[Sidefx-houdini-list] Linear Workflow

Andy Nicholas andy at andynicholas.com
Thu Aug 26 06:13:35 EDT 2010


Hi Rangi,

Following the link to Wikipedia, the first line reads:

"Gamma correction, gamma nonlinearity, gamma encoding, or often simply
gamma, "

Kapow!

That's exactly the problem. People often talk about correcting for gamma.
So does this mean what they're actually saying is that they're correcting
for gamma correction? I guess that'd be called gamma correction correction
;-)

When I read any sort of literature on this, I find myself questioning
everything that's written because of this ambiguity in the language. This
usually results in getting utterly confused about what's going on.
Everything you wrote was familiar and made sense, and I could have
probably written something similar myself. My issue is knowing what this
means in practice and resolving these ambiguities.

Maybe you could answer a couple of questions that would help my
understanding.

> Most computer monitors correct to 2.2

For me, the only way I can resolve all of these ambiguities is to ask if
this operation brightens or darkens the image?

> You can take the gamma correction 2.2 off your monitor, (man xgamma..

I did that and it says...

"xgamma allows X users to query and alter the gamma correction of a
monitor via the X video mode extension"

If I type xgamma, it returns 1.00 on all channels. Okay, so that's not the
monitor it's talking about then. It must be some intermediary colour
correction on the X server.

Now I know that the monitor has a gamma (correction??) of 2.2, so I would
assume that typing "xgamma -gamma 0.454545" would correct for that. But
that makes everything darker, so I'm guessing that's wrong. So typing
"xgamma -gamma 2.2" would be the correct thing to do?

The language is so confused on all of this, you never know if you're
applying a gamma, applying a gamma correction or if you're correcting a
gamma.

Yuck.

Yours utterly bamboozled ;-)

Andy





> 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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