Direct photon yield at midrapidity as a function of charged-particle multiplicity ${\rm{d}N_\rm{ch}/\rm{d}\eta}$. The results from this letter are shown as full blue markers and are compared to theoretical calculations for prompt (open green circle) and prompt+thermal (open red square) yields for INEL pp collisions at $\sqrt{s} = 13$ TeV . The results from the STAR, PHENIX and ALICE 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. The Pb$-$Pb results are compared with the theoretical prediction by Gale , which includes prompt, pre-equilibrium and thermal photons. The prompt-photon yield is computed with NLO pQCD using the INCNLO code  together with the nCTEQ15 nuclear parton distribution function, and the BFG-II fragmentation function.