Introduction to

Shape Coding

Outline

Introduction Shape Coding Approaches

– 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– 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 BlockSize : 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 modeCoding mode Coding type

0 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

downsampling

upsampling

MxCR

MxCR

M

M

conversion error

M

M

CAE method (6)START

N

SET CR=1/4

END

Y

N

Y

SET CR=1

SET CR=1/2

There is at least one Error-PB in MB

There is at least one Error-PB in MB

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– 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