<title>Quality improvement of low-data-rate compressed video signals by pre- and postprocessing</title>

Abstract

In this paper a technique for improving the image quality of block-based video coders is presented which combines the following pre-and postprocessing steps: Before the coding process, the image sharpness is enhanced by applying a special prefilter to the original image. By this means a possible degradation of the image sharpness by the coder will be compensated. The decoder output is smoothed by a lowpass filter which reduces artifacts such as block discontinuities.Secondly, the quantization error of the frequency DCT coefficients is reduced by predicting AC coefficients based on the mean values of the neighbouring image blocks. This step improves the luminance function within the blocks. While the original sequences show coarse blocking patterns, the images processed by this technique look smooth on the overall with wellpreserved local edge structures. The tool supports all standardised DCT codecs, such as MPEG-1/2 or H.261/3.

Reduction of ringing noise in transform image codingusing simple adaptive filter

“…been followed in [6], where the quantization noise of DCT coefficients is shifted from the block boundaries to the inner part of the block. Other proposals employ Dolby-like noise suppression techniques for reduction of blocking artifacts [7]. Although such noise shaping requires a matched receiver for best performance, a standardized receiver which does not know about the encoder modifications can still decode an image of reasonable quality.…”

Low bit-rate video coding with implicit multiscale segmentation

“…At low bit rates, when adequate residual coding cannot be performed, this results in perceptually unacceptable performance. The usual fix for this situation is to use a pre-or post-processing filter which smoothes the block artifacts [13], [14]. Such filtering typically leads to a blurring of the true image edges in addition to a reduction of the block artifacts.…”

Improving the image quality of block-based video coders by exploiting interblock redundancy

“…The prediction of the DCT coefficients is useful also in combination with other algorithms for image improvement. The authors demonstrate an example in [19].…”