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In order to specify the optimal psychovisual quantizer associated to a given visual subband decomposition scheme [1], a new methodology has been developped. Psychovisual experiments based on the visibility of the quantization noise, have been conducted. The complex signals used, have been caracterised by the local bandlimited contrast. The main finding of this study is that the quantizers are, in the chosen space contrast ,of a linear type. The quantization intervals contrast, obtained with a given observer, are of 0.039 for the subband called III-1 (radial selectivity 1 .5cy/d° to 5.7cy/d°, angular selectivity -22.5d° to 22.5) 0.03 1 for the subband called IV-1 (radial selectivity 5.7cy/d° to 14. lcy/d°, angular selectivity -15d°to 1 Sd°) 0.1 17 for the subband called V-i (radial selectivity 14.icy/d° to 28.2cy/d°, angular selectivity -i5d° to 15d°) To evaluate the importance of the "angular" aspect in this approach, further measurements have been made with the subband IV -2 (radial selectivity 5.7cy/d° to 14. icy/d°, angular selectivity 15d° to 30d°). The linearity is also observed and the quantization interval contrast, for the same observer, is of 0.122.Key words: Psychovisual quantizer,visual subband, Local band-limited contrast, Visibility of quantization noise 1 . INTRODUCTIONThe design of quantizers involves specifying thresholds and levels which minimize both entropy and some measure of distorsion. Whereas for the "classical" quantizers, such distorsion may be well measured by the rms (root mean square) error, for the psychovisual quantizers we are concerned by the visual distorsion. For the human visual system, the same error is less detectable when it is superimposed with strong signals (with high values ) than when it is superimposed with weak ones ( small values).To design a psychovisual quantizer, A.B.Watson [2] used the contrast masking function. This function represents, for a given observer, the discrimination between two images that differ only in contrast. In his experiments the first image (the background) has a contrast c and the second one (the background plus increment) has a contrast c + L\c. The increment discrimination thresholds measured have then been modeled bywhere Ec(O) represents the contrast detection threshold when the background contrast is zero. This function has been applied directly to the design of the quantizer. The quantization thresholds T and the reconstruction levels L1 have been determined by 1446 ISPIE Vol. 2308 0-8194-1638-X/94/$6.00 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/25/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx

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In order to caracterize the spatial frequency mechanisms of the visual system, we measured the visibility threshold elevation as a function of the spatial frequency cosine maskers. The stimulis and the maskers used were spatially localized and temporally weighted. The results show that the relative bandwidth ( defined as the ratio between the estimated bandwidth and the frequency of the masker )varies from 3 in low frequencies of (lie masker ( I cy/d°) to I . I 5 in high frequencies of the masker ( I Ocyld°). This is consisteiit with a model having five classes of spatial frequency mechanisms covering the band 0 -3Ocy/d°. These results, associated with those of [I 1 and [2] allow the definition of a sub-band decomposition of images iii twenty-one "visual components".