Modern computers employ graphics extensively. Window-based operating systems display the disk’s file directory graphically. The progress of many system operations, such as downloading a file, may also be displayed graphically. Many applications provide a graphical user interface (GUI), which makes it easier to use the program and to interpret displayed results. Computer graphics is used in many areas in everyday life to convert many types of complex information to images. Images are thus important, but they tend to be large! Since modern computer hardware can display many colors, it is common to have a pixel represented internally as a 24-bit number, where the percentages of red, green, and blue occupy 8 bits each. Such a 24-bit pixel can specify one of 224 ≈ 16.78 million colors. An image at a resolution of 512 x 512 made of such pixels thus occupies 786,432 bytes. At a resolution of 1024 × 1024, it gets four times as large, requiring 3,145,728 bytes. Movies are also commonly used with computers, making for even larger images. This is why image compression is so important. An important feature of image compression is that it can be lossy. An image, after all, exists for people to look at, so when it is compressed, it is acceptable to lose image features for which the human eye is not sensitive. This is one of the main ideas behind JPEG and other lossy image compression methods.
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