# Volume ray casting

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Volume ray casting

Volume ray casting, sometimes called volumetric ray casting, is an image based volume rendering technique. It computes 2D images from 3D volumetric data sets. Volume ray casting, which processes volume data, must not be mistaken with ray casting, which processes surface data.

Classification

The technique of volume ray casting can be derived directly from the rendering equation. It provides results of very high quality at the price of long runtime, usually it is considered to be both the best and the slowest technique. Volume ray casting is classified as "image based" volume rendering technique, as the computation emanates from the output image, not the input volume data as is the case with "object based" techniques.

Basic Algorithm

In its basic form, the volume ray casting algorithm comprises four steps:
# Ray casting. For each pixel of the final image, a ray of sight is shot ("cast") through the volume. At this stage it is useful to consider the volume being touched and enclosed within a "bounding primitive", a simple geometric object — usually a cuboid — that is used to intersect the ray of sight and the volume.
# Sampling. Along the part of the ray of sight that lies within the volume, equidistant "sampling points" or "samples" are selected. As in general the volume is not aligned with the ray of sight, sampling points usually will be located in between voxels. Because of that, it is necessary to trilinearly interpolate the values of the samples from its surrounding voxels.
# Shading. For each sampling point, the gradient is computed. These represent the orientation of local surfaces within the volume. The samples are then "shaded", i. e. coloured and lighted, according to their surface orientation and the source of light in the scene.
# Compositing. After all sampling points have been shaded, they are "composited" along the ray of sight, resulting in the final colour value for the pixel that is currently being processed. The composition is derived directly from the rendering equation and is similar to blending acetate sheets on an overhead projector. It works "back-to-front", i. e. computation starts with the sample farthest from the viewer and ends with the one nearest to him. This work flow direction ensures that masked parts of the volume do not affect the resulting pixel.

* [http://wwwcg.in.tum.de/Research/Publications/Raycast Acceleration Techniques for GPU-based Volume Rendering (J. Krüger, R. Westermann, IEEE Visualization 2003)]

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