

What's wrong with this figure?
So What's Going On? Although the impossible triangle certainly looks possible at each corner, you will begin to notice a paradox when you view the triangle as a whole. The beams of the triangle simultaneously appear to recede and come toward you. Yet, somehow, they meet in an impossible configuration! It is difficult to conceive how the various parts can fit together as a real threedimensional object. It is not the drawing itself that is impossible, but only your threedimensional interpretation of it, which is constrained by how you interpret a pictorial representation into threedimensional mental model. Given the chance to interpret a drawing or image as threedimensional, your visual system will do so. It does not generally take a perspective drawing and reinterpret it as flat, because there is a spatial paradox. There are many ways that this figure can be perceived as a possible as a misperceived object. In other words, it is possible to construct a physical model of the impossible triangle that looks impossible from only one angle. See the example below. The true construction is revealed in the mirror. 
Your visual system, however, is very constrainted by how it interprets twodimensional pictorial images into threedimensional mental representations. It is with the help of such constraints that your visual system assigns depth to each point in an image. Furthermore, it is more important for your visual system to adhere to these constraints than to violate them because you have encountered something that is paradoxical, unusual, or inconsistent. It would lead to biological disaster if you were blind to the unusual, inconsistent, or paradoxical. In particular, certain elements of an image correspond to similar elements in your threedimensional interpretation. For example, one constraint is that twodimensional straight lines should be interpreted as threedimensional straight lines. Likewise, twodimensional parallel lines should be interpreted as threedimensional parallel lines. Continuous straight lines are interpreted as continous straight lines. Acute and obtuse angles are interpreted as 90° angles in perspective. External lines are viewed as the boundary of the shape. This external boundary is extremely important in defining your overall mental image of the shape. This can be summed up in "The Generic View Principle," which states that your visual system assumes that you are viewing something from a nonaccidental point of view. This holds unless there is information to the contrary. Let us see how this applies to the impossible triangle.

The figure above represents the top vertice of the impossible triangle. The scene, however, is visually ambiguous. For example, the lines abb'b''a'' can define the boundary of a limb whose extension is occluded by the boundary of surface a''b''b'bcc', which is part of the right limb . There are many other possibilities. Another example can be seen in the photograph above. In this case, the information is supplied by what are known as Tjunctions. This is a junction where the lines meet. Two of the lines are collinear, forming the top of a T. Tjunctions are good (but not entirely infallible) clues to depth and occlusion. The top of the T is usually the occluding contour. The stem of the T presumed to continue behind. Occlusion, however, is a special case for your visual system. Locally, there are no cues that suggest occlusion. It is straightforward to interpret lines abc and a'b'c' as continuous straight lines, not as abrupt breaks. Therefore, lines abcc'b'a' define the boundary of one continuous surface. This would be the case with all three verticies. 

These constraints operate at different levels, first local and then global. When you examine a drawing of an impossible triangle, you first build up a global image by examing the local parts. Each corner in itself is consistent with spatial perspective, although each corner suggests a different angle of the object. Rejoin the triangle and your global perception produces a spatial paradox. Cover any corner (in the right figure) with your hand so that you can only see 2/3rds of the impossible triangle (as in the left figure). It does not matter what corner you occlude. The result is nonparadoxical. You perceive a "broken limb" model of the impossible triangle; however, notice that your perception will change about the nature of the triangle's shape. Take your hand away and the paradox resumes. It is only by joining the third limb to the other two limbs that forces the spatial paradox. If you separately occluded the three different corners, you can perceive three different consistent broken limb models. You can also break the global paradox by occluding or closing off, any part of the parallel lines that form the boundary of a limb. For example, occlude the middle of any limb. The paradox disappears when the straight lines no longer perceived as forming a continuous surface boundary. The Generic View Principle explains why your visual system does not seize upon the infinite variety of possible, but not probable, threedimensional solutions. The genericview principle is so strong at a local level that it recovers a surface representation of an image that is literally impossible from the point of view of object knowledge and spatial perspective. 

