Space, in two-dimensional design, is essentially flat; it has height and width, but no depth. There are certain visual cues, however, that can create the illusion of space in the mind of the viewer. By using those cues, artists and designers can create images that are interpreted as three-dimensional.
Size is one of the easiest ways to create the illusion of space. A larger image will appear closer than a smaller one because we observed (very early in life) that objects appear to become smaller as they get farther away.
Overlapping is another easy way to suggest depth in an image. When objects overlap each other, the viewer perceives the one that is covering parts of other to be in front and the one that is covered to be in the back.
Compositional location refers to where a form is positioned vertically in the image. The bottom is seen as the foreground, the part of the image that is nearest the viewer and the top as the background, the part farthest from the viewer. The higher an object is place in the image, the farther back it is perceived to be.
Atmospheric perspective uses value, contrast and color to give the illusion of space. Atmospheric perspective is based on the fact that the farther something is away from us, the more the atmospheric haze may obscure our view of it. By lightening the value, lowering the value contrast, softening the edges, decreasing detail and muting the color, you can mimic the effect of atmospheric haze and create the illusion of increasing distance. Increasing the bluish cast of an image also creates a sense of depth because cool colors recede and warm colors come forward.
Linear perspective is based on the visual phenomenon that as parallel lines (such as railroad tracks) recede into space, they appear to converge at a distant point. Linear perspective not only evokes a feeling of great depth, but it also creates a strong focal point at the place where the lines converge.
Using these visual cues in combination with each other strengthens the illusion of depth.