It has been reported that the luminance mechanism mediates shadow perception of binary images. We evaluated the luminance additivity law to determine whether shading, which is produced by gradual intensity changes, is mediated by luminance or brightness using pictures with tones: two still lives and a painting. They were depicted by two colors-a reference color and a test color. The reference color was white with a constant luminance profile, and the test color was a mixture of red and green in various proportions. The observer's task was to adjust the luminance of the test color in order that the impression of depth due to shading just disappeared (shading disappearance setting), where the intensity that produced the shading supposed to be equated between the test and the reference colors. The results show that the luminance additivity law holds for shading disappearance settings. This suggests that shading perception is based on luminance or a luminance-type additive mechanism as well as shadow.
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