One way to simplify adding shadows to your perspective drawing is to use the parallel shadow method. Instead of having a designated light source, the shadow is created with parallel lines and 45 degree angles. Here is a finish image of a table using this method for adding shadows.
The image below shows the preliminary steps that were taken to create the shadow area. 1. The ground shadow lines are drawn from the front corner leg parallel to the the horizon line.
2. Repeat this with the middle and back corner leg. 3. Start the angle of the light, which is represented with a dashed line, from each of the three top corners. Use a 45 degree triangle to draw these lines. 4. Draw lines connecting the intersections of the ground lines and light angles. 5. Add a variety of value to the image. The shadow is a darker value than the shade on the object.
Remember, it is important to use different line types when showing each area that has shade and shadow. This will help the viewer distinguish each of these areas. Here are the three line types that were used for the values in the table image. The top value is the table shade, the middle value is the floor shadow and the last value is the wall shadow.
How about take time to use this method to add shadows in your next perspective drawing!