Flounders and cuttlefish have an impressive ability to change colouration, for camouflage and, in the case of cuttlefish, for communication. We pursue the hypothesis that these diverse patterns are created by combining a small number of distinct pattern modules. Independent component analysis (ICA) is a powerful tool for identifying independent sources of variation in linear mixtures of signals. Two versions of ICA are used, one assuming that sources have independence over time, and the other over space. These reveal the modularity of the skin colouration system, and suggest how the pattern modules are combined in specific behavioural contexts. ICA may therefore be a useful tool for studying animal camouflage and communication.

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We are all brought up with the Amesian viewpoint, deriving from the string-chair demonstration of Adelbert Ames III, that perspective information is interpretable as an infinite set of projections of real-world objects that cannot be resolved into a particular interpretation without additional constraints. Indeed, the situation is even less constrained than is implied by the string-chair constructions because the thin strings making up the Ames chair structure were straight, whereas in the limit each line on the retina is itself ambiguous, and could have derived from an infinite array of projections of curved and wiggly lines that happened, by some implausible accident, to be wiggly only within the plane of projection through the eye, and hence to appear straight from its immediate viewpoint. In the face of this array of arrays of interpretative possibilities, is there any sense in which the information provided by a perspective scene is interpretable as just one single structure in space? Opposing the Amesian viewpoint is the Gibsonian view that there is sufficient information in the world to resolve such ambiguities, and that in general we can tell from the information available in the optic array how far away from us all the objects are located. Indeed, our general experience of the world is that we know the distance of the objects we are navigating among, and that we are rarely confused by Amesian ambiguities, so in operational terms Gibson must be right that there is sufficient information to resolve cues into absolute distance perception, but in general it remains a puzzle what actual information we use to solve the task. If we dissect it into the component types of information, the linear perspective array, the horizontal binocular disparity array, the optic flow array, the texture gradient array, and so on, each one provides only relative cues to the actual distances of objects in the scene. If we set any one of these cues up as an array on a computer screen, it provides relative depth information, but each is scaled relative to the external cues of the screen. However, in some cases, the optical array does contain some form of information that resolves the relativity to provide absolute distance information. For binocular disparity, for example, it is the array of vertical disparities introduced by the relative magnification to the two eyes in peripheral view that is unique to each distance. We can ask whether there is such an absolute depth cue for the monocular perspective information in pictures. In general, the answer is 'no'. If we view a scene composed of arbitrary objects and material textures, there is nothing to go on. It is only when there are textural regularities and matching sized objects that we have any chance of resolving the depth structure of the scene. Consider, for example, what is arguably the earliest surviving perspective diagram (Figure 1), for a painting of the 'Adoration of the Magi' by Leonardo da Vinci (1481). The architectural elements are overlaid on a classic perspective pa...

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