Datasets such as two dimensional raster images or three dimensional voxel based representations are often processed for representation using a transfer function defined by a curve. A mechanism for manually adjusting such curves is described, whereby a user adds a second curve. The transfer curve is recalculated so as to draw closer to the second curve. By drawing the second curve in the shape required for the transfer curve, and repeating this gesture as the transfer curve evolves, the user can subtly and interactively develop the transfer curve until the processed representation is exactly as required. The attractive effect of the points of the first curve on those of adjacent point on the first curve and on those of the second may be attractive or repellent, may vary in any manner as a function of distance, and in particular may imitate the effects of physical forces such as magnetic, elastic, etc.