[Sidefx-houdini-list] Linear Workflow

Gene Dreitser keyframe at rogers.com
Thu Aug 26 09:28:48 EDT 2010


Not to add to the confusion -- I'm still grokking this myself, I thought I would 
point this out just in case it was relevant to somone:

While 'most of the time' people equate the sRGB to linear color space transform 
with a application of a 0.45 gamma colour correction (and the inverse operation, 
by applying a gamma 2.2 colour correction), this is infact, an approximation.

The ACTUAL colour transformation is slightly more complex, and involves a LUT.  
you can confirm this yourself in Nuke rather easily.  There is a portion in the 
black that will get obliterated if you apply a 0.45 gamma, that won't be using a 
proper lut.  subtle, but present.

Best,

G

 -- 
Gene Dreitser
Loki Visual Effects
p: 416.532.5654 // c: 416.803.0682 // www.lokivfx.com



----- Original Message ----
From: Rangi Sutton <rangi.sutton at gmail.com>
To: sidefx-houdini-list at sidefx.com; andy at andynicholas.com
Sent: Thu, August 26, 2010 9:06:01 AM
Subject: Re: [Sidefx-houdini-list] Linear Workflow

On 26 August 2010 20:13, Andy Nicholas <andy at andynicholas.com> wrote:

> 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
> ;-)
>

Fair enough. It's all confusing as hell. I'll try and spell it out
randomly...

As far as I understand it... Your display is constantly darkening
everything. Most images we deal with are sRGB, they have actually got
brighter values.. so when they're displayed, everything turns out equal.

(ie.. a grey value of 0.5 in sRGB, should be displayed as 20% grey. There's
a stack more information in the lower-luma ranges than the higher luma
ranges.)

Of course.. terms like brighten and darken are simplifications.. you know
what I mean.. mid grey goes up == brighten, mid-grey goes down == darken ;)

Type "xgamma -gamma 2.2" to see you monitors in linear space. Your display
is now neutral, with a gamma of 1.0 (assuming it was "normally" calibrated
to begin with!) Everything looks too bright, your interface is washy, it
wasn't designed in linear space.

If you open a jpg now it will look washed out (it's in sRGB space, and your
display is now linear). Open a linear render and it will look true, although
you might need to adapt what you think is true!

Apply a gamma of 0.45 to your jpg with a gamma cop or something, and you've
converted it from sRGB to linear (pretty much). The image is darker now..
looks correct.

So  when a monitor is said to have a gamma of 2.2.. it means that 1/2.2 gets
applied to everything. You should "gamma correct" a linear image to your
viewing device by applying a gamma of 2.2 to it.

Take a linear image, apply gamma 2.2, display on a monitor with gamma 2.2..
it will look right.

Take a photograph, your camera inherently applies a gamma of 2.2, display it
on your "normal" monitor (has a gamma of 2.2), it will look right.

Have I contradicted myself? Someone correct me because in Southern
hemisphere we might have upside down denominators...  and I usually just
flip shit around until it looks right.

Nuke is a good one for this stuff.. allows you to specify what colour space
images are in on the way in (rather than assume by image type, which I think
cops does).. and allows you to simply control the viewer space. Nuke likes
to work linear internally, as far as I understand it.... which is how stuff
should be composited!

I'm going to hit send then realise I said it all backwards....

r.
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