The coefficients are retained based on
Maximum variance : zonal coding
Maximum magnitude : threshold coding
Zonal coding
Each DCT coefficient is considered as a random variable whose
distribution could be computed over the ensemble of all transformed
subimages
The variances can also be based on an assumed image model (e.g., a
Markov autocorrelation function)
Coefficients of maximum variances are usually located around the origin
Threshold coding
Select coefficients which have the largest magnitudes
Causing far less error than the zonal coding result