Lesson 3 Form Fundamentals: Rectangular Prism
The principles of drawing rectangular prisms will crop up in drawing most organic forms, particularly those that have a line of symmetry such as the torso and head.
The key message I would like to get across here is that you need to actually do the exercises yourself for this to be of much use to you. When we observe a drawing in which the forms follow the rules of perspective enough to be convincing, our eye interprets them as form in space rather than shapes, and stops noticing the relationships that make this work. In other words you look but don't see. When you actually draw them, the mistakes you will invariably make, even if you are reasonably experienced serve as feedback to remind you about how the rules work. You need to "get the feel" for form, need to experience it with your hand and pencil on the page. You need to realise that drawing in perspective does not necessarily mean complex mathematical projections from plans - instead it is learning to follow a few relatively simple rules and relying on your eye's capacity to recognise when something is off, and solve the problem with a rule of thumb.
As for the section on drawing the cylinder in Lesson 1, below you will find some rules of thumb for rectangular prisms - the relationships that I have noticed are most commonly contradicted as people are learning to draw this type of form. These mistakes I have noticed to be very consistent between students, and occur even when I explicitly warn students about them. I suspect strongly that this is to be due to the exact same cause: the automatic integration of sensory data into the perception of objects in space means that we have no conscious awareness of the two dimensional relationships that tell our eye the story. When we draw an object we tend to draw what we know the thing to be rather than how it appears, for example drawing the edges of a rectangular prism as parallel because we know them to be physically parallel, even though they may not be appearing as parallel from that particular viewpoint.
Leonardo da Vinci's advice, in his Treatise on Painting was:
"The young student should, in the first place,
acquire a knowledge of perspective, to enable him
to give to every object its proper dimensions"
Please, please, please do yourself a favour and do the exercises!
Click the images to enlarge: