Figure 17.2)The histogram reports the number of pixels for each grayscale level present in the image, as a bar graph. When an image uses onl

4、y a small portion of the available gray scale levels, histogram equalization can be used to spread out the usage of gray scale levels over the entire available range1This procedure re-assigns gray scale levels so that image contrast is improved.,Figure 17.2 A Histogram,3. Addition and Subtraction of

5、 Images Digital images can be added and subtracted pixel-by-pixel. Adding two images can combine two sets of objects into a single image. Moreover, several noisy images of the same scene can be averaged together to reduce the effect of noise. Image subtraction, on the other hand, can be used to remo

6、ve an unwanted background from an image. Subtraction of two time-lapsed photographs will show where motion has occurred between the two.,When two images are added or subtracted, the resultant matrix will frequently contain illegal gray scale values. For example, when a pixel in one 17-bit image has

7、the gray scale level 129 and the corresponding pixel in a second 17-bit image has the gray scale level 201, the sum pixel is 129+201330. This is outside the legal range for an 17-bit image, which may only contain gray scale levels between 0 and 255. When the same two images are subtracted, the diffe

8、rence pixel is 129-201= -72, again a value outside the legal range. For these reasons, scaling follows most image arithmetic. Scaling to the range 0, GSLmax may be accomplished as follows: ,4. Warping and Morphing Warping and morphing are digital image techniques that are finding application not onl

9、y in entertainment but also in medical imaging. Warping stretches or re-shapes an object in an image, while morphing transforms one image into another. These transformations may be accomplished by marking control points, control lines, or triangles in a source image and choosing their new positions

10、in a destination image. The transition between source and destination images is then accomplished by smoothly transforming not only the control element locations, but also their colors. The locations and colors of pixels not explicitly marked as control elements are determined by the locations and c

11、olors of the control elements that are nearest.,5. Image Filtering Digital images can be filtered using two-dimensional convolution with a convolution kernel. When an NN image is filtered by an MM convolution kernel, (M-1/2) rows and columns on each side of the image are lost to boundary effects. Lo

12、w pass filters blur images, high pass filters emphasize sharp changes in gray scale level, and edge filters locate edges in an image.2,6. Dilation and Erosion Dilation adds a layer of pixels to all objects in an image. Erosion removes one layer of pixels from all objects. When dilation is followed b

13、y erosion, gaps in broken boundaries identified through edge detection can be filled in. Conversely, when erosion is followed by dilation, spots of noise in an image are removed. Successfully detecting the edges in an image is the first step towards confident identification of object boundaries and

14、then objects recognition. From boundary information, shape characteristics like perimeter and area can be calculated, which can be used to classify an object.,7. Image Spectra Two-dimensional FFTs are used to analyze the spectra of digital images. Just as in the one-dimensional case, a two-dimension

15、al spectrum comprises a magnitude spectrum and a phase spectrum. The phase spectrum carries the best information about the locations of the objects in the image.3When all magnitudes are set to one, the phases alone still show a facsimile of the original image. When all phases are set to zero, the ma

16、gnitudes alone show no trace of it. Image spectra form the basis for both CT(computed tomography)and MRI (magnetic resonance imaging)scan displays. CT scans are X-rays taken in many directions in a single plane of an object.4 MRI scans depend instead on the magnetic properties of an object placed in

17、 a varying magnetic field. Both types of scans permit non-invasive investigations of three-dimensional objects.,8. Image compression In part due to the Internet, digital images are transmitted from place to place more often than ever. To save time and bandwidth(space), both images and other files ar

18、e often compressed before being transmitted. Lossless compression means that a file is compacted without losing any information, so that the reconstructed file is identical to the original.5 Lossy compression means that some information from the original file is irretrievably lost, but generally the

19、 reconstructed file is extremely close to the original. The compression ratio is the ratio of the original file size to the compressed file size.,One simple compression scheme is run-length encoding, which codes more than three repetitions of a number as three copies of the number followed by a coun

20、t of the additional copies needed. Another compression scheme is Huffman encoding, which uses shorter codes to represent the most common signal elements, and longer codes to represent the least common signal elements. JPEG, an extremely common image compression scheme, uses the discrete cosine trans

21、form (DCT) to concentrate most of the information about an 1717 sub-block of an image into a few significant coefficients.6 It then uses both run-length encoding and Huffman encoding to provide further compression.,Notes 1 When an image uses only a small portion of the available gray scale levels, h

22、istogram equalization can be used to spread out the usage of gray scale levels over the entire available range. 當(dāng)一幅圖像只使用了可用灰度級的一小部分時, 可以使用直方圖均衡的方法將灰度級的使用擴(kuò)展到整個可用的范圍。 2 Low pass filters blur images, high pass filters emphasize sharp changes in gray scale level, and edge filters locate edges in an imag

23、e 低通濾波器使圖像變得模糊, 而高通濾波器突出了圖像的灰度銳變, 邊緣濾波器則對圖像邊緣進(jìn)行定位。 ,3 The phase spectrum carries the best information about the locations of the objects in the image 相位譜攜帶著圖像中目標(biāo)位置的信息。 4 CT scans are X-rays taken in many directions in a single plane of an object. CT技術(shù)是使用X射線從不同方向?qū)δ繕?biāo)的某個平面進(jìn)行掃描。 5 Lossless compression m

24、eans that a file is compacted without losing any information, so that the reconstructed file is identical to the original 無損壓縮是不損失任何信息地將文件進(jìn)行壓縮, 重建得到的文件和原文件是完全一樣的。,6 JPEG, an extremely common image compression scheme, uses the discrete cosine transform (DCT) to concentrate most of the information abo

25、ut an 1717 sub-block of an image into a few significant coefficients. JPEG是一種極其常用的圖像壓縮方法, 該方法使用離散余弦變換將圖像中1717小塊的大部分信息集中到少數(shù)幾個重要系數(shù)上。 ,Exercises 1. Please translate the following phrases into English. (1) 灰度圖像 (2) 直方圖均衡 (3) 圖像對比度(4) 結(jié)果矩陣 (5) 邊緣檢測(6) 行程編碼 (7) 無損壓縮(8) 低通濾波器 (9) 哈夫曼編碼(10) 數(shù)字圖像壓縮,2. Pl

26、ease translate the following phrases into Chinese. (1) edge filter (2) magnitude spectrum (3) object recognition (4) phase spectrum (5) CT(Computed Tomography) (6) MRI(Magnetic Resonance Imaging) (7) DCT(Discrete Cosine Transform) (8) high pass filters (9) lossy compression,3. Translate the followin

27、g sentences into English. (1) 人眼具有這樣的特性: 圖像出現(xiàn)在視網(wǎng)膜上會保留幾毫秒, 然后消失。 如果一個圖像序列以50幅圖像每秒逐行掃描顯示, 人們就不會覺得看到的是一幅離散的圖像。 所有視頻系統(tǒng)都是利用這一原理產(chǎn)生運動畫面的。 (2) 所有壓縮系統(tǒng)均需要兩種算法: 一個在信源壓縮數(shù)據(jù), 一個在信宿對數(shù)據(jù)進(jìn)行解壓。 在文獻(xiàn)當(dāng)中, 這兩個算法分別被稱做編碼算法和解碼算法。,(3) JPEG(聯(lián)合圖像專家組)用于壓縮連續(xù)色調(diào)的靜止圖像(例如照片)。 它是ITU、 ISO和IEC共同支持的圖像專家開發(fā)出來的。 (4) 寬高比是圖像的寬度和高度之比。 常規(guī)電視的寬高比是

28、43。 高清晰度電視使用169的寬高比。 攝像機(jī)常用1.851或者2.351的寬高比。 (5) MPEG代表運動圖像專家組, 它是對數(shù)字壓縮格式視聽信息(如電影、 錄像、 音樂)編碼的一組標(biāo)準(zhǔn)的統(tǒng)稱。 與其他視頻、 音頻的編碼格式相比, MPEG的主要優(yōu)勢是相同質(zhì)量下的文件要小得多。 這是因為MPEG使用了非常復(fù)雜的壓縮技術(shù)。 ,4. Answer the questions. (1) What is the function of subtraction of images? (2) What is the function of warping? (3) What is dilation?

29、 (4) Compare lossless compression to loss compression.,,Passage B Compression/Decompression Techniques Numerous methods have been developed for the compression of digital image data. One of the principal drivers for this development is the television industry where quality image data must be transfe

30、rred to receivers using relatively simple equipment. The development of high definition television is further focusing the attention of industry and university scientists toward problems of data reduction and digital transmission. The principal evaluation criteria for the analysis of compressed vers

31、us uncompressed imagery is whether a person can tell the difference between the images. A more implemental measure is the Root Mean Square (RMS) error between the original image and the image that has been compressed. Compression rates may be generated by determining the size of the compressed image

32、 in terms of number of bits per image pixel for the original image.1,Here we only considers compression of single high resolution multi-spectral images. Higher compression rates will be achieved in a motion sequence where frame to frame variations may be quantified and only the changes from a refere

33、nce image need be coded.,There are two general types of compression: (1) loss-less, and (2)loss. Loss-less compression means that you can achieve a certain compression factor and be able to exactly reproduce the original image. Loss compression on the other hand allows some loss, but has the potenti

34、al for much higher compression rates. No matter what technique that you use, the exact rate is very dependent on the complexity of the image that you are analyzing. For example, the normal best that can be achieved with loss-less encoding in a rate of 2 bits per pixel. In fact, for some Land-sat sce

35、nes with urban areas and many small farms, the factor of 2 bits per pixel may not be able to achieved. The same technique applied to a Land-sat image of the mid-west where large fields occur and few shadows exist in images might produce a much better compression.,One loss-less technique is known as

36、run length encoding. The compression algorithm processes each line of input imagery looking for regions in which data values are the same. If ten pixels in the original image have a value of 10, then the same data may be represented as a data value, 10, and a multiplier saying how many times the val

37、ue is repeated before a changed value. Huffman encoding follows a similar process. These loss-less techniques are generally called entropy coding techniques, and have application in document imaging, desktop publishing, and GIS. It should be noted that entropy coding does not work exceptionally well

38、 in the representation of remote sensing images.,In remote sensing imagery it is well known that there may be significant correlation between different bands of multi-spectral data. In image processing, a procedure called principal components has been designed to identify correlation between image b

39、ands and to create a new set of transformed bands that represent a new color space in which the new image bands are uncorrelated.2 The procedure also provides a measure of the percent of the original variation present in the original image as found in each of the new transformed bands. For Land-sat

40、TM data, three to four of the transformed images represent 98 percent of the variance in the original images; therefore, a compression factor of 2 would be achieved with little loss.,Another type of transform coding does not involve a rotation of the color space, but instead represents images in ter

41、ms of spatial frequency of certain base functions. Fourier transforms map an image into a spatial frequency image base on sin and cosine functions. A fast computer implementation of the Discrete Fourier Transform (DFT) is known as a Fast Fourier Transform (FFT). Discrete Cosine Transforms (DCTs) map

42、 the same image to a spatial frequency image based only on the cosine function. Each pixel may be represented by a series of trigonometric functions and coefficients derived from the images. If all terms of the transforms trigonometric functions are used, compression is minimal. As more terms are de

43、leted, compression goes up, but the resulting compressed image develops certain artifacts of the procedure.,Vector Quantification (VQ) is a type of encoding that defines a vector representation of non-overlapping area blocks within an image. A vector consists of values representing the data values f

44、or each pixel within the region. Using these vectors, clusters of vectors are derived using a derived spectral distance measure.3 A codebook consisting of the clustered vectors is stored, representing the characteristics of the image. This process is numerically intensive and may be iterative. The d

45、ecoder for VQ takes an image vector and compares it to stored vectors in the codebook. A selection is made based on minimization of a distortion function between the new vector and the codebook. The VQ technique can generally achieve compression ratios of between 20 to one and 35 to one with little

46、observable distortion.,The VQ technique is an example of asymmetric compression in that considerably more time is spent deriving the codebook than in decompressing via a codebook lookup. Since different images may have different characteristics, a robust codebook is necessary to successfully code an

47、d decode Land-sat and other satellite images. A VQ technique using between channel correlation as well as spatial correlation achieves higher compression rates with lossless than independent band VQ.,The last type of compression considered is fractal compression. Based on Mandelbrot sets which take

48、advantage of a self similar, scaling independent, statistical feature of nature, fractal compression and decompression involves a clustering approach to finding regions which exhibit the same characteristics as a sample region without regard to rotation and scale. Regions within an image are related

49、 to numerous other regions within the same image, with this duplication of information being the basis of the compression potential. Fractal compression can achieve compression ratios of up to 80 to one with only moderate loss of information. The fractal technique, like the VQ technique is also asym

50、metric. Hardware implementation of the decompression of fractal images has achieved real-time rates.,NOTES 1 Compression rates may be generated by determining the size of the compressed image in terms of number of bits per image pixel for the original image.壓縮比可以通過原始圖像每像素所需壓縮的比特數(shù)來確定。 in terms of意為“根

51、據(jù)”。,2 In image processing, a procedure called principal components has been designed to identify correlation between image bands and to create a new set of transformed bands that represent a new color space in which the new image bands are uncorrelated.在圖像處理過程中，一個稱為主分量的過程被設(shè)計用以確認(rèn)圖像頻譜之間的相關(guān)性，并產(chǎn)生一組新的變換頻

52、譜，其中新圖像頻譜互不相關(guān)地表示一個新的彩色空間。 that引導(dǎo)定語從句修飾bands。in which引導(dǎo)定語從句修飾space。,3 Using these vectors, clusters of vectors are derived using a derived spectral distance measure. 使用這些矢量，用一個導(dǎo)出的頻譜距離可推導(dǎo)出矢量簇。 第二個using相當(dāng)于by using。,EXERCISES 1. True/False. (1) Loss-less and loss are two general types of compression.( )

53、 (2) No matter what technique that you use, the exact compression rate is not very dependent on the complexity of the image that you are analyzing.( ) (3) The decoder for VQ takes an image vector and compares it to stored vectors in the codebook.( ),2. Fill in the blanks. (1) Loss-less compression m

54、eans that you can achieve a certain and be able to exactly reproduce the original image. (2) A fast computer implementation of the Discrete Fourier Transform is known as a . (3) Vector Quantification (VQ) is a type of encoding that defines a of non- overlapping area blocks within an image. (4) Th

55、e fractal technique, like the VQ technique is also . (5) compression can achieve compression radios of up to 80 to one with only moderate loss of information.,3. Chose the best answer. (1) What kind of compression algorithm processes each line input imagery looking for regions in which data values

56、 are the same? a. run length encoding b. Vector Quantification c. fractal compression (2) Which type of compression take advantage of a self similar, scaling independent and statistical feature of nature? a. run length encoding b. Vector Quantification c. fractal compression,,Passage C Television A

57、television picture is built up gradually by moving a spot of light across and down a screen in a raster pattern. The video signal causes the brightness of the spot to vary in proportion to the intensity of light in the original image. The movement of the spot across the screen is controlled by the l

58、ine scan signal. Each time the spot reaches the right side of the screen, it is blanked and moved rapidly back to the left side ready to start the next line. This rapid movement back to a starting position is known as fly-back. Each complete image or frame requires a minimum of 500 lines to give a p

59、icture of acceptable quality. The present European TV system uses 625 lines per frame.,The movement of the spot down the screen is controlled by the field scan signal. When the spot reaches the bottom of the screen, it is blanked and moved rapidly back to the top of the screen. The frame must be sca

60、nned at least forty times per second to prevent the screen from flickering. The present European TV system has a frame scan rate of 50Hz. The video signal contains line and field sync pulses to make sure that the TV receiver starts a new line and a new frame at the same time as the TV camera.,To all

61、ow the video signal to be transmitted using a smaller range of frequencies, each frame is transmitted in two separate halves, known as fields. The first time the spot travels down the screen it displays the first field, which consists of the odd-numbered frame lines. The second time the spot travels

62、 down the screen it displays the second field, which consists of the even-numbered frame lines. Combining two fields in this way is known as interlacing. Although the fields are displayed one after the other, it happens so quickly that the human eye sees them as one complete picture.,In Europe, the

63、USA, and Japan, the race is on to produce a new generation of television sets. These new sets will be larger than todays models, possibly with 100-centimetre flat screens. Picture quality will be excellent, crisp, and without flicker, as good as those we are used to seeing in the cinema. Sound quali

64、ty too will be superb, thanks to digital multi-track transmissions. By the turn of the century such sets may be offering programs in a choice of languages as they will be equipped with eight sound tracks.,In Europe, the term HDTV is used. In the USA, the more generic term ATV, Advanced Television, h

65、as been adopted. The Japanese, who were the first to start work on the new technology, in 1974,called their system Hi-Vision. Whatever name is used, these new sets share certain features. The picture is displayed using more lines per frame. This means that they provide clearer, more detailed, high q

66、uality images. The picture can be displayed on large, wide screens which are flicker-free. They also provide very high quality three-dimensional sound output.,A wider range of frequencies can be used to transmit each HDTV channel. This is because they can be transmitted at high frequencies which are virtually unused at present. These wide frequency ranges make it possible to transmit digital, rather than analogue signals. Digital processing can then be used in the receivers to provide almost per