[Sidefx-houdini-list] OT: sRGB to linear question
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.
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
>>> 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
>> and similarly again for values between zero and a bajillion?
> Yup. Sorry if I wasn't clear - by rendering 'linear' - mantra is
> 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
> 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
> 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
>> 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
> 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
> 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.
> 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 -
> 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
>> 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
> However, please be aware that the author you reference says "beware
> absolutes", he's not saying converting your tmaps to linear is
> unnecessary. Most of the work we do here doesn't require that, but
> 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
> when you look at your Truelight lookup, then you know the first
> thing to
> 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.
> John Coldrick www.axyzfx.com Axyz Animation
> 416-504-0425 477 Richmond St W
> Toronto, ON
> jc at axyzfx.com M5V 3E7
> The best car safety device is a rear-view mirror with a cop in it.
> - Dudley Moore
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