Abstract
When looking at a mirror from diagonally above it, the mirror may disappear. This disappearance can be a hazard when walking; however, it also has the potential for device applications and could be used as a form of optical camouflage. The stability of this "fading mirror" phenomenon depends on the texture pattern of the ground surface on which the mirror is placed. The disappearance of a mirror is not instantaneous but begins with the disappearance of a part of the mirror; the spatial area of disappearance increases over the span of a few seconds. In this study, to investigate the basic characteristics of this disappearance phenomenon, we measured the extent and speed of disappearance when a mirror (6.4(V) x 8 (H) arc deg) was placed on different ground patterns (size: 38.4 (V) x 21.6(H) arc deg). In Experiment 1, we measured the magnitude and speed of disappearance when the mirror was located on one of several texture patterns comprising bandpass filtering noise while the center frequency of spatial filtering was varied between 0.2 -6.3 c/deg. The results revealed that the fading was the fastest (several arc deg/sec) and strongest (near complete disappearance of objects) when the center frequency was approximately 1.6 c/deg. These results indicate that the fading of a mirror is triggered by a local (within several tens arc min as a diameter) suppression of edge processing, which then extends along the contour of the mirror. In Experiment 2, wherein we measured the fading extent and speed on various naturalistic images, certain image patterns (grass, water-surface, etc.) caused fading twice as fast as the noise patterns. These results suggest that fading rate prediction could improve based on the frequency spectrum of ground textures and that other factors in the higher-order image statistics may boost this fading.