Linear perspective is very easy if all of the objects in your scene are squares and rectangles. But drawing complex shapes tends to be more intimidating. That is, it's intimidating until you realize that most shapes can be found within a rectangle of some sort.

A circle will fit inside a perfect square. An oval will fit inside a rectangle. A triangle can easily be found inside a rectangle once you know where the center of the rectangle is. Look at the illustration of ants at a picnic for examples of each of these.

Many geometric shapes can be found within a rectangle. This is especially easy if you remember to find the center of the rectangle by drawing lines that connect opposite corners inside the rectangle (as seen in the detail on the right side of the illustration).

Therefore, it's a good idea to try to use rectangles — drawn in perspective — to establish the general proportions of the object you're drawing. These rectangles can be turned into three-dimensional boxes to help you get the correct depth. Connecting the shapes by drawing lines to the vanishing points will result in nonrectangle geometric shapes, drawn in perfect perspective.