1. A system for visualizing a flow within a volume of a 3-dimensional (3-D) image, the system comprising: a first transfer unit configured to apply a first transfer function, wherein the first transfer function assigns a renderable property to each location of a first plurality of locations within the volume of the 3-D image based on a flow pattern class assigned to each location of the first plurality of locations, the flow pattern class being assigned from a plurality of flow pattern classes for classifying the flow at each location of the first plurality of locations; a second transfer unit configured to apply a second transfer function, wherein the second transfer function assigns a renderable property to each location of a second plurality of locations within the volume of the 3-D image based on a value of the 3-D image assigned to each location of the second plurality of locations; and a mixing unit configured to compute a first 2-dimensional (2-D) image based on the renderable property assigned to each location of the first plurality of locations and a second 2-D image based on the renderable property assigned to each location of the second plurality of location, the mixing unit being configured to compute the first 2-D image and the second 2-D image for displaying on a display by rendering using the renderable properties assigned by the first and second transfer functions, the mixing unit being further configured to display the first and second 2-D images alternatingly for a predetermined period of time, wherein the plurality of flow pattern classes are defined by ranges of curvature and torsion values, each of the plurality of flow pattern classes having a different range of curvature and torsion values, and wherein the flow pattern class assigned to each location of the first plurality of locations is assigned directly based on curvature and torsion values calculated for each location of the first plurality of locations.