Problem summary
When displaying an ImageRead node with a LUT applied (e.g sRGB), it should look the same as an 'ImageRead + OCIODisplay with no LUT' but these two do not match.
When the same setup is created in Nuke, both of these images match.
The images are being viewed in the Monitor tab, and are being compared with a colourspace transformation applied in the Monitor tab to an equivalent one applied in an OCIO node. ( sRGB Monitor - sRGB OCIODisplay )
This suggests that there is something in the Monitor tab LUT which is causing the slight colour differences.
Customer reported version
katana.2.6v3
Customer reported platform
fedora
Steps to reproduce
Reproduction 1:
1. Create an ImageRead node and read in a 'colorwheel.exr' file.
Set the colorspace in the properties to 'sRGB'.
2. 'Preview Render' the ImageRead node.
3. Take a screenshot of the monitor tab and save it as 'ColorWheel_LUT.exr'
4. Create an OCIODisplay node and connect it to the original ImageRead.
5. Preview render the OCIODisplay.
6. In the monitor tab, set the LUT to 'none' (As the OCIODisplay should have 'sRGB' in its parameters by default)
7. Screenshot & Save the render like in step 3 with the name 'ColorWheel_OCIO'
8. Open and switch between both images in an image viewer
Result: Both colorwheels will be slightly different, when they should be the same.
Reproduction 2:
This can also be reporduced with an 'ImageWhite' node via the following reproduction:
1. Create an ImageWhite node and preview render it.
2. In the Monitor tab, switch between the 'sRGB' LUT and 'none'
Result: There will be a slight difference between the LUT's.
You can screenshot both LUTs, save them and use the Eyedrop tool in Nuke to see both are slightly different: (Bottom right color values are different)
Workaround
2D nodes
Use OCIODisplay nodes or OCIOTransform nodes to set the correct LUT values, turn off the LUT within the Monitor tab.
3D Nodes
The OCIODisplay & OCIOTransform nodes both need images as their inputs.
This means we need to pre-render an image for the OCIODisplay.
This can be done with finished and paused renders from your Catalog.
To set this up, follow the steps below:
1. Start your Preview Render of your desired 3D node.
2. When you are happy with your render state, cancel the render (Spacebar)
3. In your Catalog, middle drag and drop your render into your Node Graph
4. Connect your render to an OCIODisplay and set it up to your desired Colorspace
5. Turn the Colorspace option in the viewer to 'None' and preview render the OCIODisplay.
Example video snippet below:
Reproduced by support
Katana2.6v3 - Windows 7 - CentOS6.9
Katana2.6v1 - Windows 7 - CentOS6.9
Katana2.5v7 - Windows 7 - CentOS6.9
Katana2.5v1 - Windows 7 - CentOS6.9
Katana2.1v6 - CentOS6.9
Katana 1.6v3 - CentOS 6.9
Earliest version tested
Katana 1.6v3
Expected behaviour
The OCIODisplay that applies the sRGB to an undetermined(none) LUT, should have no differences to an ImageRead node's render output when the monitor is set to sRGB (Without the OCIODisplay).
Actual behaviour
The Monitor tab LUT is slightly different, when it should be matching.
