Primary rays start at the viewpoint and pass through a pixel.
If objects with only opaque and non-reflecting surface characteristics are in the scene then the reflected light intensity at the pixel can be calculated at the first intersection point between the ray and an object. The first intersection between each primary ray and an object is calculated by calculating all the intersections and determining the closest one to the viewer. Finally, the normal to the surface at this intersection is determined and then the reflected intensity is calculated. Secondary Rays Secondary rays can start anywhere and may travel in any direction. Secondary rays are generated at reflective and refractive surfaces and also during calculations of shadows.