However, the pixel density, usually specified in pixels per degree or in arcminutes per pixel, is also used to determine visual acuity. 60 pixels/° (1 arcmin/pixel) is usually referred to as eye limiting resolution, above which increased resolution is not noticed by people with normal vision. HMDs typically offer 10 to 20 pixels/°, though advances in micro-displays help increase this number. Binocular overlap – measures the area that is common to both eyes. Binocular overlap is the basis for the sense of depth and stereo, allowing humans to sense which objects are near and which objects are far. Humans have a binocular overlap of about 100° (50° to the left of the nose and 50° to the right). The larger the binocular overlap offered by an HMD, the greater the sense of stereo. Overlap is sometimes specified in degrees (e.g., 74°) or as a percentage indicating how much of the visual field of each eye is common to the other eye. Distant focus (collimation). Optical methods may be used to present the images at a distant focus, which seems to improve the realism of images that in the real world would be at a distance. On-board processing and operating system. Some HMD vendors offer on-board operating systems such as Android, allowing applications to run locally on the HMD, and eliminating the need to be tethered to an external device to generate video. These are sometimes referred to as smart goggles. To make the HMD construction lighter producers may move the processing system to connected smart necklace form-factor that would also offer the additional benefit of larger battery pack. Such solution would allow to design lite HMD with sufficient energy supply for dual video inputs or higher frequency time-based multiplexing (see below). Support of 3D video formats Depth perception inside an HMD requires different images for the left and right eyes. There are multiple ways to provide these separate images: Use dual video inputs, thereby providing a completely separate video signal to each eye Time-based multiplexing. Methods such as frame sequential combine two separate video signals into one signal by alternating the left and right images in successive frames. Side by side or top-bottom multiplexing. This method allocated half of the image to the left eye and the other half of the image to the right eye. The advantage of dual video inputs is that it provides the maximum resolution for