Mumbai University > Mechanical Engineering > Sem 7 > CAD/CAM/CAE

Marks: 8M, 7M, 5M, 10M

Year: May 2012, Dec 2012, Dec 2013, May 2014

• Ray tracing is a global illumination model that accounts for the transport of light energy beyond the direct/local contribution from the light sources. Its operation is based on the principles of geometric optics. By tracing the path of light it is able to integrate the tasks of shading, shadow generation, hidden surface removal, and projection and scan conversion into one single computational process.
• In order to construct an image based on the pinhole-camera we need to determine the light rays that are responsible for the pixels. We do so by following their path in the opposite direction i.e. from the viewpoint through the centre of each pixel into the scene. These are called the primary rays. If a primary ray does not intersect any object in the scene, then the corresponding pixel is simply set to some background colour.
• On the other hand, if a primary ray intersects an object, then the colour of the corresponding pixel is determined by the surface shading of the object at the intersection point.
• Several secondary rays are used to compute the three components of this surface shading. The first component is called the local contribution i.e. direct contribution from the light source. We send a shadow ray or illumination ray from the surface point to a light source. If the ray is blocked before reaching the light source, the surface point is in shadow.
• The second component is called the reflected contribution i.e. light reflected from another object surface. This is determined by a reflected ray, a ray that represents the reflection of the primary ray with respect to the normal vector N.
• The third component is called the transmitted contribution i.e. transmission of light energy coming from behind the surface. This is determined by a transmitted ray, a ray that represents the refraction of primary ray with respect to N.

• Parametric representation of a ray

  A ray is not a vector although it may look like one. The difference between the two is that, while a vector is defined by its direction and magnitude, a ray is determined by its direction and starting point. We may represent a ray in terms of two vectors; s ⃗ to specify its starting point and (d ) ⃗ to describe its direction as shown in the figure. These two vectors are used to provide a parametric vector representation for the ray; r ⃗(t)= s ⃗+td ⃗ (0 ≤ t) Where r ⃗(t) denotes family of vectors. When the tails of these vectors are placed at the origin, their heads make up the ray.