Just in case drawing and painting weren’t difficult enough already, we will now discuss another visual anomaly that only artists think about. To most people, a cast shadow is simply a cast shadow. But as artists we must ask, “What is the light source?” If the answer is anything but, “the sun,” then we know that a different rule applies to rendering these shadows.
|Gerge Caleb Bingham, Family Life on the Frontier, before 1845|
In a previous post, we’ve discussed how to calculate and draw shadows created by the sun. When working with a local light source, the rules are slightly different.
We begin by determining how high off the ground the light source is and drawing a line from the light source directly to the ground (fig. 1). The point on the ground below the light is known as the shadow trace point. As with the sun, the light source may be on or above your paper.
From the shadow trace point, draw a straight line out through the bottom corners of all of the objects (fig. 2). This process is the same regardless of whether the objects are in one or two point perspective.
Then draw a straight line from the light source through the top corners of the objects until they intersect the shadow trace point lines (fig. 3).
Connect the intersection points to finish creating the shadows (fig. 4).
In an indoor scene, cast shadows are likely to be interrupted by walls. To show this visually, we begin again by drawing a line to the ground from the light source to find the shadow trace point. Again, draw a line from the shadow trace point through the bottom corners of the object. When they reach the wall, continue them upward as straight lines parallel to the side of the wall (fig. 5).
As before, draw a line from the light source through the top corners of the object until they intersect with the shadow trace point lines. Where they intersect will mark the corners of the object’s shadow (fig. 6).
This can become more difficult as the objects you draw get more complicated. For example, if your object has overhanging edges, take a line directly down from the edges to the ground. From there, draw a line from the shadow trace point (fig. 7).
Now draw lines from the light source through all top edges and any overhanging edges until they intersect with their corresponding shadow trace lines. Once they meet, connect the points to create the object’s shadow (fig. 8).
These are very good exercises to practice with a simple pencil and ruler. With practice, you will be eventually able to create shadows without specifically measuring the shadow trace point and taking lines back to the light source. This is particularly useful when creating shadows for organic shapes, as George Caleb Bingham has done in Family Life on the Frontier.