A system detects an object contour with an image acquisition assembly, the object moving relative to the assembly. A line detector scans the surface line by line during a scan cycle, the line being transverse to the relative motion direction. During active periods, a light source emits light synchronized with the scan cycle, allowing the line detector to acquire a first group of at least one lit scan line. A second group of unlit scan line(s) is acquired during non-emitting idle periods. The object passes between the line detector and the light source. A processor receives and analyzes acquired scan lines. For each lit scan line group and a successive second unlit scan line group, the processor identifies a token pattern with a lit segment adjoining an unlit segment. The processor searches the first and second groups for the token pattern ending or reappearing to produce an object contour.