Systems and devices for processing image or other data using non-linear methods to compensate for localized slopes are described. In one implementation, the slope of the sample values in an image or other dataset is estimated in one or more directions using a non-linear filter, such as a median filter. The values of at least some of the samples of interest are compensated using the estimated slope values to remove the effects of the slope. The compensated values may then be processed to determine if the target is present in the samples of interest, or for any other purpose.