The present disclosure relate generally to image signal processing, color science and signal encoding. One claim recites an apparatus including: an input for obtaining color image data; memory for storing a luminance contrast sensitivity function (CSF1) and a chrominance contrast sensitivity function (CSF2); means for degrading data representing color image data with the CSF1 and the CSF2 to predict visibility changes attributable to encoding plural-bit information in the obtained color image data, in which the CSF1 varies depending on luminance values associated with local regions of the color image data, in which said means for degrading data representing color image data yields results for different image areas within the color image data, and in which the CSF1 is used for degrading luminance data and the CSF2 is used for degrading chrominance data; and means for altering the color image data by encoding plural-bit information therein, in which signal embedding strength of the encoding within the different image areas varies based on the results. Of course, other features, combinations and claims are disclosed as well.