There are two display windows appearing on the left with one above the other. With this tutorial, you see how the shading of a face is controled by the direction of its vertex normals. We use two rectangles, which we call left and right, for this purpose. The material color for diffused and ambient reflection is set to white for both faces.
The bottom display window shows a top view of the two faces with controlable normals at the common edge. Although each face has two vertices and, therefore, two normals at the common edge, we set them equal and show only one controlable normal for each face. The normal at the common edge for the left face is drawn in red and can be rotated through dragging with the left button down provided that the radio button Left is clicked in the Selection Panel. The vertex normals at the common edge for the right face are drawn as one in green and can also be rotated provided that the radio button Right is clicked. By clicking the Both radio button, the normals at the common edge for both faces are set equal and rotated as one. Vertex normals are also drawn at the other edge of each face but these are fixed and cannot be rotated.
The bottom display window also shows other information. Initially five lights are enabled, one of which is a gray ambient light. Two other lights are a red positional (omni) light on the left and a green positional (omni) light on the right. Two white vectors are drawn at the vertices of the left face and point to the red omni light, while two are drawn at the vertices of the right face and point to the green omni light. The scene is also illuminated initially by red and green directional lights. The direction of the red light is towards the left face as depicted by two white arrows. The direction of the green light is towards the right face.
The top display window shows a front view of the two rectangles (faces). Also, their rendering is based on the enabled lights and vertex normals.