Editing Volume rendering (section)

{{short description|Representing a 3D-modeled object or dataset as a 2D projection}}
{{for|[[Rendering (computer graphics)|rendering]] of 3D [[wire frame model]]s|3D rendering}}
{{3D computer graphics}}
[[File:Image of 3D volumetric QCT scan.jpg|thumb|Multiple [[CT scan|X-ray tomographs]] (with [[Quantitative computed tomography|quantitative mineral density calibration]]) stacked to form a 3D model]]

[[Image:CTWristImage.png|250px|thumb| Volume rendered [[Computed tomography|CT]] scan of a forearm with different color schemes for muscle, fat, bone, and blood]]

In [[scientific visualization]] and [[computer graphics]], '''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 by a [[computed axial tomography|CT]], [[magnetic resonance imaging|MRI]], or [[Microtomography|MicroCT]] [[Image scanner|scanner]]. 
Usually these are acquired in a regular pattern (e.g., one slice for each millimeter of depth) 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 space|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.

Another method of volume rendering is [[Ray marching]].