Volume ray casting, sometimes called volumetric ray casting, volumetric ray tracing, or volume ray marching, is an image-based volume rendering technique. It computes 2D images from 3D volumetric data sets (3D scalar fields). Volume ray casting, which processes volume data, must not be mistaken with ray casting in the sense used in ray tracing, which processes surface data. In the volumetric variant, the computation doesn't stop at the surface but "pushes through" the object, sampling the object along the ray. Unlike ray tracing, volume ray casting does not spawn secondary rays. When the context/application is clear, some authors simply call it ray casting. Because ray marching does not necessarily require an exact solution to ray intersection and collisions, it is suitable for real time computing for many applications for which ray tracing is unsuitable.

Classification

The technique of volume ray casting can be derived directly from the rendering equation. It provides results of very high quality rendering. Volume ray casting is classified as an image-based volume rendering technique, as the computation emanates from the output image and 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:

Advanced adaptive algorithms

The adaptive sampling strategy dramatically reduces the rendering time for high-quality rendering – the higher the quality and/or size of the data-set, the more significant the advantage over the regular/even sampling strategy. However, adaptive ray casting upon a projection plane and adaptive sampling along each individual ray do not map well to the SIMD architecture of modern GPU. Multi-core CPUs, however, are a perfect fit for this technique, making them suitable for interactive ultra-high quality volumetric rendering.

Examples of high quality volumetric ray casting

This gallery represents a collection of images rendered using high quality volume ray casting. Commonly the crisp appearance of volume ray casting images distinguishes them from output of texture mapping VR due to higher accuracy of volume ray casting renderings. The CT scan of the crocodile mummy has resolution 3000×512×512 (16bit), the skull data-set has resolution 512×512×750 (16bit).

Ray marching

The term ray marching is more broad and refers to methods in which simulated rays are traversed iteratively, effectively dividing each ray into smaller ray segments, sampling some function at each step. These methods are often used in cases where creating explicit geometry, such as triangles, is not a good option.

Other examples of ray marching