If objects are located in a turbid medium, such as a fog or murky water, their hue, chroma and lightness all converge on those of the medium with increasing distance from the viewer. In Photoshop this effect can be emulated very simply by either interposing layers at low opacity between the successively more distant objects (Figure 10.15), or by achieving the same result with a single layer modified by a mask.

This basic principle, like many here, was clearly set out be Arthur Pope. Pope also noted that the situation is more complex for atmospheric perspective, where the blue colour is derived by scattering from the light passing through the medium. In that case, for light toned objects the addition of bluish light from the atmosphere is often exceeded by the removal of bluish light by scattering on the way to the eye. Thus light toned objects tend to become somewhat warm-hued, at least up to the middle distance (Pope, 1931). This effect can be emulated in Photoshop by using an orange layer in multiply mode, masked to respond to depth and lightness (link)

Figure 10.15. Effect of coloured fog. Left: emulated in Photoshop CS2 using interposed layers in normal mode at low opacity. Right: Colors from these spheres viewed from two directions in YCbCr space.

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