Re: [Lcms-user] Creating equivalent matrix-shaper and 3D LUT profiles, problem with BPC

[<mar...@li...>]

Hi,

> I have understood that LittleCMS has already made that decision for me,
for
> the v4 perceptual rendering intent:
> https://github.com/mm2/Little-
> CMS/blob/46355888b823b563db928faec59b0312a05e1143/src/cmscnvrt.
> c#L1129-L1133

That was a long time ago.  You probably know about V4 and perceptual PCS.
For V4, ICC decided to define a different PCS for perceptual and saturation
intents. This PCS is almost identical to relative colorimetric but with a
perceptual black point. This decision messed out things to incredible
levels, converting some combined transforms in a big mess. Just consider a
scanner to printer going rel. colorimetric in scanner and perceptual on
printer. 
To make things worse, I found a lot of matrix-shaper profiles claiming to be
V4 and providing only one set of matrix-curves. Obviously, this cannot give
zero and perceptual black at same time, so I reported the issue to ICC (I
was in the committee at that time) and get the answer "all those profiles
are wrong".
Then, since I could not discard such big number of profiles, I tried to
imagine a way to automatically do the rescaling from different PCS. And
found BPC to work fine for that. So, the idea behind this change is to
increase inter-operability and fix broken profiles.

Regards
Martí