A method of controlling an additive manufacturing process in which a directed energy source is used to selectively melt material to form a workpiece, forming a melt pool in the process of melting, the method comprising: using an imaging apparatus to generate an image of the melt pool comprising an array of individual image elements, the image including a measurement of at least one physical property for each of the individual image elements; from the measurements, mapping a melt pool boundary of the melt pool; computing a geometric length of the melt pool boundary; and controlling at least one aspect of the additive manufacturing process with reference to the geometric length.