1. A computer enabled method of depicting an image, comprising the acts of: providing a data-set representing an image in three dimensions, wherein the data-set includes a plurality of elements; and applying a plurality of transfer functions to the data set, each defining a correspondence of scalar field value and a correspondent color and opacity, wherein: each of multiple transfer functions is associated with subset of elements of data for controlling the color and opacity for a corresponding group of pixels, each of the plurality of transfer functions is associated with a correspondent group of pixels via an extended octree structure comprising a bit-field extension with min/max values for each node of the extended octree structure, and each bit in the bit-field extension represents a presence or absence of at least one pixel of data in correspondent sub-volume associated to the correspondent transfer function.