Multimedia Systems Chapter 7.2: • Interactive versus non-interactive Layer 6: Compression server client Video Archive CS/ECPE 5516: Comm. Network Stored Prof. Abrams, Spring 2000 server client Based in part on material from Scott F. Midkiff Live CS/ECPE5516 1 2 CS/ECPE5516 Example Multimedia Applications Need for Video Compression • Multimedia-on-demand • Video characteristics n Demanding with respect to storage and/or data rate n Video-on-demand n Audio-on-demand (640x480 pixels/f)(24 b/pixel)(30 f/s) = 221 Mbps • Live video n Highly redundant -- duplicated information n Meetings n Compression ratios of 200:1 or even 2000:1 are possible n Collaboration • Compression is needed to enable n News n Storage • Remote sensing and imaging n Transmission 3 CS/ECPE5516 4 CS/ECPE5516 Compression Example (JPEG) Compression Techniques • Information may be lost (but not missed) n Lossy compression -- information is lost Quality: 100% Quality: 90% Quality: 60% n Lossless compression -- no loss Size: 57459 Size: 20525 Size: 8293 • Lossy techniques: drop info not important to human perception. Examples: Quality: 25% Quality: 10% Quality: 5% n Images: changes in high frequency brightness Size: 4984 Size: 3338 Size: 2551 changes as you move across image JPEG = Joint Picture Experts Group n Audio: low frequency sounds in woman’s voice 5 CS/ECPE5516 6 CS/ECPE5516 1
Lossless Compression Lossless Compression Algorithms (1) Algorithms (2) • Dictionary-based methods • Run Length Encoding n Build table of variable length strings “AAABB” ⇒ “3A2B” “to be or not to be is Shakespeare’s line – is it not?” Can actually increase file size Dictionary: Can be applied to images by comparing adjacent pixels 0=“to be” • Differential Pulse Code Modulation 1=“not” 2=“is” “AAABBC” ⇒ “A00112” since B is 1 away from A, … ⇒ “0 or 1 0 2 Shakespeare’s line – 2 it 1?” • Delta encoding n Cost: must send dictionary before file “AAABBC” ⇒ “A00101” since C is 1 away from B, … n Examples: Lempel-Ziv (Unix compress) 7 CS/ECPE5516 8 CS/ECPE5516 Lossy Compression – Images (1) Lossy Compression – Images (2) • GIF JPEG • n Given 24-bit pixels, pick the 256 most used colors. DCT Phase: n Map each 24-bit pixel into 1-of-256. Divide image into 8x8 pixel blocks. n n Achieves 3x compression. If you move across x-axis, you see pixels vary with some n frequency. n Then run Lempel-Ziv, maybe achieving 10x DC Compute something like compression. n Fourier transform, called Discrete Cosine Transform high (DCT) – maps intensity to frequency domain with 64 intensities. 9 CS/ECPE5516 10 CS/ECPE5516 JPEG Video Compression Techniques DCT Phase • Scope of compression • Quantization Phase • n Intraframe -- eliminate or reduce redundancy within a single frame Use table of coefficients; divide step 1 values by coefficients. • Maps many frequencies to zero. n Interframe -- eliminate or reduce redundancy between Encoding phase • consecutive frames 3 7 11 15 Huffman code: use few bits n Prediction, interpolation – n for most popular numbers 7 11 15 21 predict frame based on previous/subsequent values Use delta encoding for 11 15 21 27 n n Sample to take advantage of human perception subsequent array values 15 21 27 33 Color: repeat 3 times (RGB) • 11 CS/ECPE5516 12 CS/ECPE5516 2
MPEG Overview (1) MPEG Overview (2) • Features • Compression techniques n Uses DCT for intraframe compression n Can achieve compression ratios of 200:1 n Uses interframe compression by storing differences between n Would reduce data rate to around 1.2 Mbps for a 640x480 successive frames image • There are three frame types n MPEG-1 compresses 320x240 images and requires at n Intraframes (I frames) are encoded using intraframe least 1.5 Mbps compression n Predicted frames (P frames) are predicted from previous I n Also includes audio compression with compression frames ratios of 5:1 to 10:1 n Bidirectional frames (B frames) are interpolated from previous and future frames 13 CS/ECPE5516 14 CS/ECPE5516 MPEG Overview (3) Transmission of MPEG • Repeated pattern of frames (pictures) is a group • If stored video, send IBBBPBBBI as of pictures (GOP) IPBBBBBBI n Example: IBBBPBBB • Might use Differentiated Services, with I’s and forward prediction P’s as high priority • Can change quantization matrix during video I B B B P B B B I 1 2 3 4 5 6 7 8 9 time bidirectional prediction 15 CS/ECPE5516 16 CS/ECPE5516 Some Video Compression Standards • MPEG-1, MPEG-2, MPEG-3, and MPEG-4 • ITU-T (CCITT) standards n H.320 (H.261) — ISDN (64 Kbps increments) n H.323 — LAN n H.324 — POTS • MJPEG (Motion JPEG) MPEG: Motion Pictures Expert Group ITU-T: International Telecommunication Union -- Telecommunication JPEG: Joint Photographic Experts Group 17 CS/ECPE5516 3
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