H. Paek et al., ON THE POCS-BASED POSTPROCESSING TECHNIQUE TO REDUCE THE BLOCKING ARTIFACTS IN TRANSFORM CODED IMAGES, IEEE transactions on circuits and systems for video technology, 8(3), 1998, pp. 358-367
In this paper, we propose a novel postprocessing technique, based on t
he theory of projections onto convex sets (POCS), to reduce the blocki
ng artifacts in transform-coded images. It is assumed, in our approach
, that the original image is highly correlated. Thus, the global frequ
ency characteristics in two adjacent blocks are similar to the local o
nes in each block. We consider the high-frequency components in the gl
obal characteristics of a decoded image, which are not found in the lo
cal ones, as the results from the blocking artifact. We employ N-point
discrete cosine transform (DCT) to obtain the local characteristics,
and 2N-point DCT to obtain the global ones, and then derive the relati
on between N-point and 2N-point DCT coefficients. A careful comparison
of N-point with 2N-point DCT coefficients makes it possible to detect
the undesired high-frequency components, mainly caused by the blockin
g artifact. Then, we propose novel convex sets and their projection op
erators in the DCT domain. The performances of the proposed and conven
tional techniques are compared on the still images, decoded by JPEG, T
he results show that, regardless of the content of the input images, t
he proposed technique yields significantly better performance than the
conventional techniques in terms of objective quality, subjective qua
lity, and convergence behavior.