translucency research

Low-Level Image Cues in the Perception of Translucent Materials

ROLANDW.FLEMINGandHEINRICHH.BU ̈LTHOFF Max Planck Institute for Biological Cybernetics

Extracts:

1.2 Translucent Materials

Many of the materials that we encounter on a daily basis are partially permeable to light. For example, fruit flesh, wax, cheese, and human skin are all somewhat translucent. The light that bleeds through these objects gives them a characteristic visual softness and glow that plays a major role in their distinc- tive appearance. Here, we study some of the cues that give translucent materials their characteristic “look.”

Translucency is potentially a very important physical property for an organism to identify. It can help us to distinguish between materials, such as milk and white paint, and can even inform us about the functional state of an object. For example, proteins, such as albumen or meat flesh, become more opaque when cooked, while fruits become increasingly translucent as they ripen, growing quite transparent as they begin to rot. Absinthe acquires a distinctive opalescent louche when mixed with water and transparent plastic becomes white and opaque when subjected to repeated stresses. How do we recognize translucent objects? What image cues do we use to distinguish translucent from opaque?

1.4 Subsurface Scattering and the BSSRDF

When light strikes a real translucent object, it passes through the surface, refracts and scatters multi- ple times within the body of the medium, before emerging at some other location on the surface. This phenomenon, called subsurface scattering, causes light to spread out into a diffuse region around the point of illumination.

[discusses importance for perception of highlights, colour, size of object]

Light diffuses through translucent materials, much like dye diffusing through a fluid. Translucent objects become “filled” with light when illuminated. An important consequence of this is that points on the surface that do not receive any direct illumination (i.e., they are in shadow) can nevertheless receive light from within the body of the object. Conversely, regions that receive strong direct illumination tend to dissipate the incident light by transmitting it to other parts of the object. This has the effect of reducing the overall contrast of translucent objects, as shown in Figure 16(a).

Although blur clearly appears to be related to translucency, we have found that it is insufficient, on its own, to produce a vivid percept of translucency.

To summarize our main findings, we will conclude with a few suggestions for artists and animators who wish to render translucent materials.

Translucent objects look most realistic when they are glossy. Although highlights are not a direct consequence of subsurface light scatter, nevertheless, most translucent materials that we commonly encounter (e.g., fruit flesh, gemstones, and mucos), are somewhat glossy. Thus, the visual system “ex- pects” translucent objects to have specular highlights. Glossiness also aids the perception of shape, by recovering detail that is lost by the softening effects of subsurface scatter.

Color can be used to modify the subjective quality of a translucent substance. If we wish translucent objects to look “glowing” and “warm,” color saturation should be positively correlated with intensity. By contrast, if we wish them to look “icy” or “dilute,” the correlation should be negative.

Translucent objects should generally be lower contrast than their opaque counterparts under similar lighting conditions. However, the relationship between translucent and opaque versions of an object is generally nonlinear. Thus, to portray a translucent object realistically, it is not sufficient simply to reduce an opaque object’s contrast (using Photoshop, for example). It is necessary to modify the entire distribution of intensities. As a simple heuristic, objects can be made to look somewhat more opaque by passing them through a sigmoidal nonlinearity, or more translucent by passing them through the inverse, i.e., an “N-shaped” nonlinearity.

ACM Transactions on Applied Perception, Vol. 2, No. 3, July 2005.Perception of Translucent Materials • 379

Sharp cast shadows should be avoided as they make objects appear hard and opaque, while blur and loss of detail gives translucent objects their characteristic soft appearance. However, as with contrast, we cannot make an opaque object appear translucent simply by blurring out the details. Attention must be paid to the global effects of light bleeding through the shadowed region of an object if we wish to portray a translucent material. Haze around the shadow boundary and blurry fringes around sharp corners can add a sense of diffusion and glow.

Translucency can also be exploited to indicate physical size. The thinner or smaller an object is, the more light bleeds through, providing information about physical scale.

Finally, we have found that translucent objects appear more translucent when illuminated from behind, rather than from the front. One consequence of this is that if we wish to enhance or emphasize the apparent translucency of an object, we should organize the scene lighting so that the object is illu- minated predominantly from behind. It is the light that has traveled all the way through a translucent medium (rather than scattering back in the direction of the light source) that is responsible for the material’s characteristic visual appearance.

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