Hi
Can someone clarify what is the proper color space for importing greyscale textures which drive effects such roughness, bump etc.

Example:
1. Substance Designer exports roughness maps as actual greyscale (one greyscale channel not rgb) - they are made in linear space.
2. Poliigon workflow - they've got greyscale textures as jpgs so the image is in fact made of RGB channels, which is usually treated by DCC app as sRGB space and in order to be properly applied need to be linearised. But in their example they converted it in Hopudini via color convert node from linear to sRGB and then plugged into material.
Reference: https://help.poliigon.com/other-3d-packages/how-to-use-poliigon-textures-in-houdini [help.poliigon.com]

So my question is what color space is proper to plug image into i.e. roughness channel of material?
Does it make any difference if texture is plugged via texture node to material or directly path to image is indicated in textures tab in material?
In shader on textures there is option to choose between automatic and linear color space. How automatic treats images then?

I appreciate clarification of that matter.
It would be even better if you (SideFX team) can make a proper note in the documentation page related to principle shader.

Many thanks in advance

There's no way to know. It depends on the author of the textures. JPG files are assumed to be sRGB when ‘automatic’ colorspace is selected, but some apps are in fact exporting jpgs in linear space. It really depends on what application exported them.

This is why using textures in non-linear formats is guaranteed to be a nightmare unless you tightly control all aspects.