Illusory gloss

Maarten Wijntjes & Sylvia Pont

Delft University of Technology, The Netherlands

The movie shows a Brownian surface (which has a fractal structure, but this does not seem to be important for the illusion). The appearance seems to change between matte and glossy, as the ruler illustrates. However, the surface is continuously rendered with matte (Lambertian) reflectance. What is happening is that the height of the surface is stretched in the viewing direction. The illumination direction is transformed together with the stretching to keep the cast shadows constant. This combined transformation of the light and surface height results in an illusory gloss. We initially though that the effect could be attributed to a luminance histogram skew transformation, but this seems not the case. A simple version of the current explanation is that stretching a surface together with the light direction (the so-called Bas-Relief transformation), induces illusory highlights due to darkening at points where the surface normal points away from the optical axis.

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