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Several recent image compression standards rely upon the Discrete Cosine Transform (DCT). Models of DCT basis function visibility can be used to design quantization matrices for arbitrary viewing conditions and images. Here we report new results on the effects of viewing distance and contrast masking on basis function visibility. We measured contrast detection thresholds for DCT basis functions at viewing distances yielding 16, 32, and 64 pixels/degree. Our detection model has been elaborated to incorporate the observed effects. We have also measured detection thresholds for individual basis functions when superimposed upon another basis function of the same or a different frequency. We find considerable masking between nearby DCT frequencies. A model for these masking effects will also be presented.