Re: [Lcms-user] Negative channel values are clipped upon floating point conversions to profiles with true gamma TRCs

[Noel C. <NCa...@Pr...>]

Thanks for the additional input, James.

 

I'll be honest - that individual channels may go negative while the overall luminance remains positive represents a new level of awareness for me.  We strive to get our color-management right and up to now I was considering negative values (observed to be very small in absolute value) as a bug or side effect of processing inaccuracy (we know Photoshop doesn't always get things perfect).  

 

We *haven't*, however, been using the cmsFLAGS_NONEGATIVES flag as we never observed negative values out of LittleCMS transforms.  We use LittleCMS to convert to our internal working space...  It's possible that without the clamping code our software will just work.  We already handle HDR (values greater than 1.0).

 

With this new level of understanding I'm going to look over whether the reasons we had been clamping negative values even still exist - it's possible because of my misunderstanding that they were only needed during development before our color-management infrastructure was fully implemented and matured.  We did not start out using a floating point internal representation at first, though we do now.

 

Thank you all for prompting me to revisit this.

 

-Noel