Examples of a method of operating an additive manufacturing system, a three-dimensional (3D) printing system and a non-transitory machine-readable medium are described. In an example, a build material is supplied to a print region of an additive manufacturing system. A temperature distribution, corresponding to a pattern, of at least a surface of the build material is generated. An image of the pattern is captured using a thermal sensor. Image data representative of the image of the pattern is compared with data representative of an expected position of the pattern. On the basis of the comparing, difference data indicative of a difference between a position of the thermal sensor during capture of the image and an expected position of the thermal sensor associated with the expected position of the pattern is generated. Operation of the additive manufacturing system is controlled at least in dependence on the difference data.