Vertices, Primitives, Fragment and Pixels
OpenGL adopts the Right-Hand Coordinate System (RHS). In the RHS, the x-axis is pointing right, y-axis is pointing up, and z-axis is pointing out of the screen. With your right-hand fingers curving from the x-axis towards the y-axis, the thumb is pointing at the z-axis. RHS is counter-clockwise (CCW). The 3D Cartesian Coordinates is a RHS.
Some graphics software (such as Microsoft Direct3D) use Left-hand System (LHS), where the z-axis is inverted. LHS is clockwise (CW). In this article, we shall adopt the RHS and CCW used in OpenGL.
The inputs to the Graphics Rendering Pipeline are geometric primitives (such as triangle, point, line or quad), which is formed by one or more vertices.
OpenGL supports three classes of geometric primitives: points, line segments, and closed polygons. They are specified via vertices. Each vertex is associated with its attributes such as the position, color, normal and texture. OpenGL provides 10 primitives as shown. Sphere, 3D box and pyramid are not primitives. They are typically assembled using primitive triangle or quad.