[Sidefx-houdini-list] Linear Workflow

Andy Nicholas andy at andynicholas.com
Thu Aug 26 15:23:21 EDT 2010


Thanks Edward, yep, I think this is the piece of the jigsaw I'm missing. 

I need to go away and do some research around all this, particularly the perceptual bit. Once I set it straight in my head, I'll write a summary on my website.

Thanks again,

A



On 26 Aug 2010, at 19:39, Edward Lam <edward at sidefx.com> wrote:

> 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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