[Sidefx-houdini-list] Linear Workflow

Francois Duchesneau sidefx at trinix.ca
Thu Aug 26 11:49:35 EDT 2010

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 :)



> 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
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