A planar image is one that renders shadows as a solid dark color, and lighter parts as a solid light color or white, with no shading (1-10). A silhouette is similar but uses only the contour of the whole subject and disregards any light falling on it (11-20).
Both of these are common techniques for making logos with strong imagery in solid colors. Most of the samples are photographically realistic drawings; however, some (8, 10, 14, 20) use a non-realistic style.
Even though some of these designs use more than one color (3, 6, 7, 8 17, 18), the secondary colors could be printed in white or black.
A separate consideration from the overall drawing style (realistic or non-realistic) is the edge quality. This can add distinct flavor without changing the drawing style. Notice how the edges of each shape can be rectilinear — (all of 2), or (just the face 6). On the other hand, all edges can be made curvilinear with either an s-curve or graceful arc (4, 11) and just the cloak (6).
Planar and silhouette renderings can also use other systems of shapes. For instance, (14) is an interesting combination of almost geometric curves with straight lines. The gold halo used on (18) can easily be omitted in one-color situations.
Adapted from Logo Design Theory: How Branding Design Really Works