The Ace of Colour Man­age­ment

ACES, the colour man­age­ment sys­tem hail­ing from the Os­cars has spread across the VFX in­dus­try with great suc­cess, Greg Barta from scivfx de­scribes its ba­sic prin­ci­ples with 3D vi­su­al­i­sa­tions

3D Artist - - CONTENTS -

In­side the lat­est in­dus­try stan­dard and tool that’s rock­ing Hol­ly­wood

The na­ture of the tasks that 3D artists usu­ally do is kind of a mix­ture of many art forms such as pho­tog­ra­phy and paint­ing. Painters start with a blank white can­vas and com­pile the wide range of colour val­ues of re­al­ity into their much nar­rower pal­ette to achieve a com­pelling im­age in its view­ing en­vi­ron­ment and light­ing. They can achieve brighter colours than the white of the can­vas us­ing spe­cial pig­ments, but it does al­most noth­ing if we hang that paint­ing on the wall of a movie the­atre with the screen­ing light set­tings. How­ever if we put it in front of the screen and light it with the pure white of the pro­jec­tor, this can make a good paint­ing a quite im­pres­sive ex­pe­ri­ence. Be­sides hav­ing a multi-thou­sand watt light source in the pro­jec­tor that de­fines the dy­namic range in this case, the wide range of com­pounds, ori­gins and dif­fer­ent man­u­fac­tur­ing pro­cesses of the paint pig­ments pro­vide the rich colour pal­ette for the pain­ter to cre­ate scenic paint­ings. Film­mak­ers have just three colours, but the fea­tures of pho­to­chem­i­cal pro­cesses, like cross-talk­ing, give them a unique, cin­e­matic look. In con­trast, to­day’s dig­i­tal imag­ing tech don’t just use only three colours, but these are just data most of the time and con­verted to an im­age sig­nal when dis­played.

The Prob­lems

Ev­ery 3D artist prob­a­bly re­mem­bers a time when they achieved a good-look­ing ren­der then changed some ba­sic pa­ram­e­ters, like ex­po­sure, and the new re­sult ended up look­ing a bit odd. There could be many rea­sons for this but the most ob­vi­ous prob­lem is the lack of proper colour man­age­ment. In this case, artists tend to use phys­i­cally-in­ac­cu­rate val­ues to com­pen­sate for the in­ap­pro­pri­ate ren­di­tion of the colours on the dis­play, which is the worst idea when work­ing with phys­i­cally-based ren­der­ers.

The 3D/CG com­mu­nity woke up a long time ago and re­alised that di­rectly dis­play­ing the raw scene colour val­ues on the dis­play is a tech­ni­cal glitch, thus it is nor­mally com­pen­sated with the well-known gamma cor­rec­tion.

How­ever the pres­ence of a proper colour man­age­ment sys­tem was still the priv­i­lege of

VFX houses un­til re­cently and is still not a widely adopted prac­tice.

Our cul­tur­ally-af­fected col­lec­tive colour mem­o­ries strongly rely on pho­tos and paint­ings we’ve pre­vi­ously seen, it can make the colours of a raw – and even gamma cor­rected – 3D ren­der un­friendly. There are also more ba­sic prin­ci­ples, which af­fect the per­cep­tion of the im­age on a dis­play. To­day’s dis­play tech­nolo­gies, even HDR mon­i­tors/tvs, have a max­i­mum achiev­able bright­ness, which are or­ders of mag­ni­tudes lower than in re­al­ity, and the sat­u­ra­tion of each colour com­po­nent lim­its the gamut. Thus, we should rein­ter­pret all or al­most all the colour val­ues of the scene prop­erly into the dy­namic and gamut range of the dis­play to get a com­pelling im­age. Firstly we set the ex­po­sure of the ren­der – or worse the lights – how­ever this is just a mul­ti­pli­ca­tion of the RGB val­ues. With such sim­ple maths, a day­light scene tends to be dim, flat and washed out on our rel­a­tively dark dis­play. Luck­ily the na­ture of pho­to­graphic pro­cesses usu­ally com­pen­sate for these au­to­mat­i­cally and we get a com­pelling im­age at the end. If we use dig­i­tal cam­eras, the cam­era’s raw devel­oper soft­wares and colour-grad­ing suites of­fer sim­i­lar op­por­tu­ni­ties.

Most ren­der­ers have ad­vanced colourtweak­ing ca­pa­bil­i­ties like high­light com­pres­sion and LUT im­port but these are still far away from the ca­pa­bil­i­ties and ben­e­fits of a stan­dard­ised colour-man­age­ment sys­tem like ACES.

To­day’s dis­play tech­nolo­gies have a max­i­mum achiev­able bright­ness, which are or­ders of mag­ni­tudes lower than in re­al­ity, and the sat­u­ra­tion of each colour com­po­nent lim­its the gamut

Guardians Of The Gal­axy Vol 2 used ACES colour man­age­ment

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