4.3 Parallactic Angle

The parallactic angle, also known as the convergence angle, is formed by the intersection of the left eye's line of sight with that of the right eye. The closer this point of intersection is to the eyes, the larger the convergence angle. The brain perceives the height of an object by associating depth at its top and its base with the convergence angles formed by viewing the top and base.

parallactic angle

• A = arbitrary point at higher elevation
• B = arbitrary point at lower elevation
• Ø_a = larger convergence angle for higher elevation point
• Ø_b = smaller convergence angle for lower elevation point
• D_A = apparent vertical distance to point "A"
• D_B = apparent vertical distance to point "B"
• D_B- D_A = difference in apparent vertical distances to points "A" and "B"

The X parallax and the parallactic angle are related. As X parallax increases, so too does the parallactic angle.

As the eyes scan overlapping areas between a stereo image pair, the brain receives a continuous 3-D impression of the ground. This is caused by the brain constantly perceiving the changing parallactic angles of an infinite number of image points making up the terrain. The perceived 'virtual' 3-D model is known as a stereomodel.