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

John Coldrick jc at axyzfx.com
Tue Jul 8 10:14:57 EDT 2008


On Tuesday 08 July 2008 09:51, Sean Lewkiw wrote:
> A question then... let's say I want an object that is a nice purply blue
> at the top and a greenish orange at the bottom.  In one render I achieve
> this using point colours.  In another I achieve exactly the same thing
> using an sRGB image I downloaded off the interweb.  How is the one
> derived from the sRGB image any worse or less accurate than the one
> derived from the point colours?  Both final renders are linear.

	If they are the same colour, then they're the same.  sRGB doesn't inherently 
taint anything - if it looks correct, and it goes out the final pipeline 
looking correct, then it's no more right or wrong than the point colour 
version.

	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).  Now, you don't see 
any problem because the monitor you're screening mplay on *is* sRGB(most 
likely, unless you're using a calibration program).  If you're sending that 
image, like we frequently do here, to an inferno suite that's also working 
with tiff plates that have sRGB burned into it, you can merrily carry on and 
not worry about things.  Because mantra is linear by default, it's 
essentially passing things through it without changing anything.

	Strictly speaking, though, this is 'wrong'(and is where you get much finger 
wagging and tsk-tsk'ing).  You should be converting the tmap to linear, and 
rendering it, then viewing it through a calibration program that expects the 
incoming imagery to be linear, and will convert it to sRGB or whatever gamma 
your display device is using(or you burn it back in for final should that be 
the pipeline).  Again, if you're all video all the time, you never do film, 
you never get sources from cineon format, etc...then this all seems like a 
lot of worry over nothing, and in fact it is, as long as your pipeline works 
like that.  It's more work to convert it, render it, comp it, then re-convert 
it again.  You can probably safely do without it(although some people will 
still finger wag you regarding the colourspace you're compositing in).  The 
instant you add other variables to the pipeline, though, you need to be aware 
of it.

> I'm not trying to be provocative, just trying to get an understanding of
> this whole thing.

	Believe me, I hear you.  :)  It's a complicated topic and non-obvious, with 
lots of misinformation out there.

	Andy got to that link before me, it's very useful.  Hopefully this post is 
somewhat complentary as a 'why' explanation as opposed to 'how'.  I found 
the 'why's to be poorly explained much of the time, based on a film 
assumption.

	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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