Continuing from Chapter 5: Ray-Tracing Algorithms
Ray-tracing algorithms highlight the vital importance of lighting to the interpretation of a scene, whether it be the stark bright light of the sun, the more diffuse glow of the blue sky, indirect light reflected by other surfaces, or light glowing or shining from luminiferous objects. We see the light not merely as a feature in the scene, but as a directed, causal process, that intimately links the light source with the patch of the world that it illuminates: we perceive the light as the causal source of the illumination of the scene.
However at the same time we also perceive the reverse inference: The illumination of the scene implicates a light source illuminating it. And the nature of the illumination reveals information about the light source, even if the source itself is currently invisible, or out of our field of view.
An overall color cast across the whole scene implies that it is the illumination source that accounts for the color across the scene. The color that is distributed across the whole scene is attributed to a single causal agent, the light source.
The direction of the shadows in the scene point backward from the occluder back toward the light source, even if the light source is outside the field of view, giving a distinct amodal percept of the invisible light source that can be localized with some precision.
This spatial inference is exposed by smoke or mist in the air that reveals the invisible beams of light as a glowing shaft of illumination slanting through empty space, making explicit that which is perceived only implicitly in clear air. The haze turns the invisible amodal percept of the directed beams of light into a visible modal experience.
Multiple shadows cast from every object implicate multiple light sources, perhaps with different colors and properties. Sharp stark shadows indicate a point-like light source. Softer shadows indicate an extended source.
The causal connection between a light source and the world it illuminates becomes even more explicit in the case of local light sources that illuminate only local patches of the world.
We are not at all surprised to see the patch of illumination vanish as the lamp disappears. But we are puzzled if the lamp disappears but its illumination remains! There is no clearer example of the perception of direct causality than that which connects the light source to the patch of the world that it illuminates.
Consider this ray-traced scene of a dollhouse, from the POV-Ray gallery. Notice how each illuminated patch on a wall is accounted for, or “explained” by the light source pointing at it. Without the lamp, the bright patch on the wall might be misinterpreted as an actual discoloration, or bleached stain on the wall. The presence of the lamp helps explain each otherwise anomalous patch of brightness, and at the same time, the patches of brightness reveal and explain the presence of the lamps that are causing them. The cause reveals its effect, at the same time that the effect implicates its own cause.
Clearly what is happening in perception is a factoring of the intrinsic color or brightness of an object, and the pattern of illumination shining on it. When all the surfaces pointing in one direction are brighter than the surfaces pointing in other directions, that suggests that those surfaces are oriented toward the light, thus implicating the direction of illumination. Knowing the direction of illumination, in turn, helps disambiguate the perception of otherwise ambiguous forms.
This exemplifies a recurring theme of one of the most puzzling aspects of perception, of a kind of circular inference, where A implies B, but only if B also implies A, and thus the result cannot be computed in a single pass, but must emerge by a kind of resonance between all parts of the scene, to equilibrate to a globally consistent state. This reciprocal causality is demonstrated most explicitly in the ray-tracing model of mirroring, for example with two mirrors pointing at each other to create an “infinite tunnel” of reflections. In the first iteration each mirror records an image of the other by reflection. In the second iteration, each mirror sees its own reflection in the other mirror, and thus each mirror contains the other mirror in their reflections. In the third iteration each mirror sees the reflection of the other mirror in their own reflection producing three reflections in each mirror, and so on through more iterations potentially to infinity. Performed in analog with two real mirrors and actual light, the computation occurs virtually instantaneously, literally at the speed of light. In the ray-tracing algorithm this kind of cyclic reflection can require some of the most computation intensive processing. In fact the user must specify a limit to the number of cycles if the computation is ever to come to an end. And yet in perception, our automatic and instinctive, almost “unconscious” computation of the illumination source when viewing a scene, also seems to occur virtually instantaneously, thus strongly implicating a parallel analog wave-like computation not unlike the actual light reflecting back and forth between two real mirrors.
The perception of light propagating through space is a necessary prerequisite to the reliable perception of objects in that space, if the sensory evidence for those objeects is the result of their illumination by a light source, and their optical projection on the retina.
Continued Chapter 7: The Inverse-Optics Problem