In 3D graphics, an imaginary line that is perpendicular to the surface of a polygon. It may be computed at the vertex of a triangle, in which case it is the average of all the vertices of adjoining triangles. Or, it may be computed for each pixel in the triangle as in Phong shading. Surface normals are used to derive the reflectivity of a light source shining onto an object. See tessellation, triangle, Phong shading and Gouraud shading.