In certain cases, images of different scenes can be mixed to produce an image of a novel scene. For example, an image of a pink sphere can be additively mixed from suitable images of a red and a white sphere. Three ways in which scenes can differ are considered: in the spectral composition of the illuminant and in the spectral and the geometric reflectance of scene objects. Sufficient conditions are given for mixing to produce images that correspond to possible scenes. Examples illustrate ways

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... hat mixtures can be used as stimuli in psychophysical experiments concerned with pictorial perception. A. Partitive Mixtures In abstract terms, a partitive mixture ( p mixture) is a weighted sum of basis items, with weights summing to unity. 7 The operationalization of weighting and summing depends on the nature of the basis items. For example, if the basis items are images projected on a screen, then weighting can be achieved by using neutral-density filters to attenuate each image suitably, and summation by superimposing the projected images. Alternatively, if the basis items are digital images, then weighting and summing may be performed on the numerical image values pixel by pixel and independently on each color channel. Although the optical method of p mixing has been successfully implemented, 8 it has several problems (alignment of slides, many processes to gamma correct, etc.).