The design of automatic transfer functions for volume rendering is a perennial problem in volume visualization. Over the last three decades, a variety of design methodologies have been proposed. However, sensitive adjustment of related control parameters remains entrusted to users, because rendering conditions, such as the thickness of emphasized subvolumes in the ray direction and the size of a target dataset, differ on a case-by-case basis. Our group previously proposed one-dimensional transfer functions to accentuate topological changes in the scalar field of the target dataset. However, the method forces us to determine the actual control parameter values for the transfer functions in an empirical manner. In this paper, we propose a supplementary mechanism with which to judiciously define an appropriate profile of the opacity values. More specifically, the height and width of the hat opacity transfer functions that accentuate feature isosurfaces are determined according to the number of voxels belonging to the relevant topologically equivalent scalar field interval. The feasibility of the proposed method is evaluated by its application to five kinds of volume datasets.