Visual discrimination of spectral distributions

Human observers seem to be able to use different features that classify materials with a large degree of accuracy. In this paper, we look at human perception of statistical properties of the spectral distribution in a scene. We investigated whether human observers can discriminate just as accurately between coloured textures that have a spectral distribution due either to shading only or to both shading and specular reflectance as between uniform colours. Thresholds for the discrimination of coloured textures are about 15 times as high as thresholds for the discrimination of uniform colours, provided there is a sharp transition between the two colours. However, the coloured texture thresholds are only 1.5 times higher when we introduce a gradual transition between the two colours. There are also distinct qualitative differences in discrimination thresholds for different base colours. These differences cannot be predicted from discrimination thresholds for uniform colours. Human observers are surprisingly good at discriminating between a material edge and a shadow edge in complex scenes. Statistical differences in the orientation of the colour distributions in colour space might be used to accomplish this. In a second experiment we investigated how well observers can discriminate between two linear distributions in colour space that have the same base colour but different orientations. When we vary the line-length in R, G, B space, thresholds cannot be predicted completely by the conservation of the average distance between the two distributions. This means that observers use not only the maximum colour difference in the stimulus to do the task, but other cues are also involved.