电子信息类专英语Unit 17 Images and Televisions.ppt

Unit 17 Images and Televisions 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 assigned 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 217＝256 possible gray scale levels. The number of row and column are 303×228. Figure 17.1 An 303×228×17 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 only 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 range［1］．This procedure re-assigns gray scale levels so that image contrast is improved. Figure 17.2 A Histogram 　　3. Addition and Subtraction of 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 remove 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 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