1. SIMS-201 Image Compression • JPEG and GIF

2. Overview • Chapter 8: • Introduction to image compression

3. Image Compression • Near Photographic Quality Image • 1,280 Rows of 800 pixels each, with 24 bits of color information per pixel • Total = 24,576,000 bits • 56 Kbps modem • 56,000 bits/sec • How long does it take to download? 24,576,000/56,000 = 439 seconds/60 = 7.31 minutes Image compression would be necessary.

4. Images are well suited for compression • Images have more redundancy than other types of data. • Human eye is very tolerant of approximation error. • 2 types of image compression • Lossless coding • Every detail of original data is restored upon decoding • Examples – Run Length Encoding, JPEG, GIF • Lossy coding • Portion of original data is lost but undetectable to human eye • Useful for images and audio • Examples - JPEG

5. The two compressed image formats most often encountered on the Web are JPEG and GIF. • JPEG -Joint Photographic Experts Group • 29 distinct coding systems for compression, 2 for Lossless compression • Lossless JPEG uses a technique called predictive coding to attempt to identify pixels later in the image in terms of previous pixels in that same image • Lossy JPEG consists of image simplification, removing image complexity at some loss of fidelity (very common) • GIF – Graphics Interchange Format • Developed by CompuServe • Lossless image compression system. • Uses of Lempel-Ziv-Welch (LZW) algorithm

6. Example: Lossy Compression • DCT (discrete cosign transform) (or DWT – discrete wavelet transform) are often used as transformation • Example: • How DCT compresses images • Image and • pixel values • in space • domain • Coefficients in DCT domain • Most coefficients in DCT • domain are zero or close • to zero 120 108 90 75 69 73 82 89 127 115 97 81 75 79 88 95 134 122 105 89 83 87 96 103 137 125 107 92 86 90 99 106 131 119 101 86 80 83 93 100 117 105 87 72 65 69 78 85 100 88 70 55 49 53 62 69 89 77 59 44 38 42 51 58 700 90 100 0 0 0 0 0 90 0 0 0 0 0 0 0 -89 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

7. Digital Video Compression (MPEG) Motion Picture Expert Group (MPEG) standard for video compression. • MPEG is a series of techniques for compressing streaming digital information • DVDs use MPEG coding • MPEG achieves compression results on the order of 1/35 of original • If we examine two still images from a video sequence of images, we will almost always find that they are similar • This fact can be exploited by transmitting only the changes from one image to the next • Many pixels will not change from one image to the next. Called IMAGE DIFFERENCE CODING, and achieved by subtracting one image from the next. For example:

8. Example: Image Subtraction • Subtraction & change detection • In many applications, it is desired to compare two images • A simple method is to align and subtract them • g(x, y) = f(x, y) - h(x, y) • Example: • Blood stream is injected with a dye and X-ray images of arteries are taken before and after the injection • f(x,y): image after injecting a dye • h(x,y): image before injecting the • dye • g(x,y) yields a clear display of the blood flow paths.