The paper focusses on gas-velocity retrievals in emission plumes using optical flow (OF) algorithms applied to remote sensing imagery. OF algorithms can measure the velocities on a pixel level between consecutive images. An issue of OF algorithms is that they often fail to detect motion in contrast-poor image areas. A correction based on histograms of an OF vector field is proposed. The new method is applied to two example volcanic data sets from Mt Etna, Italy and Guallatiri, Chile.