Re: vd16 and alpha channels PS -> Nuke


Ashley Retallack <ashl...@...>
 

Hi Troy,

Thanks for the reply. If I was to do this in a nuke context what would the process be, it this no different to doing an unpremult before color transform and a premult afterwards? In which case it does not work as the colors are so different now the alpha associated with it multiple against the wrong value. 

Or do you mean that the alpha it'self needs to be treated first, in which case how do I go about doing this to compensate for the values?

Many thanks

Ash



On 22 November 2013 02:24, Troy Sobotka <troy.s...@...> wrote:

Assuming I understand the issue...

On Nov 21, 2013 10:02 AM, "Ashley Retallack" <ashl...@...> wrote:
> Thanks for the reply. Yes that's what I thought. so it's more that comp and reprojections need to over in the space space as the matte painting was done in rather than trying to correct the alpha to work with the converted color.

If the alpha is associated, it must be predivided prior to linearization.

The reason is due to a luminance collision. A standard sRGB or 709 inverted TRC looks at a value of 0.5 identically when alpha is associated.

For example, a value of red 0.2 looks exactly the same as an associated alpha version of 1.0 red at 0.2 alpha. The resultant luminance will be off when linearized via an inversion of the TRC.

Predivide if associated and you shouldn't have any issues.

With respect,
TJS

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Ashley Retallack | Pipeline Engineer 

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