Illusions of colour and shadow
Colour (chromatic) vision not only tells us about the colour of surfaces but about the structure of the visual world. One way that colour vision informs us about scene structure is by helping decompose the scene into its material and illumination layers ability. This is underpinned by the visual system’s in-built knowledge that colour variations, and those luminance variations that are aligned with them, tend to be material in origin, whereas ‘pure’ luminance variations tend to arise from illumination. Evidence for such in-built knowledge is that shadows appear to be material in origin when strongly coloured, and colour variations enhance or suppress perceived shape-from-shading in luminance variations depending on their spatial relationships. These findings remind us that many of the benefits of colour vision only emerge when studying the interactions between colour and luminance rather than colour vision in isolation.