Introduction to Shape Coding

Outline Introduction Shape Coding Approaches Conclusions Block-based methods CAE, Modified MMR, ….. Contour-based methods Chain code, BBSC (baseline-based) , ….. Vertex-based binary shape coding Conclusions

Introduction Why shape coding? Binary Alpha Plane describes the shape of one VOP Akiyo QCIF format (176 x 144)

Introduction Shape coding modes Lossy coding Intra mode ( I VOPs) Inter mode ( P, B VOPs) Lossy coding Progressive (scalable encode)

CAE method (1) Context-based Arithmetic Encoding Intra mode BAB : Binary Alpha Block Size : 16 x 16 pixels

CAE method (2) Binary arithmetic encoding using the context of the neighboring pixels A template of ten pixels is used to predict the current pixel

CAE method (3) Inter mode MVDs=0 & no inter update 1 Coding mode Coding type MVDs=0 & no inter update 1 MVDs!=0 & no inter update 2 Transparent (all 0) 3 Opaque (all 255) 4 Intra CAE 5 MVDs=0 & inter coded 6 MVDs!=0 & inter coded

CAE method (4) InterCAE

CAE method (5) Lossy coding Conversion Ratio (CR) : ¼, ½ or 1

CAE method (6)

Contour_based method (1) Chain code

Contour_based method (2) DCC (differential chain code) encode the differential between two consecutive directional links DCC with Huffman Coding Characteristics of one contour

Contour_based method (3) EBDCC

Contour_based method (4) Starting point Turning points

Contour_based method (5) The decoder should know The starting point of tracing Locations of turning points Sequences (one bit per step)

Contour_based method (6) Compression results for intra coding MPEG-4 Our method Akiyo 574 542 Bream2 771 606 weather 519 563

Contour_based method (7) Lossy coding Reduce the number of TPs ! The slope between two neighboring TPs is compared with Hori_TH : viewed as a horizontal line Vert_TH : viewed as a vertical line

Contour_based method (8) 563 bits / per VOP 497 bits / per VOP

Contour_based method (9) Scalable shape representation TPs are important information! TPs Seq. (1/2) Seq. (additional) Local TPs & seq.

Contour_based method (10) H : 9 V : 7 H’ : 5 V’ : 4 

Conclusions Block-based method Contour-based method High coding efficiency Suitable for hardware implementation Irregular boundary effect & blocking effect Contour-based method Smaller blocking effect on the reconstructed shape Suitable for specific application where semantic shape features are utilized Hardware optimization