Human observers were shown projected angles, embedded in solid cross-like figures and wen: asked whether these projected angles could be the projection of an orthogonal angle in 3-D space (i.e. whether the two legs of lie cross were orthogonal to each other). We found that performance depended on the viewpoint at which the angle was viewed: Both slant (i.e. the angle between the normal of the target angle relative to the plane of projection) and roll (i.e. the rotation around the normal of the target angle) had a systematic effect on the proportion of errors when observers were shown non-orthogonal angles. With orthogonal angles, however, this effect was absent (i.e. very low error rate with no systematic effect of slant and roll). Instead of assuming a viewpoint-dependent bias towards orthogonality, a computational analysis of the task; using a Bayesian approach, and a computer simulation showed that the viewpoint-dependency can be modelled by a fixed set of biases in order to constrain the set of possible scenes that could give rise to the projection. (C) 2000 Elsevier Science Ltd. All rights reserved.