Ray Tracing
Introduction
Ray tracing is a rendering technique for generating an image by tracing the path of light as pixels in an image plane and simulating the effects of its encounters with virtual objects. The technique is capable of producing a very high degree of visual realism, more so than typical scanline rendering methods, but at a greater computational cost. This makes ray tracing best suited for applications where the image can be rendered slowly ahead of time, such as in still images and film and television visual effects, and more recently, in real-time applications such as video games and VR.
History
The concept of ray tracing comes from as early as the 16th century with Albrecht Dürer's work on perspective drawing, and later with da Vinci's studies on light. The modern version of ray tracing was first described by Arthur Appel in 1968 in his paper "Some Techniques for Shading Machine Renderings of Solids". In 1980, Turner Whitted improved upon Appel's ray tracing algorithm to include effects such as reflection and refraction.
Mathematical Foundation
Ray tracing is based on the geometric optics approximation of Maxwell's equations, which states that the wavelength of light is much smaller than the smallest feature of interest in the scene being rendered. This allows for the simplification of light to a straight line, or ray. The ray is traced backwards from the viewpoint (the eye), through a pixel in the image plane, and into the scene. The color of the pixel is computed based on the objects encountered by the ray.
Ray Casting
Ray casting is the first step in the ray tracing process. It involves shooting a ray from the eye, through the pixel, and into the scene, and determining the first object in the path of the ray. This is typically done using a bounding volume hierarchy or a similar spatial data structure to speed up the intersection tests.
Shading
Once the first intersection point has been found, the next step is to compute the color of the pixel. This is done by simulating the interaction of light with the material at the intersection point. This involves calculating the direct illumination (light that comes directly from the light sources) and the indirect illumination (light that has bounced off other objects).
Reflection and Refraction
Ray tracing can simulate reflection and refraction by recursively tracing rays from the intersection point. For reflection, a ray is traced in the mirror direction. For refraction, Snell's law is used to trace a ray through the object.
Shadows
Shadows are computed in ray tracing by shooting a shadow ray from the intersection point towards each light source. If the shadow ray intersects an object before reaching the light source, then the intersection point is in shadow with respect to that light source.
Anti-Aliasing
Ray tracing can produce jagged edges or aliasing artifacts in the image. Anti-aliasing techniques, such as supersampling and adaptive sampling, can be used to smooth out these edges and produce a higher quality image.
Performance
Ray tracing is computationally expensive due to the large number of rays that need to be traced and the complex calculations for each ray. However, advances in hardware and algorithms have made real-time ray tracing possible in certain applications.
Applications
Ray tracing is used in a variety of fields, including computer graphics, optical system design, and radio wave propagation. In computer graphics, it is used to produce high-quality images and animations. In optical system design, it is used to simulate the propagation of light through an optical system. In radio wave propagation, it is used to predict the path of radio waves in urban environments.
Future of Ray Tracing
With the advent of more powerful hardware and more efficient algorithms, ray tracing is becoming more prevalent in real-time applications. Future developments in ray tracing could lead to more realistic and immersive virtual reality experiences, and more visually stunning video games.