[Sidefx-houdini-list] Linear Workflow

Gene Crucean emailgeneonthelist at gmail.com
Thu Aug 26 12:09:26 EDT 2010


Just don't ask me how to do that yet ;)

I'm still learnin' houdini.





On Thu, Aug 26, 2010 at 9:08 AM, Gene Crucean
<emailgeneonthelist at gmail.com>wrote:

> 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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>> >
>>
>>
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>
>
>
> --
> [Gene Crucean] - [VFX & CG Supervisor/Generalist]
> ** Freelance for hire **
>



-- 
[Gene Crucean] - [VFX & CG Supervisor/Generalist]
** Freelance for hire **



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