The perception of colour gradients

Chromatic shading on a surface (spatial gradients in colour) may arise from several physical factors, e.g.: variations in surface pigment concentration, shadows caused by occlusion of a spectrally non-neutral light source, and mutual reflections between surfaces of different spectral reflectance. We have shown that chromatic shading due to mutual illumination can provide cues to object spectral reflectance and scene geometry (Bloj et al 1999). To explore the role of chromatic shading in providing such information, we are investigating the sensitivity of the human visual system to chromatic shading, under varying image contexts. In a simultaneous discrimination task we measure and compare sensitivity to chromatic only and luminance only gradients. On each trial, a reference and test image (each 10 degree square) are presented centrally at a separation of 5 degrees (with side of reference presentation randomised between trials) for 500ms, followed by a static mask for 500ms and then by a neutral full field which remains until observers indicate with a button-press which of the two images contained a larger change in hue or brightness over the extent of the shading. The reference image contains a one-dimensional gradient (vertical or horizontal) in hue or luminance extending over the central 5 degrees, with a 2.5-degree border of a baseline neutral colour. For different image contexts, the edges of the gradient are either gaussian-blurred with the border colour or sharpened by a luminance edge. Within each block, the upper limit of the hue or luminance gradient remains fixed, as does the border colour everywhere, while the lower limit of the gradient is varied between the test images. For blurred-edge gradients, thresholds for discriminating changes in chromatic gradients are lower than for luminance gradients, measured as the just-discriminable difference in total cone contrast between the limits of the gradient. Discrimination thresholds for sharpened-edge chromatic gradients are lower than for blurred-edge chromatic gradients. The results are consistent with the hypothesis that chromatic gradients whose edges coincide with luminance edges are more likely to be due to intrinsic surface properties than to extrinsic shading, and that therefore sensitivity to chromatic gradients in that context is increased.