Perspective

• Perspective is the flat representation of a three-dimensional image. The three most important principles of perspective are foreshortening, the horizon line and vanishing points. If you take a pencil and look at its full length and then turn it to point forward, the pencil appears shorter even though its actual length doesn't change. Elements within a painting contain imaginary lines that converge to a vanishing point on the horizon line, which is usually assumed to be at the viewer's eye level.
  
  

Color

• Color saturation decreases the farther an object is from the viewer. Color saturation is the purity of a hue; low saturation means a hue doesn't show up much in an object, whereas more saturation means the object appears as a purer hue. Normally, when you look at a distant object, the color saturation decreases because air disperses light waves and color. Brightness also affects apparent depth; distant objects are less bright for the same reason. Also, you can give distant objects a slightly blue hue in outdoor scenes to simulate the effect of the sky.
  
  

Value

• Value, commonly known as shading, refers to the darkness of a hue on a scale ranging from white to black, with the various tones falling in the middle. A hue is the true color, shades are darker versions of the color and tints are lighter versions. Mix a hue with its complementary color — for example, red and green — to produce a shade of the base hue. Mix the hue with white to produce a tint.
  Different values are used to portray highlights and shadowed areas of an object, determined by the location and intensity of a light source. A single light source, for instance, casts shadows on the opposite side of objects.
  
  

Line Definition

• You can also use decreasing line definition at distance to create the illusion of depth. Instead of making objects in the distance clearly defined, you can smudge the borders slightly to create a smooth transition from one area to another. Use a thicker paintbrush to paint objects in the distance and don't include as many details. For example, if you paint a brick house closer to the viewer and one near the horizon of the image, the one that's farther away will appear as more of a red-brown blur than have any individual bricks showing.