Import from darktable in linear prophoto colorspace seems not be properly implemented

[Vasto7]

I exported from darktable a 16Bit tif in linear prophoto rgb colorspace.  I opened this document in AP (first attachment as screenshot).  AP seems to be crushing the blacks with a purple cast in the shadows.   I've opened the same file in several other programs (Capture One, Capture NXd) and the file seems to be rendered much closer to what it does in darktable.  Then I convert the file to ROMM RGB and it seems to lighted the shadows and remove the color cast (second attachment as screenshot.  Is it possible that AP is not recognizing the tif file color space properly, i.e., it doesn't recognize linear prophoto rgb as ROMM RGB?  I've also attached the 2 tif files.   

DSC_2409_Prophoto.tiff DSC_2409_ROMM.tiff

[walt.farrell]

Aren't linear profiles supposed to be used with 32-bit images, rather than 16-bit?

[Vasto7]

Walt, thank you for you quick response.  You make an excellent point.

When it comes to colorspace and profiles, I am out of my depth.  I'm not sure linear profiles are limited to 32 bit.  At least that doesn't seem to be the case for darktable's raw processing.  Nonetheless, the only built-in Prophoto profile in darktable is linear.  It would appear that AP isn't recognizing this profile.  I am not able to locate the linear prophoto profile in the darktable installation files in order to provide it to AP.  As a work around, I have taken the ROMM RGB profile from AP and loaded it into darktable as a new output colorspace.  This seems to have done the trick.   This seems to give the same result as converting the file to ROMM RGB within AP.  I am also able to convert from linear prophoto to ROMM in ACDsee and Krita.  However, Krita seem to be able to recognize a correctly display the linear prophoto profile.  In the end, I am unlikel to work in ROMM very often, but wanted to be able to in there was a need.

Thanks again and Happy New Year.

[walt.farrell]

You're welcome.

Perhaps another user or Serif staff member will have additional comments on the use of linear profiles for 16-bit images.

[Gabe]

Hi both,

Sorry for the delayed reply. 

Linear profiles are meant to be used for linear compositing only. If you don't have a linear workflow, I suggest you stay away from them. They are not limited to 32bit though. Converting your 16bit tiff to 32bit shows the correct values though (which it's expected, as it's a linear workflow). I'll get this logged, as the colours seem to be different between Mac and Windows. 

[Vasto7]

Thanks for the response.  Darktable is implementing a linear workflow as their preferred raw development method (filmic RGB module).  I suggest Affinity Photo developers check it out as I suspect many Affinity Photo users may use darktable to develop raw files non destructively, before doing additional work in Affinity.