Photometric measurements. Top: intrinsic shape of an example galaxy from TNG100 as a function of the intrinsic major axis distance r/R_e; axis ratios and triaxiality parameter are shown in separate panels. This simulated galaxy is oblate with q ∼ 0.4 and p ∼ 0.95 in the central 3 R_e, and becomes near-prolate (T >  0.7) in the outer halo. In this galaxy 9 r_soft correspond to 0.48 R_e. Bottom: ellipticity and photometric position angle profiles for two example galaxies from TNG100 as a function of the projected major axis distance R/R_e. The quantities derived from the inertia tensor are shown with solid symbols, those from the mock images with open symbols.