In this work, we present results from a new formulation for determining certain class of optical flow fields. The formulation is particularly efficient, as the flow field is either a global 90 degrees rotation applied to the gradient of a scalar function, or is identical to the gradient of a scalar function. The formulation is general: it is applicable whenever the velocity field is incompressible, or irrotational. We are interested in the study of nonrigid motion of incompressible fluids, and as such will restrict most of the discussions to the case of divergence-free velocity fields. Starting from the conservation of mass principle, we derive a motion constraint equation for x-ray projection pictures, a special case of which is shown to be the Horn and Schunck's optical flow constraint. It is shown that if specific criteria are met, in addition to the normal component of the velocity field, the tangential component is recoverable, without the need for smoothness. An algorithm is presented to illustrate this. The techniques are applied to synthetic images, as well as contrast-injected x-ray images of flowing fluid, in a cylindrical phantom.