A relatively moving surface is illuminated with a laser. Light from the laser is reflected by the surface into an array of photosensitive elements; the reflected light includes a speckle pattern. Sums are calculated for outputs of pixels perpendicular to a first dimension along which motion is to be determined. Motion along the first dimension is then determined based on spatial and temporal gradients of the calculated sums. Sums are also calculated for outputs of pixels perpendicular to a second dimension along which motion is to be determined. Motion along the second dimension is then determined based on spatial and temporal gradients of those sums. The array may be rectangular, or may contain arms separated by a pixel-free region.