[Sidefx-houdini-list] Linear Workflow

Gene Crucean emailgeneonthelist at gmail.com
Thu Aug 26 12:08:15 EDT 2010


These threads are always so interesting :)

All you should have to do in Houdini is use a LUT for test renders, and
apply a gamma correction to input texture maps that aren't linear. Then
render to a linear format like exr.



On Thu, Aug 26, 2010 at 8:49 AM, Francois Duchesneau <sidefx at trinix.ca>wrote:

> Great clarification. I'll keep that as a reference because when I will
> think I've figureg it out I'll realize there's still something wrong :)
>
> Thanks
>
> François
>
> > On Thursday 26 August 2010 08:27:06 am François Duchesneau wrote:
> >> This subject confuses me.
> >>
> >> I thought Houdini already worked in linear space?
> >
> >       It does, inasmuch as it doesn't assume anything other than linear.
>  It
> > won't
> > *add* to any existing LUT/sRGB issues, unless you ask it to.
> >
> >>
> >> I thought Photoshop only generated sRGB texture so we have to apply a
> >> lut (or a gamma correction to approximate it) when we read those
> >> textures in our shaders?
> >
> >       Well, you should, assuming you want to work in linear.  When you
> look at
> > your
> > PS image on your computer, it looks "right", that's precisely what
> > Microsoft
> > invented sRGB for, so that would happen. Pop it over to mplay, by
> > default(gamma 1), it looks the same.  Thing is, that's incorrect, it's
> not
> > linear.  It *should* look "dark" at gamma 1 in order to be linear.  The
> > confusion comes about because you're looking at the image through the
> > filter
> > of a monitor that isn't linear, you want to pull that out of the
> pipeline.
> >  If
> > you set mplay gamma to 2.2, that's a decent approximation of what a
> linear
> > image looks like, and by default your PS export will look washed out.
>  You
> > want to convert it to linear first(as Gene correctly points out, running
> > it
> > through Nuke and using a Colourspace node to do it is more accurate than
> > simply inverse 2.2, but it's close).
> >
> >>
> >> When I read a Houdini image in Nuke. I set the read to linear and my
> >> viewer color space to "none" and my image is exactly how I see it in
> >> Mantra. Nuke works in linear so the gamma correction should be at the
> >> end of it?
> >
> >       To clarify, Nuke makes assumptions about the colourspace of your
> image
> > based
> > purely on the file format(this can be tweaked in prefs).  By default, a
> > tiff
> > image is assumed to be sRGB and an exr is linear.  If you rendered a
> > constant
> > shader of that tiff image in mantra and rendered out an exr, then read
> > both in
> > Nuke, they will look different(assuming default prefs).  You should be
> > looking
> > at *everything* in houdini with gamma 2.2, the viewport, mplay, IPR(which
> > you
> > can do now in H11!).  You convert your incoming sRGB images to linear
> > first,
> > they will look correct in all those contexts.
> >
> >       The above example, you want your Nuke viewer(as a rule, mind you)
> to be
> > sRGB,
> > not none, because just like with mplay set to 2.2 gamma, you want to
> strip
> > out
> > all srGB references, then at the tail end you pop your 2.2 gamma or
> > similar
> > LUT in there have it look 'correct'.
> >
> >       All of this assumes you want to be working linear, of course.
>  There's
> > lots
> > of people out there that never do film that live in an sRGB world and
> they
> > can
> > happily go about working as if sRGB is an invisible filter laid over top
> > of
> > every last thing they're working on, since in the end it gets displays on
> > an
> > sRGB buffer(or close).  The one downside I would point out, though, is
> > that
> > light behaviour with PBR won't be the same, really I would argue this is
> a
> > good reason to consider linear workflow even outside of film.
> >
> >>
> >> Why do we have to set the gamma correction to "Gamma 2.2" to match XSI
> >> linear space????
> >
> >       If you're referring to the setting in the ROP that Mario suggested,
> all
> > that
> > does is pour more sampling into the dark areas, it doesn't change
> > colourspace
> > per se.  It's a handy trick especially if your final product ends up on
> > TV.  I
> > suspect even in film it's handy for certain shots.
> >
> >       Nuke's assumption about colourspace based on format drove me mad
> when I
> > first
> > encountered it.  Now the Foundry has added the ability to tweak those
> > assumptions and I understand the pipeline, it's actually rather handy for
> > detecting things you're missed.  I went through plenty of hair pulling on
> > this
> > topic, believe me.  I understand your pain.  ;)
> >
> >       Cheers,
> >
> >       J.C.
> >
> > ---
> > John Coldrick
> > 416-504-0425
> > jc at axyzfx.com
> > -----------------------------------------------------------------------
> > Life does not cease to be funny when people die any more than it ceases
> > to be serious when people laugh.
> >   - George Bernard Shaw
> >
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-- 
[Gene Crucean] - [VFX & CG Supervisor/Generalist]
** Freelance for hire **



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