Parreira, Pedro F.Mendes, Ana R.Lopes, Daniel SimõesJorge, JoaquimDias, Paulo and Menezes, Paulo2020-09-242020-09-242020978-3-03868-128-1https://doi.org/10.2312/pt.20151198https://diglib.eg.org:443/handle/10.2312/pt20151198The visual quality of volume rendering images and the contrast between different anatomical structures strongly depends on the type of transfer function that is applied by the volume renderer. While conventional windowicon-menu-pointer approaches refer the user to apply pre-defined transfer functions for isolating specific tissues, these are not always desirable for many clinical and surgical scenarios. This leaves the user with the tedious and time consuming task of manual transfer function design. In order to make this task more natural, we propose a sketch-based interface where touch gestures are used to directly control voxel opacity and colour. We also present new transfer function design approaches that aim to reduce the number of trial and error iterations, which usually are necessary for finding appropriate transfer functions. By following our approach, users were able to more rapidly explore and understand volume data, as the number iterations necessary to obtain a desirable transfer function where reduced. For both transfer function design and volume manipulation tasks users responded more favourably to our sketch-based interface when compared to standard mouse and keyboard controls, as the former approach promotes reduced design iterations and improve the user's overall understanding of the data.Transfer FunctionVolume Rendering3D Medical Images10.2312/pt.2015119813-19