[Sidefx-houdini-list] Linear Workflow

Rangi Sutton rangi.sutton at gmail.com
Thu Aug 26 04:13:41 EDT 2010


On 26 August 2010 17:45, Andy Nicholas <andy at andynicholas.com> wrote:

> Hi Craig, thanks, yep, that's what I'm doing already.
>
> You know, I thought I understood all this linear colour workflow business.
> What endlessly confuses me is when people talk about gamma and gamma
> correction, as I'm sure these terms are often used interchangeably in
> articles when they shouldn't. One puts the colour to the power of the value,
> while the other puts it to the power of the reciprocal of the value, and I
> can never remember which. (Jordi did send me a pretty good link to a GPU
> Gems article that seems to explain it, so I guess I need to commit that to
> memory)
>
> 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. I still haven't honestly figured out
> why 2.2 is the correct value to be using, except that it matches our
> existing workflow in XSI which we've been using for ages. If anyone has a
> good explanation of that, then I'm all ears.
>


Most computer monitors correct to 2.2, while most images are stored
compressed at the recipricol gamma of ~0.45. Most images we deal with these
days are sRGB, which is gosh darn close to 2.2. I don't know if much
software actually expands out the real sRGB curve.. I think mostly stuff
gets displayed "close enough" with a monitor correcting to 2.2.

http://en.wikipedia.org/wiki/Gamma_correction

You can take the gamma correction 2.2 off your monitor, (man xgamma on
linux, clicky clicky montior setting on them other OSs..) but pretty much
everything else will look like puke except for your linear renders! Favoured
approach is to leave your monitor at 2.2, and gamma correct in mantra so you
have a linear looking result, as you've got!

Well, real favoured approach is to use colour management software, so you
have a 3D LUT for your particular montitor. But that's for someone who
actually knows their colour to rant about..

HTH,
r.



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