All Forms are Created From Five Basic Forms
There are only five basic forms from which all other forms are created. They are the sphere, the cone, the cylinder, the cube, and the doughnut shaped torus. Parts of these forms combine to create everything we see. Imagine a half cylinder on top of a cube and you have the shape of a mailbox, a half sphere and a cone make a teardrop form, a fir tree is a cone an oak is a half sphere. The cylindrical coffee mug has a half torus handle.
Values Create Form
Each of these forms has distinctive light and dark value shapes that define them. Spheres are recognized by crescents and ovals. Cones have triangular light and dark value shapes. Cubes and flat surfaces are even blends. Cylinders are stripes. The torus is crescents and stripes. Concave versions of these forms have the same value shapes but without reflected light. (See Shadows/Reflected light.) When you can paint these five forms you can paint all other forms.
A SPHERE is defined by CRESCENTS AND OVALS. Sphere forms are painted with crescent and curved brush strokes. (See Brushes)
CONES are defined by TRIANGULAR values of light and dark. Cones are painted and blended using triangular brush strokes.
CYLINDERS are defined by light and dark value STRIPES. Cylinders are painted with parallel brush strokes.
CUBES and all FLAT surfaces are governed by the same rules. GRADUAL EVEN BLENDS depict a receding flat surface. If there is a flat surface parallel to your canvas, it may be painted with a single color or value. CUBES are various receding flat surfaces. Each surface is a gradual blend. Cubes are painted with parallel brush strokes.
TORUS value shapes combine aspects of two other basic forms. They take the parallel STRIPES of a cylinder for the middle and the CRESCENTS of a sphere for the ends. The torus is painted using crescent and curved brush strokes.
Here you can see that value shapes are stronger than contour lines for the creation of form.
Lighting can be misleading in seeing forms, particularly flat surfaces. Try to see the form first. Then see the lighting on it.