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anon_user_09389530

April 1, 2021 21:14:14

Hi,

I need to recreate a color chart in Houdini.
I'm using values from this table.
https://xritephoto.com/documents/literature/EN/ColorChecker24_After_Nov2014.zip [xritephoto.com]

And convert the values from that table like this in VEX.

v@colorChartValue_sRGB = ctransform("cspace:Lab", "cspace:rgb",  set(20.64,  0.07, -0.46));

And then convert that color to ACES

v@colorChartValue_ACES = ocio_transform("Utility - Linear - sRGB", "ACES - ACEScg", v@colorChartValue_sRGB);

Does this seem like the correct method or not?

Thanks

jsmack

April 1, 2021 22:39:21

deviner
Does this seem like the correct method or not?

Perhaps, but I don't think cspace:rgb is sRGB. I think it's most likely NTSC, because of the legacy of Prisms.

The other complication is the rather aggressive tone curve built into the ACES RRTODT which will affect the result. To get the colors looking the same you would need to apply the inverse RRT as well, but this might result in impossibly bright colors.

anon_user_09389530

April 2, 2021 00:52:48

Ok, I replaced the LAB to sRGB conversion with just the sRGB values from this table [xritephoto.com].

At least I know that I use sRGB now

Thank you