Reasons for the biased distribution of projected directions in an aperture. A) Because longer lines necessarily includes shorter ones, the occurrence of projections generated by the red line moving in the direction indicated by the black arrow on the left will always be greater than the occurrence of satisfactory projections generated by the blue line moving in another direction (black arrow on the right). Thus the most frequently experienced projected direction when viewing linear objects that satisfy a circular aperture will always be the direction orthogonal to the orientation of the projected line (gray arrows). B) Further biases generated by perspective.  Two physically different lines, x and y, can generate projected lines of the same (red) or different (red vs. blue) lengths on the image plane. Because the longer line, y, can, in a different 3-D orientation, generate the same projected length (red) as the shorter line, x, projected lines of different lengths have differerent numbers of possible real-world sources. The same linear source, z, can generate different projected lengths (red vs. blue) depending on its distance from the image plane. Due to the shape of visual space (see <a href="#Fig2">Figures 2A</a> and <a href="#Fig8">8A</a>), more distant sources are available, leading to a further bias in favor of shorter line projections. Both these geometrial considerations lead to more short than long line projections. (After Sung et al., 2008)