Direct photon yield at midrapidity as a function of charged-particle multiplicity $\mathrm{d}N_\mathrm{ch}/\mathrm{d}\eta$. The results from this letter are shown as two blue markers that correspond to the measurements in INEL and HM pp collisions at $\sqrt{s} = 13$ TeV, with the corresponding $\mathrm{d}N_\mathrm{ch}/\mathrm{d}\eta$ values taken from Ref.[59]. The results in INEL event class are compared to theoretical calculations for prompt (open green circle) and prompt+thermal (open red square) yields [35]. The results from the STAR [36], PHENIX [91,92] and ALICE [9,10] collaborations are shown as full markers of various colours, and the band shows $N_{\rm coll}$-based extrapolation of pQCD calculations for pp collisions at $\sqrt{s} = 200$ GeV [93]. The Pb$-$Pb results are compared with the theoretical prediction by Gale [94], which includes prompt, pre-equilibrium and thermal photons. The prompt-photon yield is computed with NLO pQCD using the INCNLO code [95] together with the nCTEQ15 nuclear parton distribution function [96], and the BFG-II [84] fragmentation function.