The detailed technology relates to optical sensors, and more particularly relates to characterizing local behaviors of a sensor, and using such characterization information in a sensor-based system. In a particular arrangement, output signals produced from each of plural sensing elements in a 2D optical sensor are checked to determine whether they tend to differ from output signals produced by one or more neighboring sensing elements—when combined across plural captured image frames. The results are stored in an associated memory, and can be consulted in determining how image data captured from the sensor should be used. The technology is particularly illustrated in the context of a watermark decoder for a cell phone camera. A variety of other features and arrangements are also detailed.