Re: reversable, but not 1D color transforms in config.ocio


Sean Cooper <se...@...>
 

OCIO is just a LUT and simple math processor(+,-,*,**,log, etc.), so its not really intended to implement algorithms like L*a*b*. Some equations are more LUT friendly than others something RGB-like to RGB-like are generally ok, so I'm not sure how well fit CIELab transforms are for LUT quantization (though at first glance it feels like fitting a square peg into a round hole).

As per Troy's suggestion, you can't directly implement CTL in OCIO. So he was saying essentially you could do the mathematically faithful forward and inverse transform on a sample image (like Nuke's CMSTestPattern) and from them you could use Nuke's make LUT functionality, to LUT-ize the Lab calculation and THEN put than in OCIO. But I'd test the resulting LUT pretty well as I'd imagine it fails in a number of spots.

On Thu, Feb 2, 2017 at 5:55 PM, Andrew Wood <andre...@...> wrote:
Can you expand on that?    Looking into CTL now, does look like I could define L*a*b* there, but how would that tie in to a config.ocio?  I haven't used it before, got some reading to do



On Thursday, February 2, 2017 at 5:16:37 PM UTC-8, Troy James Sobotka wrote:
Would be more optimal to define L*a*b* according to the proper formulas in a CTL and generate the to and froms from there on an EXR, using a suitable shaper to the 3D LUT.

With respect,
TJS

On Thu, Feb 2, 2017 at 6:05 PM Andrew Wood <andr...@...> wrote:
hmm, I tried that but it didn't line up.  I'll poke around with other lut formats maybe.  I used nuke to generate two .cube files




On Thursday, February 2, 2017 at 2:54:20 PM UTC-8, dbr/Ben wrote:
You can use a 3D LUT but still have a reversible colorspace transform by defining both the to-reference and from-reference transforms (I.e to reference would use lab_to_xyz.lut and from-ref would use xyz_to_lab.lut)

Sent from my phone

On 3 Feb 2017, at 09:03, Andrew Wood <andr...@...> wrote:

Hey all!

So I'm trying to get my team off of doing so much in photoshop, and have written some tools using oiio to replace a lot of the workflow.  Right now, they like to do an unsharpen mask in photoshop in LAB space on just the Lightness channel for some images.

I was hoping to represent this transform using ocio, but so far am at a loss.  I can use a MatrixTransform to get to XYZ, but I don't want to use a 3d LUT to get to LAB because it needs to be fully reversible.

Is there some magic that I'm missing, or will I just have to do this in code?

thanks!
Andrew

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