Manual/Materials
From BlenderWiki
Introduction
Before you can understand how to design effectively with materials, you must understand how simulated light and surfaces interact in Blender's rendering engine and how material settings control those interactions. A deep understanding of the engine will help you to get the most from it. The rendered image you create with Blender is a projection of the scene onto an imaginary surface called the viewing plane. The viewing plane is analogous to the film in a traditional camera, or the rods and cones in the human eye, except that it receives simulated light, not real light. To render an image of a scene we must first determine what light from the scene is arriving at each point on the viewing plane. The best way to answer this question is to follow a straight line (the simulated light ray) backwards through that point on the viewing plane and the focal point (the location of the camera) until it hits a renderable surface in the scene, at which point we can determine what light would strike that point. The surface properties and incident light angle tell us how much of that light would be reflected back along the incident viewing angle (Rendering engine basic principle.).
Two basic types of phenomena take place at any point on a surface when a light ray strikes it: diffusion and specular reflection. Diffusion and specular reflection are distinguished from each other mainly by the relationship between the incident light angle and the reflected light angle. The shading (or coloring) of the object during render will then take into account the base color (as modified by the diffusion and specular reflection phenomenon) and the light intensity
Using the internal raytracer, other (more advanced) phenomena could occur. In raytraced reflections, the point of a surface stroke by a light ray will return the color of its surrounding environment, according to the rate of reflection of the material (mixing the base color and the surrounding environment's) and the viewing angle. On the other hand, in raytraced refractions, the point of a surface stroke by a light ray will return the color of its background environment, according to the rate of transparency (mixing the base color and the background environment's along with its optional filtering value) of the material and the optional index of refraction of the material, which will distort the viewing angle.
Of course, shading of the object hit by a light ray will be about mixing all these phenomena at once during the rendering.
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