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Dave Cook avatar image
Dave Cook asked ·

Going a bit crazy ... a color/colour management question!

Hi

I have recently noticed that the id mattes that I have been creating that I thought were 'pure' rgb are in fact not. We have colour management enabled and set to and ocio config written in house but based around ACES. So rendering space is ACEScg and the view transform is Rec709(ACES). This produces a consistent result between Maya and Nuke.

However I have noticed that when I map a mask to an id channel, colours that are created as pure RGB in Photoshop or Mari are not rendering as that. This happens whether I read in a texture as sRGB or RAW. It even happens when I create a ramp (see attached files) and map it to a flat shader.

If I change the view transform to RAW in Maya that fixes it, but I don't see how to get that to work for Nuke.
And anyway it makes no sense to me how a Red channel that is created as [1,0,0] in colour terms can output information in the green and blue channels...what am I missing here?

Using Maya 2018.4 MtoA 3.0.1.1 and 3.1.1

Any pointers would be lovely!

Thanks

Dave

Scene:

rgb-test.zip

Info in green:

capture.jpg

But it's just blue and red:

ramp.jpg

color space
capture.jpg (42.3 KiB)
ramp.jpg (65.1 KiB)
rgb-test.zip (8.8 KiB)
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Stephen Blair avatar image
Stephen Blair answered ·

In general, it's not unexpected for a color transform to convert (1,0,0) to data in all channels, because the transform changes the definition of what R G and B mean

When you define the color for a mask, it needs to be done in ACEScg space, or in raw space so that there’s no color transform

For picking a color in the Maya UI, if your rendering space is ACEScg, then the numeric color values that you set are interpreted as already being ACEScg (according to the Maya docs).

When I create a ramp that goes from red to blue, I don't see green pixel values. I don't have your OCIO config, so I just used ACEScg for the Rendering Space and Rec 709 gamma as the View Transform.

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Dave Cook avatar image
Dave Cook answered ·

Thanks again @Stephen Blair for your answer, sorry a bit slow at my end.

Using ACEScg on the file nodes is working out for me. There were two things that were tripping me up.

Firstly our local OCIO implementation of ACES was clearly not as vanilla as I thought it was and it does have different results from the standard one.

Secondly the default behaviour 'Auto-convert textures to .tx' being on meant that someone who was not part of the colour workflow inadvertently made tx files in the wrong colour space, and of course no amount of changing the input colour space on the file node changed anything until I remade the tx files. I understand the reasons for having it on as default, but it hasn't always been my friend...

Cheers

Dave

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Dave Cook avatar image
Dave Cook answered ·

Thanks @Stephen Blair for the response.

I guess what I didn't anticipate is that the transforms would affect channels that are 1 or 0 before transformation, or introduce for example information from the red channel into the green channel. 'Pure' refers just to the 1 or 0 raw colour values in the ramp in the sample scene, (also how my id matte textures are).

The question is then: how to achieve a proper rgb values to give a clean id matte when rendered through a colour transform? Just to go back to the ramp as a basic example: at the moment when I use the maya colour picker to choose a red or blue, no matter whether I have the mixing colour space enabled I will end up with some pixel data in the green channel which obviously I don't want... so what is the correct approach?

Thanks again

Dave

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Stephen Blair avatar image
Stephen Blair answered ·

For the ramp, where you enter/select a color in the UI, that will depend on the color mixing space. You could enter 1 0 0 and then get something else after the transforms are applied. eg

0.914300025 0.137500003 0.0666000023

Whats "pure RGB" ? Maybe you could upload one of these id matte textures?

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