Thursday, 17 June 2010

Reading Group - Gibson (1979) Chapter 5 Part 3

Chapter 5 Part 3

So far, Gibson has explained how invariant structure can emerge from changes in perspective (see here and here). How he gets down to the real problem. What are the consequences of this structure for the ambient optic array. What is it that I see, as a perceiver, that specifies things about the environment?

How is ambient light structured? A theory

Ambient light is structured by the environment, by surfaces that emit or reflect light. Because the environment is the direct cause of the structure, ambient light contains information about the environment. The colour of things in the environment also contributes to the ambient array. A strange feature of perception is that we perceive light and dark coloured things differently than lighted and shaded things. In other words, we don’t think that a shadow changes a thing’s colour. Indeed, if we think of perception as static, then darkness caused by pigment and darkness caused by shadow are not distinguishable. However, if we think of perception as extended in time then differences become apparent. The colour of an object in the environment is not absolute. It is only specified with respect to relations between the reflectance ratios (colours) of all currently visible objects. We don’t see an object’s colour in isolation of everything else that’s visible. While this might seem strange, anyone who has painted a room has experienced the transformation of a lovely colour on a paint chip to a completely different (frequently hideous) colour in the context of a room. The paint hasn’t changed, but its context has.

The sources of invariant optical structure

In our environment, surface layouts and reflectances tend to persist – things stay the same shape, texture, and colour. So, there is an invariant structure to be perceived.

The sources of variant optical structure

One source of variant optical structure is a moving point of observation – we walk around, turn our heads, move our eyes. The other primary source of variant optical structure is a moving source of illumination, like the sun. We’ve gone through variance caused by locomotion here. Now, we should spend some time thinking about variance caused by moving illumination. When the sun moves across the sky, the lightedness and shadedness of surfaces in the environment changes. So, the layout and colour of things persists, but things come into and go out of shadow throughout the day.

Variants and invariants with a moving source of illumination

Surfaces that face the sun are directly illuminated, surfaces that face the sky (but not the sun) are somewhat less directly illuminated, and surfaces inside a semi-enclosure are even less directly illuminated. Regardless of whether surfaces are directly or indirectly illuminated, there will be a direction from which most of the light comes – a direction of prevailing illumination. When a surface is perpendicular to the prevailing illumination it gets more light than when the same surface is inclined with respect to the prevailing illumination (even less light falls on surfaces parallel to this illumination). The direction of illumination helps us to perceive texture. Imagine shining a flashlight on a roughly textured surface like concrete. If the surface of the concrete is inclined with respect to the flashlight’s beam, then the peaks in the surface facing the light will be more illuminated than the peaks facing away from a light. If all the peaks in the concrete’s texture were evenly illuminated, we would have no basis for perceiving its texture. You can scale this example up to large environmental features like hills and valleys.

So, there's a lot of information about the environment that is available through structure in ambient light. Certain facts about the world (we have a sun that moves across the sky everyday) and certain consequences of being an animal (we move around) help to reveal invariant information about the layout, texture, and colour of surfaces.

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