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 represents a single volume and includes a plurality of elements; applying a plurality of transfer functions to the data set by casting rays through the single volume in a volume rendering process, each of the plurality of transfer functions defining a correspondence of scalar field value and a correspondent color and opacity, wherein: each of the plurality of 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 multiple bit values, each bit value defining a transfer function in the plurality of transfer functions, at 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; and displaying an image at a display device based on the data set after the applying the plurality of transfer functions to the data set.