[Sidefx-houdini-list] OT: sRGB to linear question

Antoine Durr antoine at floqfx.com
Tue Jul 8 13:01:25 EDT 2008


If an image is in an 8-bit format, the reader/writers of that image  
will often do a gamma correction of the data before they hand it to  
you.  Thus, if you now have file reader that is *not* doing that same  
correction, ergo is treating the data linearly, then you'll need to  
apply the 2.2 yourself.

-- Antoine

On Jul 8, 2008, at 9:56 AM, John Coldrick wrote:

> On Tuesday 08 July 2008 12:32, Sean Lewkiw wrote:
>> Sorry to hog the list, but I'm still struggling a bit....
>>
>>> 	It seems obvious, but where it gets tricky is that when you  
>>> render that
>>> sRGB texture in mantra, what you're doing is rendering a texture  
>>> map that
>>> has a gamma burned into it(sRGB) that is different than what the  
>>> renderer
>>> is using(probably linear assuming you've changed nothing).
>>
>> I don't understand this.  How is the texture that mantra is using to
>> render different from the texture from the original (sRGB) texture?
>
> 	It's not(mip mapping aside).
>
>> Isn't is the same, gamma-burnt-in image I've been happily  
>> viewing?  That
>> is, if I multiply Cf by my gamma encoded image's zero value, I get  
>> zero,
>> and similarly again for values between zero and a bajillion?
>
> 	Yup.  Sorry if I wasn't clear - by rendering 'linear' - mantra is  
> *not*
> tainting anything passing through it, it's the same burnt-in,  
> lovely image
> you liked from the net.  The only downside is that you're using an  
> image with
> a hard coded gamma as a texture map.  Being aware of that is all  
> that's
> important.  There's nothing inherently wrong with the image, unless  
> you were
> rendering it in the same shot with another texture map that was a  
> cineon or
> linear image!  Again, even in that case, if you hit render and you  
> like what
> you see, then you're OK.  However, once you start mixing  
> colourspaces(which
> by all accounts you *don't* seem to be doing), you're likely in for  
> a world
> of hurt.
>
>
>> (I should mention that we have a mplay LUT that closely mimics the  
>> look
>> of the Truelight film LUT which our compositors use, into which we
>> render our images, and this is how we judge colour.)
>
> 	OK, in that case it's assuming you have a linear incoming OExr  
> render(for
> example) and your LUT utility shows you what it will look like on your
> Truelight.  Let's say you have a flag in the picture with your inet  
> image
> displayed on it.  It will look like it will look like on the  
> Truelight in
> that case, there's nothing 'hidden' in there that would hurt you.   
> However,
> keep in mind that you've lit your scene to have it look the way you  
> want.  A
> better approach would be to convert the image to linear and light  
> it so that
> looks correct.  If leaving it at sRGB gets you what you want  
> faster, then
> fine.  In your pipeline, for many cases, it wouldn't be the first  
> thing I'd
> do however given it sounds like you've got more than sRGB in there.
>
> 	The thing is, as a rule you want to unwire yourself from the  
> tyranny of sRGB.
> It's everywhere because you use computer monitors to view images -  
> sRGB
> doesn't really belong in a film world.
>
>> In a nutshell, my point is that when I think of what my shaders are
>> doing mathematically to the texture map anyway, (power functions,
>> clamps, remaps, etc), whether it's linear or sRGB seems to be the  
>> least
>> of my worries!  ;-)
>
> 	It's really important to be aware of it.  Working all-linear will  
> give you
> more control IMHO.
>
>>
>> Is the thinking that the need for truly linear images as textures so
>> that physically accurate materials and lights perform the way they
>> should, (EX:  HDR lighting)?
>
> 	That too, yes.  Again, not *strictly* necessary unless that sort  
> of thing is
> important to you or your pipeline.
>
> 	To sum up, if it looks good and you like it, there's no hidden  
> gotchas.
> However, please be aware that the author you reference says "beware
> absolutes", he's not saying converting your tmaps to linear is  
> categorically
> unnecessary.  Most of the work we do here doesn't require that, but  
> some
> does.  It sounds like you'll probably be OK with what you're  
> using.  If you
> start having trouble getting your lighting/shading solution to look  
> correct
> when you look at your Truelight lookup, then you know the first  
> thing to
> try...;)
>
> 	Oh, and one more thing - you'll find people that will jump up and  
> down and go
> blue with screaming insisting this is all wrong and you *must* do  
> this or
> that.  I guess it makes sense, this touches on an area that people  
> will get
> very passionate about - the ability to perceive colour.
>
> 	Cheers,
>
> 	J.C.
>
> -- 
> John Coldrick                  www.axyzfx.com        Axyz Animation
> 416-504-0425                                         477 Richmond St W
>                                                      Toronto, ON  
> Canada
> jc at axyzfx.com                                        M5V 3E7
> ---------------------------------------------------------------------- 
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>   - Dudley Moore
>
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-- Antoine

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