A method of operating a grain harvesting machine includes: capturing images of a crop material flow in an image capture area of the grain harvesting machine; identifying a plurality of grain elements in the images; determining a velocity of each of the grain elements; determining whether each of the grain elements is likely to be overblown out of the grain harvesting machine based at least in part on the velocity of each of the grain elements; and controlling a subsystem of the grain harvesting machine at least in part based upon the determination of whether the grain elements are likely to be overblown. A grain harvesting machine for performing such a method is also disclosed.