The extensive use of discrete cosine transform (DCT) techniques in image coding suggests the investigation on filtering and downsampling methods directly acting on the DCT domain. As DCT image transforms usually operate on blocks, it is useful that the DCT filtering techniques preserve the block dimension. In this context the present paper first revises the intra-block filtering techniques to enlighten the limitations implied by small block dimensions. To overcome the artefacts introduced by this method and to satisfy the filtering design constraints which are usually defined in the Fourier domain, inter-block techniques are developed starting from the implementation of FIR filtering. Inter-block schemes do not exhibit any limitation but their computational cost has to be taken into account. In addition, hybrid techniques, using variable length FIR filters after the discard of low order DCT coefficients, are introduced to increase the computational efficiency; in this case, the introduced aliasing has to be kept at tolerable values. The amount of the tolerable aliasing strictly depends on the subsequent operations applied to the filtered and downsampled image. The numerical examples reported could form a basis for error estimation and evaluation of trade-off between performance and computational complexity. © 1994.
|Titolo:||Inter-block Filtering and Downsampling in DCT Domain|
|Autori interni:||NERI, Alessandro|
|Data di pubblicazione:||1994|
|Rivista:||SIGNAL PROCESSING-IMAGE COMMUNICATION|
|Appare nelle tipologie:||1.1 Articolo in rivista|