电子信息类专业英语(西电第二版)Unit 17 Images and Televisions

Unit 17 Images and Televisions
电子信息类专业英语（西电第二版） Unit 17 Images and Televisions
Unit 17 Images and Televisions
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.
Unit 17 Images and Televisions
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 129+201＝330. 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 difference 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:
ScO alu etd n p ]u M Q t(O [u am tx u n p )- ,M ] tM up(t( O O u [iin m n tu u ))G t,tp p m S u u aL nd Televisions
Figure 17.2 A Histogram
Unit 17 Images and Televisions
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 timelapsed photographs will show where motion has occurred between the two.
Unit 17 Images and Televisions
Figure 17.1 An 303×228×17 Digital Image
Unit 17 Images and Televisions
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.