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Re: Reversed transform for a OCIO::DisplayTransform

Joseph Slomka <joseph...@...>
 

Remember that colorspaces can be defined to and from the reference. You can manually define the inverse and come up with an inverse 3d lut for the color picker that is close enough to fully inverted.

I don't believe that OCIO inverts any 3d luts, even if you had a unity lut where rgb in = rgb out.

Typically the design of OCIO does not automatically do anything that will get you into trouble. You have to deliberately make those choices. 


Example code below.

-Joseph

cs = OCIO.ColorSpace(family='kodak',name='kdk2383')
cs.setDescription("kdk2383: Kodak Vision stock")
cs.setBitDepth(OCIO.Constants.BIT_DEPTH_UINT10)

# this sets the conversion to reference XYZ from film log
groupTransform = OCIO.GroupTransform()
groupTransform.push_back(OCIO.FileTransform('2383_to_bt1886.3dl',interpolation=OCIO.Constants.INTERP_LINEAR))
groupTransform.push_back(OCIO.ColorSpaceTransform(src='bt1886', dst='lxyz'))
cs.setTransform(groupTransform, OCIO.Constants.COLORSPACE_DIR_TO_REFERENCE)
config.addColorSpace(cs)

# this sets the conversion from reference XYZ to film log values
groupTransform = OCIO.GroupTransform()
groupTransform.push_back(OCIO.ColorSpaceTransform(src='lxyz', dst='bt1886'))
groupTransform.push_back(OCIO.FileTransform('bt1886_to_2383.3dl',interpolation=OCIO.Constants.INTERP_LINEAR))
cs.setTransform(groupTransform, OCIO.Constants.COLORSPACE_DIR_FROM_REFERENCE)
config.addColorSpace(cs)


On Fri, Apr 11, 2014 at 4:29 PM, Kevin Wheatley <kevin.j....@...> wrote:
As Piotr points out this is in general not mathematically possible, in particular if any LUTs are not invertible, you'll not be able to do this. in a 1D sense imagine if you have a curve which has a turning point which results in there being multiple cases where two values of x give y having the same value, when generalised into a 3D LUT this problem becomes even more difficult to invert in a general sense even in typical outputs this is not necessarily going to give you predictable outcomes (typical for a display transform emulating another device/output especially a subtractive one like film prints or other CMY(K) outputs). These 3D luts often feature gamut compression which is where a lot of the loss comes from.

In specific cases there can be techniques to invert, but these need hand selecting, which is probably why OCIO won't do it, something like Argyll CMS or another ICC profile tool might do something for you, which might be interesting to implement, alternatively the CTL code has a scattered data interpolation code that might work.

Kevin


On Fri, Apr 11, 2014 at 10:29 PM, Piotr Stanczyk <piotr.s...@...> wrote:
From what I remember, you can do this if the transforms are analytical. If you are referring to a baked in file based one then you will have to provide your own from for that. 

Piotr




On 11 April 2014 10:24, Dmitry Kazakov <dimu...@...> wrote:
Hi, all!

I am a developer of Krita [0] painting application and we use OCIO for displaying wide range images. I'm having a bit of a problem now. Since we are a painting application, we need to be able not only to display a color on screen (which we do using DisplayTransform), but also to pick an already displayed color back into app (for usage in color selectors or for drag-and-dropped colors). So, effectively, I need to get a reversed transform for a DisplayTransform. But it seems to be explicitly prohibited (it throws an exception in the case). I'm wondering, is it possible to get a reversed transform for a DisplayTransform without reimplementing it completely using GroupTransform?

Thank you in advance!

[0] - krita.org

--
Dmitry Kazakov

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