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{ Lossy Compression William Dayton Nick Trojanowski

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Lossy file compression compresses files by trimming out data, and therefore quality, from the original data. The loss of data is not usually enough to be noticeable by the user, but can immensely save on file size. Definition

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Lossy compression loses data; lossless does not When encoding data, lossy takes up less file space Lossless compression 9/10 original size Lossy compression 1/10 original size Lossy vs Lossless

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Transform Codecs Samples taken, separated, quantized, and entropy encoded Predictive Codecs Data based off preceding data, by doing next byte based off previous Lossy Compression Types

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chroma filtering 4:2:2 means every 4 pixels are 4 luminance samples (Y), and 2 chrominance samples (RGB) 4:2:2 reduces by 33%, 4:1:1 & 4:2:0 by 50% hue change not typically seen, can affect certain effects YXbXr; Y = (R+B+G)/3, Xb = Y-B, Xr = Y-r 4:2:2 4:1:1 4:2:0 Image Compression

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File Types JPEG DjVu AT&T Research Labs PGF (Progressive Graphics File) BTC (Block Truncation Coding) Image Compression cont.

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Best known image compression type, mostly commonly used on Internet Used for digital photography Based on discrete cosine transform (DCT) Has varying compression ratios First converted to YCbCr Chroma downsampling to RGB DCT Joint Photographic Experts Group (JPEG)

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Group of functions that express a signal in sum of different sinusoids 8 types DCT-II most common Discrete Cosine Transform

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Audio Compression Higher quantization at extreme ranges 10:1 compression ratio almost indistinguishable from original Less accuracy after loud sections Bad compression around 3kHz

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Audio Compression cont. File Types Opus MP2 MP3 MP4 OGG WMA Musepack

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Uses psychoacoustics Based on Musicam format Sub-band coding Ex: 128kbit/s 1/11 file size Can use variable bit rate for encoding .mp4 can also do video MP2, 3, & 4

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MP3 vs WAV Can you hear the difference?

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Video Compression Mix between audio & image compression Compression ratio between 10:1/200:1 Reduces redundancy & framerate Bases image on macroblocks No standard uncompressed format

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File Types Motion JPEG MPEG1 MPEG2 MPEG4 OGG Video Compression cont.

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MPEG is flexible & error resilient Max bitrate for MPEG4 is 8000kbits/s Achieves compression ratio 20:1 to 200:1 OGG is open source Does audio, video, & text Theora, Tarkin, Dirac for video Theora made as competitor to MPEG4 MPEG1, 2, 4, & OGG

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The one on the right has over triple the bitrate of the one on the left.