For rendering in medical imaging, a transfer function is determined. A simple approach to setting the transfer function uses a combination of a rendered image and the voxel data, providing a hybrid of both image and data-driven approaches. A region of interest on a rendered image is used to select some of the voxel data. A characteristic of the selected voxel data is used to determine the transfer function for rendering another image. Both the visual aspect of the rendered image and the voxel data from the scan are used to set the transfer function.