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Accurately depicting the 3D objects on a 2D surface is a challenge faced by many artists, regardless of media. We wallers, with our digital canvases, are no exception and so chances are that the word perspective is already your acquaintance.

Because the application of perspective is ultimately a representation of what we see, it is somewhat intuitive. However, instincts will only take you so far and observation can be an inadequate teacher. No approach compares to building a solid technical foundation.

Terminology
These are terms that you've probably encountered at some point. But for those to whom they are unfamiliar, let's start out with some definitions:

Rectilinear perspective is the most commonly known form of perspective--a depiction of 3D objects in space as we see them using a 2D surface. Straight lines always appear straight regardless of whether or not they are the X-, Y-, or Z-axis.

Curvilinear perspective is usually known as the fish eye effect. In contrast to rectilinear perspective, only the X-, Y-, and Z-axis appear straight. All the other lines parallel to them appear curved (even though they represent straight lines in the 3D world).

Vanishing point is a point at which a set of parallel lines in an image appears to converge. These perspective-defining points may be inside or outside the canvas.

Horizon Line
The limit of how far we can see along the ground till the earth meets the sky is called the horizon. This is what you should draw first when setting out to design a scene. Because vanishing point(s) that are used to define all horizontal lines must be located along the horizon, the horizon line is an essential guide in defining perspective.

So what in turn serves as the reference for the placement of the horizon itself? Well, the key factor is the viewing angle.

A. Bird’s eye view (a high angle): When looking downward from an elevated location, the horizon line is located in the upper regions of the image.

B. Eye level (a neutral angle): A horizon line located between one-quarter and one-half of the way up from the bottom canvas edge is used to create the impression that the viewer is at the average human height and looking straight ahead. (Of course, this estimate doesn't account much for topography.)

C. Worm’s eye view (a low angle) When looking upward, above your level line of sight, the horizon line used is very low.

Point Perspective

Zero-point
This special case is commonly seen in nature scenes where organic shapes dominate. The illusion of depth is created purely through scale, blur, color, saturation, and contrast.

scale: Every object has an "expected size". When it is larger than that standard, we know it is very close to us. When it is increasingly smaller, we perceive it as being increasingly more remote.

blur: Typically, only objects within a certain distance appear focused. Objects that are either too close or too far will look out of focus. And so greater blur, when combined with scale, can imply greater distance.

color, saturation, and contrast: More distant objects have lower saturation and contrast. Their range of colors also diminishes and they start to take on the color of their backdrop--typically the sky.



While Zero-Point perspective is commonly used, most will think of the three more traditional Rectilinear cases when thinking about perspective.

One-point: As the name implies, there is only one vanishing point. This is common in images featuring a frontal view of an object or an object with no or minimal height.

Two-point: Two vanishing points are used when looking at objects "at an angle." This is applied when one set of edges' convergence is negligible.

Three-point perspective is applied when looking at objects "from the corner." Three vanishing points are often used in images that depict tall objects extending far above and/or below eye level.

After deciding on point perspective, you can apply knowledge of field/angle of view to place your vanishing points.



Curvilinear perspective uses four, five, or even six vanishing points to define the same Cartesian axes normally defined by the three vanishing points of rectilinear perspective.

Instead of each vanishing point being the only apparent convergence point for a unique set of parallel lines, one (in four-point) or two (in five-point) or three (in six-point) pairs of vanishing points work together. In such a partnership, the lines originate from one point and converge at the second on the other side of the image. This type of perspective is applied in creating panoramic views, for its warped effect, or to simulate how things look through fish eye lenses.



While I have made the best attempts to cover a vide array of topics, this article only goes so far, so I encourage you to look at other tutorials dealing with perspective. Where Part I dwells mainly upon theory, Part II will attempt to fill in quite a few gaps between these concepts and the actual walling process.