Metasurfaces have opened the way for gaining control over the frequency and the polarization responses of electromagnetic waves. However, it is still challenging to control their electromagnetic angular response. Here, the potential of diffractive surfaces for breaking the symmetry of the angular transmittance is revealed. By engineering an asymmetric angular response for the scattering particle in each unit cell of the surface, it is possible to break the angular transmittance symmetry in a reciprocal, passive and lossless fashion. In this work, we introduce a metagrating which can filter the positive (or negative) side of the momentum space for obliquely impinging TM waves. Such a system could find several applications in optical analog signal processing. For example, the Schlieren imaging technique, which consists in filtering out half of the momentum space, would be feasible without the need for a bulky free-space system composed of multiple lenses or mirrors.