Volume Visualization: Direct Volume Rendering

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Volume visualization is the technique of displaying two-dimensional projections of three-dimensional (volume) data.
Volume rendering techniques of 3D data are classified as direct and indirect. Indirect volume rendering methods, such as Marching Cubes [1], extract intermediate geometric meshes of iso-surfaces from volume data and render them using surface rendering methods. Indirect methods are more suitable for applications where the visualization of surfaces is important. Direct Volume Rendering (DVR) techniques render the volume data without generating any intermediate representation; thus facilitating the visualization of the material inside. By specifying a proper transfer function in the DVR process, voxels are assigned with certain optical properties and different structures are revealed in the resulted images.
The techniques of DVR are useful for various kinds of scientific visualization, such as medical diagnosis and life science research. However, structures may not be clearly shown in the resulted images, due to various reasons like poor lighting and reflection parameters. Thus, the pixel values cannot provide enough implication on existences of structures. For example, a homogeneous region in a DVR image may represent the fine details of a structure. The image should have displayed a variation to indicate this feature. Meanwhile, many data processing operations may have been applied to the data set before the volume rendering process taken place, including

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