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is a small program that defines the properties of a surface for OpenGL or Direct3D rendering.
Usually this program is written in one of shading languages
The "materials" you see in games are actually shaders. Examples can include glass, water, car paint shaders. Shadows can be done with shaders too.
The most basic task that shader achieves is determining how much light from a light source is reflected to the observer in any particular point. During the rasterization
process, each point is goig through the shader, though actually up to 17000 points in parallell