GO­ING FOR GAMMA

3D Artist - - THE ACE OF COLOUR MANAGEMENT -

In CG the gamma means the power func­tion of how com­put­ers en­code the colour data, but its past traces back to be­fore the era of com­put­ers

Gamma orig­i­nated from the pho­to­chem­i­cal era of pho­tog­ra­phy and shows the re­la­tion­ship be­tween the orig­i­nal in­com­ing light val­ues and the dark­en­ing of the film emul­sion – the den­sity. Thus we can al­ter the gamma of the fi­nal re­sult with dif­fer­ent tech­niques. How­ever the crys­tals in the film emul­sion are noth­ing to do with maths and their sen­si­tiv­ity to the light isn’t lin­ear nor ex­press­ible with the maths power func­tion, which is the gamma value in dig­i­tal imag­ing. Ad­di­tion­ally the say­ing is that paint­ing and film-based pho­tog­ra­phy use sub­trac­tive colour mod­els. If we put lay­ers of evenly ex­posed film frames on top of each other and each of them halves the tres­pass­ing light amount, then two lay­ers will mean a quar­ter light, three means one eighth and so on. This is a non-lin­ear mul­ti­plica­tive sit­u­a­tion, and not sim­ple sub­trac­tion. Sum­ming all of the fea­tures of the process, the den­sity of the film be­comes more like an S-shaped curve in the func­tion of the ex­po­sure level at the cap­ture side, which is not lin­ear but log­a­rith­mic.

In video and dig­i­tal imag­ingm CRT mon­i­tors had spe­cial re­sponse char­ac­ter­is­tics to the in­put sig­nal, the gamma curve – which has to be com­pen­sated to more or less have phys­i­cally-lin­ear gra­da­tion at the end. LCD and other con­tem­po­rary dis­plays adopted this be­cause it is a ben­e­fit – there are more dig­i­tal sig­nal steps in the low and mid-tones than in the high­lights, which is in cor­re­la­tion with the sen­si­tiv­ity of the hu­man vi­sion.

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