Editing Tomography (section)

==Volume rendering==
{{Main|Volume rendering}}
[[File:Image of 3D volumetric QCT scan.jpg|thumb|Multiple X-ray [[CT scan|computed tomographs]] (with [[Quantitative computed tomography|quantitative mineral density calibration]]) stacked to form a 3D model]]
Volume rendering is a set of techniques used to display a 2D projection of a 3D discretely [[Sampling (signal processing)|sampled]] [[data set]], typically a 3D [[scalar field]]. A typical 3D data set is a group of 2D slice images acquired, for example, by a [[CT scan|CT]], [[MRI]], or [[MicroCT]] [[Image scanner|scanner]]. These are usually acquired in a regular pattern (e.g., one slice every millimeter) and usually have a regular number of image [[pixel]]s in a regular pattern.
This is an example of a regular volumetric grid, with each volume element, or [[voxel]] represented by a single value that is obtained by sampling the immediate area surrounding the voxel.

To render a 2D projection of the 3D data set, one first needs to define a [[Virtual camera|camera]] in space relative to the volume. Also, one needs to define the [[opacity (optics)|opacity]] and color of every voxel.
This is usually defined using an [[RGBA color model|RGBA]] (for red, green, blue, alpha) [[transfer function]] that defines the RGBA value for every possible voxel value.

For example, a volume may be viewed by extracting [[isosurface]]s (surfaces of equal values) from the volume and rendering them as [[Polygon mesh|polygonal meshes]] or by rendering the volume directly as a block of data. The [[marching cubes]] algorithm is a common technique for extracting an isosurface from volume data. Direct volume rendering is a computationally intensive task that may be performed in several ways.