[Sidefx-houdini-list] Linear Workflow

Magnus Pettersson magnus at stormstudios.no
Fri Aug 27 08:10:09 EDT 2010


Just to summarize for some people who might still be confused by all this
(most likely :P) what to actually do. This is "the ballpark" linear workflow
without LUTs and so on. I assume all textures and colors is already sRGB
(2.2) space (if stuff looks good on your monitor, it is most likely in sRGB
and need conversion to linear):

1. ALL shaders should have their input colors/textures converted to linear
-> apply 1/2.2 to all textures and color parameters (including point
colors). In houdini right now it could equal to add a multconstant node
after a color parameter before its piped into something (like a mult etc).

2. Ok now all your inputs should be correct, time to Render. Now to view
this "in the ballpark sRGB" put the mPlay gamma to 2.2 and this should give
you a good appreciation of how your render looks (if your monitor colors
isnt totally wacky then you should go buy/borrow a monitor calibrator!). And
this ofcourse means if you watch the picture in mPlay gamma 1.0 it will look
dark (because of your monitors gamma correction making it darker).

3. When happy with the render looking at it with this mPlay gamma 2.2 you
can send it off to comp. Like people said Nuke comp artists just take in all
your renders with linear space and everyone should be happy :)

The reason we have this very confusing gamma 2.2 here and 0.454545 there and
some makes stuff darker (monitors) and other brighter (like mPlay gamma) etc
is because of the EVIL old CRT screens had a negative gamma (technically
limitation) and to correct for this every picture produced, every GUI
produced and most likely every photo taken and every webpage have been
having a 2.2 positive gamma applied (mostly in background without user
knowledge) to correct for the CRTs shortcomings. BUT now a days with our new
technology our LCD screens have no problem showing right colors in linear
(which would be the best for us in this business so we wouldnt need to
care).
But if LCD screen manifacturers from the beginning would give you a screen
in linear space as default noone would buy them because people would go "omg
this new technology is crap, everything is ugly!" so they have applied this
negative gamma curve to display the content of all programs and all internet
"correct".

Myself would like to see linear becoming the new standard, but that would
require all digital content that is shown on a monitor to be chaged. So most
webpages on the internet would have to change its colors, all pictures
produced until now need to be converted to linear and so on, which you can
understand is a massive undertaking. Not to mention all new content made
needs to be made in linear (photos, programs gui, webpages etc)

Because of a very small procentage of the people who owns a monitor or
creates the GUI/webpages/images found in this digital age is actually aware
of this linear space mumbo jumbo, so seeing a change in this in the
near/distant future is very unlikely so we will have to continue our sRGB to
linear (to sRGB) pipeline.. i blame CRT!

Magnus Pettersson | Effects TD
Storm Studios AS
www.stormstudios.no


On Thu, Aug 26, 2010 at 9:23 PM, Andy Nicholas <andy at andynicholas.com>wrote:

> 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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