Abstract: The force-constancy hypothesis states that the user of a force-feedback device maintains a constant penetration force when stroking virtual surfaces in order to perceive their topography. The hypothesis was developed to address a real-world data perceptualization problem where the perception of surface topography was distorted when the surface stiffness was nonuniform. Two experiments were conducted. In Experiment I, we recorded the penetration depths of the probe tip while the user stroked two surfaces with equal height but different stiffness values. We found that the data could be quantitatively modeled by the force-constancy hypothesis when the virtual surfaces were neither too soft nor too hard. In Experiment II, we demonstrated that given two adjacent surfaces, their perceived height difference depended on both the surface stiffness values as well as the relative heights of the surfaces. Specifically, we showed that the higher but softer surface could be perceived to be lower, at the same height, or higher than the other surface, depending on how much higher it was than the other surface. The results were consistent with the predictions of the force-constancy hypothesis. Our findings underscore the importance of understanding the interplay of haptic rendering parameters.