The horopter refers to the set of points in space that are perceived as being at the same depth or distance as the fixation point when both eyes are focused on that point. The horopter is an important concept in binocular vision and stereopsis, as it helps explain how the brain uses the disparity in the images captured by the two eyes to perceive depth and create a 3D representation of the environment. When both eyes are focused on a single point in space, the visual system aligns the corresponding retinal points of the two eyes. Points on the horopter fall on these corresponding retinal points in both eyes, meaning they have zero disparity and are perceived as being at the same depth as the fixation point. The horopter is not a fixed geometric shape but changes depending on the position of the fixation point and the angle of convergence of the eyes. ![[ETH/ETH - Computational Vision/Images - ETH Computational Vision/image123.png|500]] **The Horopter and Panum's Fusion Area** ![[ETH/ETH - Computational Vision/Images - ETH Computational Vision/image124.png|500]] When both eyes are focused on a point in space, the visual system aligns the corresponding retinal points of the two eyes. Points on the horopter fall on these corresponding retinal points and are perceived as being at the same depth as the fixation point. However, the visual system can tolerate small disparities and still fuse the images from the two eyes into a single perception of depth. This tolerance is described by Panum's fusion area, which is the region around the horopter where the disparities between the images are small enough for the brain to combine the images and perceive them as a single 3D image. Outside of Panum's fusion area, the disparities between the images from the two eyes become too large for the visual system to fuse them effectively, leading to a phenomenon called diplopia, or double vision. This occurs because the visual system cannot reconcile the differences between the two images and perceive them as a single, coherent 3D image. Panum's fusion area is important in understanding how the brain processes depth information from the two eyes and forms a coherent, single perception of the environment. It highlights the visual system's ability to tolerate and fuse small disparities, which is crucial for achieving binocular depth perception and stereopsis.