什麼是量化表(Quantization Tables )

Quantization Tables

• In JPEG, each F[u,v] is divided by a constant q(u,v).

• Table of q(u,v) is called quantization table.

  ----------------------------------
  16  11  10  16  24   40   51   61   
  12  12  14  19  26   58   60   55   
  14  13  16  24  40   57   69   56   
  14  17  22  29  51   87   80   62   
  18  22  37  56  68   109  103  77   
  24  35  55  64  81   104  113  92   
  49  64  78  87  103  121  120  101  
  72  92  95  98  112  100  103  99   
  ----------------------------------
  

• Eye is most sensitive to low frequencies (upper left corner), less sensitive to high frequencies (lower right corner)

• Standard defines 2 default quantization tables, one for luminance (above), one for chrominance.

• Q: How would changing the numbers affect the picture (e.g., if I doubled them all)?

  Quality factor in most implementations is the scaling factor for default quantization tables.

• Custom quantization tables can be put in image/scan header.