<title>Multigeneration of transform-coded images</title>

Abstract

Multigeneration is the repeated compression-decompression of images. Under a compression scheme like transform coding, reconstructed images suffer further degradation at each generation, even though there is no manipulation of the image data. This paper describes multigeneration characteristics of transform coded images. We also study motion compensated coding in a multigeneration environment. We present five mechanisms that contribute to the continued degradation in multigeneration: pixel domain quantization (PDQ), pixel domain clipping (PDC), compression control parameters variation (CCPV), motion vector re-estimation (MVR) and error propagation due to motion compensation (EPMC). For PDQ, we show that it is not the step seize of the DCT domain or the pixel domain quantizers that contributes to the saturation of degradation in multigeneration, but instead the ratio between these tow quantizers. We observe that PDC mainly affects the DC coefficient. In CCPV and MVR, multigeneration error is reduced when the quantization scaling parameters and/or the motion vectors of the first generation are used for each subsequent generation. For EPMC, multigeneration errors in reference pictures propagate to frames that are predicted form them. COnsequently, EPMC only multiplies the effect of other mechanisms.