A cube has 8 unique vertices. Is it true that each of these 8 vertex normals (unit vectors) is making 135 degree angle to each of the edges which shares that vertex? And the vertex normal pointing outward/out of the cube? Your answer should be technically correct. Or it depends on how the cube is defined (drawn) like using triangle strips or indices that define 2 triangles for each side of the cube? The purpose of the vertex normal is smooth shading and lighting in OpenGL ES application.
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If the cube is defined by 8 unique vertices, then the normals will likely be making a 135 degree angle to each edge, as you mentioned.
However, a cube is often defined using 24 vertices for exactly this reason. This allows you to have vertex normals that are perpendicular to each face, by “duplicating” vertices at each corner. Defining a cube this way is, effectively, just defining 6 individual faces, each pointing outwards appropriately.