what is the usual workflow of using custom LUT?

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  06 June 2012
what is the usual workflow of using custom LUT?

hi guys,

recently i am sticking into a problem of using a LUT(and i am using nuke6.3v4),
so now i have a custom LUT for my dpx which is with raw setting, and i import it into a vectorfield node.
then when i render it to dpx format, i just cannot get back the original color of my dpx.
so the above seems not to be the proper workflow of using a custom LUT for dpx, right?
then actually what is the correct workflow of using a custom LUT?

would be very grateful if someone can answer my question, thank you
 
  06 June 2012
you hook it into a Vectorfield node. Make sure it's not before your write - it's used only for display.
 
  06 June 2012
you could import the LUT into the viewer settings and choose it as display LUT while your working

or you could use your vectorfield node. dont connect any in or outputs, rename it to "VIEWER_INPUT" and the nuke will use that node (or even group of nodes) as InputProcess = the "IP" icon in the viewer

then turn the display LUT or the IP on and work while you just make changes to the displayed picture and not the data itself
 
  06 June 2012
cgbeige and pingking,

thank you for both of your responses, im really touched
but here i still have a question about using the lut for display.
so let's say if i am using the VIEWER_INPUT... i thk i should still read my DPX as raw, right?
however, the problem is i need my final DPX to be rendered as log color after combining it with some linear cg footages. in this situation, even though i set the colorspace of write node to cineon, the output is still not in a log color....

so how can i ensure all the output are in log color when i am working with a VIEWER_INPUT?
very appreciate for your help!
 
  06 June 2012
you should read your dpx in with the settings they got, not raw. because nuke does all the math under the hood in linear space and you will get "wrong" or unexpected results when not converting everything to linear

then you should also have no problem with converting back to Log at the end

if your LUT expects and Log input then you can make a group call it VIEWER_INPUT and in the group add a colorspace node befor your vectorfield with the lut
 
  06 June 2012
pingking, your explanation is useful to me.
so in conclusion, my custom 3d lut should be made based on the linear color but not log, right?

on the other hand, even i put a colorspace node into VIEWER_INPUT as your mentioned. is it true that i still cannot output log dpx? because nuke expects it is input as linear( also i have given it a try and i fail to do so)

and there is 1 more question about import the 2d lut into the colorLookup. when i when click the colorLookup graph, i could see "import ascii" and "import discreet lut". what are there different actually? i think both of the format to be imported are .lut, but i just don't know what difference there is.

thanks again
 
  06 June 2012
you can export any colorspace you want. just set it correct in the write node or use raw in the write node and a colorspace before it.

nuke needs all images in linear internally to work correct. so it tries to convert everything, when you load it, to linear. this is done via the colorspace settings in the loader.
then you work in linear with a viewer LUT (sRGB, rec709, your pregrade, ...). the data stays in linear only the viewer is affected.
when you write out your results you convert it to the colorspace you need it. here nuke tries also to help you and set the default colorspace depending on the file format.

did you take a look on the videos from the foundry about colorspaces?
youtube
it will a lot of your questions clear


the different LUT formats have different formating inside.
 
  06 June 2012
great! thank you very much pingking, i thk my problems are solved and i have learn a lot from you
 
  06 June 2012
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