A system(1)for determining three-dimensional spatial information about an object(2), such as a fish, comprises a projector(4), a camera(5, 6), and a processing subsystem (3). The projector(4)emits structured light towards the object(2). The light has a periodic intensity pattern, having a first spatial frequency in a first direction. The camera(5, 6)captures images of the object(2)over time. The processing subsystem (3)determines aspeed of the object(2), relative to the projector(4), and instructs the projector(4)to control the emitted structured light in dependence on the speed, such that the intensity pattern is shifted, relative to the object(2), between successive images, by a spatial-phase step that is independent of the speed of the object(2). The processing subsystem(3)then usesimages from at least two time frames to calculate spatial phase values for points on the object(2), and uses these spatial phase values to determine three-dimensional spatial information about the object (2).
