Fluence field modulation (FFM) using dynamic pre-patient attenuators could reduce radiation dose while preserving image quality by optimizing the distribution of x-ray photons incident on the patient. However, past dynamic attenuators require mechanical action that is challenging to implement in diagnostic CT scanners and generate a large variety of system states that makes calibration complex. To circumvent these difficulties, we propose a motion-free mechanism for FFM that uses electromagnetic deflection of the focal spot, also called flying focal spot (FFS), together with interference patterns generated from fixed metal gratings. Our proposed design is digital in that only a limited number of fluence fields is possible, but the fluence field from each focal spot position is stable with respect to source perturbation, simplifying calibration. Intermediate fluence fields can be virtually achieved during reconstruction by using either statistical reconstruction or rebinning. We describe the geometric constraints for our FFM mechanism and illustrate some of the possible fluence fields that can be achieved.