VIDEO COMPRESSION & MPEG BY ttchen 2003/4/13
Outline VIDEO COMPRESSION MPEG FAMILY VIDEO CODING BASICS MOTION COMPENSATION MPEG FAMILY MPEG-1 VIDEO standard MPEG-2 VIDEO standard MPEG-4 H.264 MPEG-7
VIDEO COMPRESSION Why do we use compression? Temporal Redundancy Spatial Redundancy y x time
Two Stage Coding Process
Reducing Temporal Redundancy Segment a frame into macroblocks, not search by pixels Output energy( bit rate) is increased with the degree of temporal redundancy Interframe coder ( only use temporal redundancy reduction)
Reducing Spatial Redundancy Using DCT coding Intraframe coder (use only spatial redundancy reduction, only for current frame) Hybrid( intra-/ inter - frame) coding method
Motion Compensation Often used and developed by MPEG family 將相鄰的frame中出現的移動物件進行互相補償的過程，因為該物件移動範圍不大且形狀在短時間內不易變動 It is preceded by motion estimation Search Range Motion Vector Reference Frame Current Frame
MPEG-1 Introduction DCT for intra- and inter- frames Block-based motion compensation Huffman Coding Source input format : YUV, why not RGB Y : luminance component(亮度) U(Cb), V(Cr) : color difference or chrominance component（色差）
MPEG-1 Picture Type Intra-Picture( I ) Predicted- Pictures( P ) Bidirectional-Predicted-Picture( B )
MPEG-2 與 MPEG-1 通用 Difference from MPEG-1 好很多的畫質 更多的輸入格式( ex: RGB, more YUV form) 在雙向傳輸時有較短的delay time Higher bit rate We can adjust our pictures quality
MPEG-4 針對整合多媒體、通訊、電腦之應用上，具體內容則是以內容為基礎(Content-based)的應用上。 將影片區分為前景及背景（Object-Based），只要記錄動態圖像的軌跡即可，因此在壓縮量及品質上，能較MPEG-1及MPEG-2來得更好，每秒動態資料處理速率彈性更大。
Some MPEG-4 Video Feature Segmentation Sprite Tracking
MPEG-1 vs. MPEG-2 vs. MPEG-4 3Mbps 80Mbps 5~10Mbps 1.38Mbps 6.5Mbps 352X288 1920X1152 720X576 352X288 720X576 720X576
MPEG-1 vs. MPEG-2 vs. MPEG-4 影像規格 Mpeg-1 Mpeg-2 Mpeg-4 壓縮能力 中 強 檔案大小 大 影像品質 可 很好 可至很好 網路傳輸 勉強 不可能 最佳化
展望~H.264 傳說中輕易KO MPEG-1,2,4 的超強壓縮軟體 Use Simplified DCT Quantization up to 51( MPEG-4 : 31) Very very low bit rate
展望~ MPEG-7 MPEG-7, 又可稱為“Multimedia Content Description Interface”, 注重在多媒體內容的描寫，旨在描述出一個物件的特性 不著重在影像壓縮技術( ex: MPEG-1,2,4 and H.264)