Re: Affinity Photo and colorsync issue, maybe
Re: Affinity Photo and colorsync issue, maybe
- Subject: Re: Affinity Photo and colorsync issue, maybe
- From: Graeme Gill <email@hidden>
- Date: Wed, 30 Mar 2016 12:57:49 +1100
edward taffel wrote:
please don’t get me wrong, graeme: i view ben’s comments as very encouraging,& assumed
your general statement an intimation of possible rationale for “ArgyllCMS is so far
superior to anything and everything else out there”.
Not my words.
Ben will have to explain what he implies by "edge cases" - that term could apply to a
multitude of different aspects of color management.
i have found that colorsync may fail to match a color it reported out-of-gamut,
i.e. transforms the coordinates, but the transformed color remains out-of-gamut.
Practical gamut checks are typically not very precise. The ICC profile has a gamut tag,
but it is a notoriously inaccurate representation, and seems to be rarely used.
Many systems use a "round trip check" instead. Take the PCS color, convert
to device space via the B2A table and back to PCS via the A2B. If the delta
E is over a threshold, then regard it as out of gamut. The use of B2A
tables and an arbitrary threshold makes the check imprecise.
If ColorSync is either using the gamut tag or doing a round trip check,
then it would be no surprise for some colors to be regarded as "out of gamut",
even after converting it to device space and back to PCS, since B2A tables
have a degree of inaccuracy, particularly near the gamut boundary, which
could lead to a noticeable round trip delta E.
Doing a precise gamut check is more work. The ArgyllCMS xicclu
tool for instance will do a precise gamut check when doing an "-fif"
lookup (inverse A2B table lookup). This is "accurate" in the sense that
it determines whether the color is in the A2B table somewhere to a high
numerical precision. A subtlety for > 4 color profiles is that a color may
not be reproducible in the B2A table, even though it is present in the A2B
table, due to smoothness/interpolation continuity constraints.
(For the purposes of the discussion I've also ignored any consideration of ink
limits.)
Graeme Gill.
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