Length and Direction of Cast Shadows
To calculate the length and direction of a cast shadow, follow the steps described in the sequences of illustrations shown below. Further examples of how cast shadows work show you what happens in a variety of lighting and compositional situations. Knowing how to do this is useful when you are drawing from your imagination or when cast shadows are obscured by objects you don't want to include in your picture.
1. To calculate length and direction of a cast shadow, first locate the light source, and then locate the point directly beneath the light on the surface (plane) receiving the cast shadow. Make a mark ot the imagined point of contact. (When the sun is close to the horizon, the horizon is the point of contact.)
2. From this mark, draw lines through the outer edges of the part of the object touching the ground. This will give us the direction and width of the cast shadow. (Treat the object as though it were transparent to locate the edges on the back side.)
3. Draw lines that start from the light source, touch the top of the object, and continue until they intersect with the lines thar define the edges of the cast shadow. Where these lines meet marks the length of the cast shadow.
4. Place the cast shadow on the ground within these lines.
The cast shadow of each object in a composition is calculated separately.
If the light source is central to a group of objects, all the cast shadows will radiate from that central point.
When the light source is off the page, its position must be imagined to calculate the length and direction of cost shadows within the picture.
Notice how a cast shadow can be calculated even when the surface receiving it changes direction.
To find a point along the perimeter of the cast shadow, draw a vertical line from the point on the object you want to locate to the ground beneath it. (In the tirst of these examples, the tree trunk is the vertical line and the top of the tree the point we want to locate.) Then draw lines from the light source and the point beneath the light source (you may have to imagine the exact location of this point on the ground) through the top and bottom of this vertical line. Where these lines cross will be that particular point in the cast shadow. This is helpful in locating the corners of cubes in cast shadow or the shadows of floating objects, and in plotting the cast shadows of curving objects.
When the sun is directly overhead, as at midday, the lines you draw from the light source to the ground to calculate the length of cast shadows are virtually parallel.
For close up views of small objects depicted in midday sunlight, I draw the lines from the light source exactly parallel to calculate the length of the cast shadows.