Real image (such as the images on the right), real rays of light are always represented by full, solid lines; perceived or extrapolated rays of light are represented by dashed lines. A real image occurs where rays converge, whereas a virtual image occurs where rays only "appear" to converge. Real images can be produced by concave mirrors and converging lenses, only if the object is placed further away from the mirror/lens than the focal point and this real image is inverted. As the object approaches the focal point the image approaches infinity, and when the object passes the focal point the image
Light field function of points in a particular four-dimensional manifold). Parry Moon dubbed this function the "photic field" (1981), while researchers in computer graphics call it the "4D light field" (Levoy 1996) or "Lumigraph" (Gortler 1996). Formally, the 4D light field is defined as radiance along rays in empty space. The set of rays in a light field can be parameterized in a variety of ways, a few of which are shown below. Of these, the most common is the two-plane parameterization shown at right (below). While this parameterization cannot represent all rays, for example rays parallel to the two planes if