[Sidefx-houdini-list] Linear Workflow

Edward Lam edward at sidefx.com
Thu Aug 26 14:39:49 EDT 2010


On 8/26/2010 3:45 AM, Andy Nicholas wrote:
>
> What also didn't help my understanding was by putting Houdini's
> colour correction gamma setting to 1/2.2 it made a pure linear ramp
> (defined by a shader) in Houdini look correct in the render (i.e. an
> even distribution of black through white), but using a value of 2.2
> made it look anything but linear and over exposed the dark areas.

You're missing the fact that the eye itself also has a nonlinear 
response. When you put in a 2.2 value into mplay, you're now undoing the 
power curve that the monitor applied, giving you linear _luminance_. Do 
not confuse this with _perceptual_ lightness.

If you want *perceptual* (lightness) linearity, then you need to 
generate an approx. 0.4 power luminance curve [1], explaining why 
putting an mplay gamma value of around 0.4 "works".

Lastly, this issue has nothing to do with Houdini in particular. Take a 
linear gradient in another package (say Photoshop) and you will see the 
same thing.

Regards,
-Edward

1. http://www.poynton.com/notes/colour_and_gamma/GammaFAQ.html#lightness



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