Visual quality improvement of image subband coder with dithering

This paper investigates effect of dithering in improving the visual quality of an image subband coder. A subband coder with scalar quantization of lowest frequency subband and pyramid vector quantization of high frequency subbands was simulated. For subband quantizer both types of subtractive and non-subtractive dither were examined, and the coder performance was compared with the non-modified coder by subjective and objective tests. In a series of pair comparison subjective tests, coder with subtractive dither gained a total average score of 0.32 (in the scaled range of [-1,1]) and the coder with non-subtractive dither gained an average score of 0.24 compared to non-dithered coder. As an objective measure the correlation between reconstruction error and original image was calculated, which shows that dithering will result an average of 10 percent reduction in case of non-subtractive dither and 12 percent reduction for subtractive dither.