Transfer function (TF) specification for 3D visualization can be improved by incorporating more than just intensity information. Gradient magnitude is an additional property that has been used predominantly for defining TFs based on 2D histograms. Recently, feature size has been introduced as an alternative. In this work we compare the usability of such 2D histograms for the visualization of 3D scalar data. By giving visualization examples for well-known volume data sets we conclude that

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... ng feature size for TF specification is more intuitive than gradient magnitude based approaches.