Volume Rendering Techniques

Representing a surface contained within a volumetric data set using geometric primitives can be useful in many applications; however, there are several main drawbacks to this approach. First, geometric primitives can only approximate surfaces contained within the original data. Adequate approximations may require an excessive amount of geometric primitives. Therefore, a trade-off must be made between accuracy and space requirements. Second, since only a surface representation is used, much of the information contained within the data is lost during the rendering process. For example, in CT scanned data useful information is contained not only on the surfaces, but within the data as well.

In the next subsections various volume rendering techniques are explored. Volume rendering is the process of creating a 2D image directly from 3D volumetric data. Although several of the methods described in these subsections render surfaces contained within volumetric data, these methods operate on the actual data samples, without the intermediate geometric primitive representations used by the surface rendering techniques.

Volume rendering can be achieved using an object-order, an image-order, or a domain-based technique. Object-order volume rendering techniques use a forward mapping scheme where the volume data is mapped onto the image plane. In image-order algorithms, a backward mapping scheme is used where rays are cast from each pixel in the image plane through the volume data to determine the final pixel value. In a domain-based technique, spatial volume data is first transformed into an alternative domain, such as compression, frequency, and wavelet, and then a projection is generated directly from that domain.

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