VIDEO COMPRESSION & VIDEO COMPRESSION & MPEGMPEG

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OutlineOutline

• VIDEO COMPRESSIONVIDEO COMPRESSION– VIDEO CODING BASICSVIDEO CODING BASICS– MOTION COMPENSATIONMOTION COMPENSATION

• MPEG FAMILYMPEG FAMILY– MPEG-1 VIDEO standardMPEG-1 VIDEO standard– MPEG-2 VIDEO standardMPEG-2 VIDEO standard– MPEG-4MPEG-4– H.264H.264– MPEG-7MPEG-7

VIDEO COMPRESSIONVIDEO COMPRESSION

• Why do we use compression?Why do we use compression?– Temporal RedundancyTemporal Redundancy– Spatial RedundancySpatial Redundancy

time

x

y

Two Stage Coding Two Stage Coding ProcessProcess

FRAME ( t - 1 ) FRAME ( t )

Processing for reducing temporal reduncy

Framedifference

Processing for reducing spatial reduncy

Stage 1

Stage 2

Reducing Temporal Reducing Temporal RedundancyRedundancy

• Segment a frame into macroblocks, not Segment a frame into macroblocks, not search by pixelssearch by pixels

• Output energy( bit rate) is increased witOutput energy( bit rate) is increased with the degree of temporal redundancyh the degree of temporal redundancy

• Interframe coderInterframe coder ( only use temporal red ( only use temporal redundancy reduction)undancy reduction)

Reducing Spatial RedundancyReducing Spatial Redundancy

• Using DCT codingUsing DCT coding• Intraframe coderIntraframe coder (use only spatial redun (use only spatial redun

dancy reduction, only for current frame)dancy reduction, only for current frame)• Hybrid( intra-/ inter - frame) coding metHybrid( intra-/ inter - frame) coding met

hodhod

Motion CompensationMotion Compensation• Often used and dOften used and d

eveloped by MPEeveloped by MPEG familyG family

• 將相鄰的將相鄰的 frameframe 中中出現的移動物件進出現的移動物件進行互相補償的過程，行互相補償的過程，因為該物件移動範因為該物件移動範圍不大且形狀在短圍不大且形狀在短時間內不易變動時間內不易變動

• It is preceded by It is preceded by motion estimatiomotion estimationn

Motion Vector

Search Range

Current Frame

Reference Frame

MPEG-1 IntroductionMPEG-1 Introduction• DCT for intra- and inter- framesDCT for intra- and inter- frames• Block-based motion compensationBlock-based motion compensation• Huffman CodingHuffman Coding• Source input format : YUV, why not RGBSource input format : YUV, why not RGB• Y : luminance component(Y : luminance component( 亮度亮度 ))• U(Cb), V(Cr) : color difference or chrominaU(Cb), V(Cr) : color difference or chromina

nce componentnce component （色差）（色差）

MPEG-1 Picture TypeMPEG-1 Picture Type

• Intra-Picture( I )Intra-Picture( I )

• Predicted- Pictures( P )Predicted- Pictures( P )

• Bidirectional-Predicted-Picture( B )Bidirectional-Predicted-Picture( B )

MPEG-2MPEG-2

• 與 與 MPEG-1 MPEG-1 通用通用• Difference from MPEG-1Difference from MPEG-1

– 好很多的畫質好很多的畫質– 更多的輸入格式更多的輸入格式 ( ex: RGB, more YUV form)( ex: RGB, more YUV form)– 在雙向傳輸時有較短的在雙向傳輸時有較短的 delay timedelay time– Higher bit rateHigher bit rate– We can adjust our pictures qualityWe can adjust our pictures quality

MPEG-4MPEG-4

•針對整合多媒體、通訊、電腦之應用上，具體內針對整合多媒體、通訊、電腦之應用上，具體內容則是以內容為基礎容則是以內容為基礎 (Content-based)(Content-based) 的應用上。的應用上。

•將影片區分為前景及背景將影片區分為前景及背景（（ Object-BasedObject-Based ）），，只要記錄動態圖像的軌跡即可，因此在壓縮量及只要記錄動態圖像的軌跡即可，因此在壓縮量及品質上，能較品質上，能較 MPEG-1MPEG-1 及及 MPEG-2MPEG-2 來得更好，每來得更好，每秒動態資料處理速率彈性更大。 秒動態資料處理速率彈性更大。

Some MPEG-4 Video FeatureSome MPEG-4 Video Feature

• SegmentationSegmentation

• SpriteSprite

• TrackingTracking

MPEG-1 vs. MPEG-2 vs. MPEG-4MPEG-1 vs. MPEG-2 vs. MPEG-4

一般解析度

最大解析度

一般資料量

最大資料量

Mpeg-1

Mpeg-2

Mpeg-4

3Mbps80Mbps

5~10Mbps

0.88Mbps6.5Mbps

1.38Mbps

720X5761920X1152

352X288

720X576720X576

352X288

MPEG-1 vs. MPEG-2 vs. MPEG-4MPEG-1 vs. MPEG-2 vs. MPEG-4

　　　

影像規格影像規格 Mpeg-1Mpeg-1 Mpeg-2Mpeg-2 Mpeg-4Mpeg-4

壓縮能力壓縮能力 中中 中中 強強

檔案大小檔案大小 中中 大大 小小

影像品質影像品質 可可 很好很好 可至很好可至很好

網路傳輸網路傳輸 勉強勉強 不可能不可能 最佳化　最佳化