L'ombre et la lumière ne sont généralement pas les premières choses auxquelles nous pensons lorsque nous essayons de maîtriser la perspective pour nos dessins, il s'agit pourtant d'une des manières les plus efficaces de créer un sentiment de profondeur dans vos dessins. Comme on peut le voir dans cette illustration, ce qui ressemble à un cercle aplati et incomplet devient une sphère en trois dimensions lorsqu'on y place l'ombre correctement, ce qui a pour effet de révéler la part de lumière du dessin.
En fait, l'ombre et la lumière sont des notions plus complexes qu'il n'y parait. Regardons mon dessin de sphère de plus près.
In general terms, we can say that the sphere has a light side and a dark side; that dark side is made dark by what artists call the “form shadow”.
Form shadow is the shadow that on object casts on itself. In our example of a sphere, as the ball curves away from the light, it darkens because its own shape is blocking the light falling on it from the upper right. We are familiar with this idea from what we learn in school about our Earth; in fact, we have light and day because the globe’s own form blocks the light from the sun.
Let’s go back to the sphere. We see a shape because the form is created by four degrees of either light or shadow, that are called “values”: the highlight, the mid-tone (or halftone), the core shadow and the reflected light.
The highlight is where the light hits the object most directly, and in your drawing will look like a spot of white; as you work on your drawing, be careful to keep that place on the paper pristine clean from the beginning so that white will be as brilliant as you can make it.
The mid-tone or halftone is where the light grades away from the highlight, and where the object is curving away from the light source.
Then comes the core of the form shadow, the darkest dark on the sphere.
Last is the tricky bit: the reflected light. You will have a natural tendency to want to fill this area in with the same black as the core shadow, carrying the core shadow right to the edge of the object. However, there will be light bouncing from the tabletop back up into the shadow in most usual lighting conditions, and it’s your job to look carefully and see it. This is what will make your drawing look like one done by a professional, and really make the object have depth.
It is key as you draw to make these four values of the light and shadow softly grade into each other, yet to create four distinctly different degrees of light and shadow; in order of light to dark, the highlight will be brightest, then the halftone of the lit side, then the area of the reflected light, then the core shadow; the halftone and the area of reflected light should be different values.
Cast shadow is the shadow created when one object blocks the light to another. In our example of the sphere, the cast shadow is created on the tabletop by the sphere blocking the light to the tabletop. If we think about astronomy, the shadow created on the Earth during an eclipse by the moon blocking the sun’s light is one big cast shadow. Therefore, when drawing an object, bear in mind that the cast shadow will fall under the dark side of the object away from the light source.
Like form shadow, cast shadow is not consistently black. It will be at its darkest where it meets the object, and grade away into a grey as it gets further from it.
Also, contrary to the soft edges of form shadows, cast shadows have hard edges, especially near the object casting the shadow. The edge will soften as it moves away from the object casting it.
This is one of the reasons that Ancient Egyptian or Early Medieval art looks as flat as it does – there is no development of light and shadow to create the illusion of depth. By the Renaissance, however, light and shadow was used to great effect to create perspective.Take a look at my drawing of the four basic shapes, and note the difference between the form shadows and the cast shadows.