Metasurface research has shown significant potential for controlling the polarization, amplitude, phase, and propagation direction of light. Nevertheless, control over the angular response of incident light remains a long-standing problem. In this work, the potential of diffractive systems for obtaining a step function in momentum space is shown, where the mirror symmetry of the angular transmittance is broken. By engineering the scattering response of an asymmetric particle in a metagrating, such a step function can be obtained in a passive, reciprocal, and lossless fashion. More specifically, the metagrating performs filtering in momentum space with an abrupt switching from reflection to transmission for an incident electromagnetic wave with an arbitrary spatial profile. This metagrating may find diverse applications in optical spatial analog computing. Moreover, it paves the way for exploring the capabilities of diffractive systems for gaining full control over the angular response of light using arbitrary momentum transfer functions.