In making the visual introduction to shaders we introduced the concept saying:
Synthetic images, as we are calculating with Blender, are rendered with different types of algorithms; results vary the final representation of the objects in the scene.
The mathematical models which allow a synthetic image are grouped into what is called “Shaders” the Shader.
A pixel Shader (the English pixel shader, abbreviated PS) is a shader program, typically run in the graphics processing unit. In OpenGL is called fragment shader.(W)
The basic shader using Cycles, el motor de render the Blender, is called “spread” based on the model of Lambert or of Oren-Nayar, based on the value that is used for the parameter “Roughness” ( 0 = Lambertian).
Increasing Roughness parameter harden shadows, sharply increasing the contrast between the different parts of the model as a function of illumination.