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Expression for Alexa footage. Example expression included.

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Simon Björk
Expression for Alexa footage. Example expression included.
on Sep 12, 2011 at 11:10:58 am

At the moment there isn't a simple way to do a logC to linear color conversion of Arri Alexa footage in After Effects. The LUTs provided by Arri doesn't seem to work for this. In Nuke, you just use a simple curve created be this expression:

AlexaV3LogC =

(x > 0.1496582 ? pow(10.0, (x - 0.385537) / 0.2471896) : x / 0.9661776 - 0.04378604) * 0.18 - 0.00937677

As I'm no expert on these type of expressions, I'm unsure how to convert it to use with a levels or a curve effect in AE. All I know is that it's a if/else statement where ? is "then" and the : means "else".

Anyone?


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Dan Ebberts
Re: Expression for Alexa footage. Example expression included.
on Sep 12, 2011 at 3:21:50 pm

I don't know that much about LUTs, but is that formula for a pixel-by-pixel manipulation of luminance? If so, I would think you could do it with a PixelBender effect. Convert RGB to HSL, run luminance through the formula, then HSL to RGB. Something like that.

Dan



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Simon Björk
Re: Expression for Alexa footage. Example expression included.
on Sep 14, 2011 at 8:11:05 am

Thanks for your reply Dan, I will try to do some more research on the subject. Out of curiosity, how come you don't think it's possible with expressions?


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Dan Ebberts
Re: Expression for Alexa footage. Example expression included.
on Sep 14, 2011 at 5:57:42 pm

My assumption is that it needs pixel-by-pixel processing, which you can't do (at least in a practical way) with expressions. Of course, I could be completely wrong. I mean, can you manually set up a Levels effect to do this conversion? I guess if you could, you could just save that as a preset.

Dan



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