Re: ArgyllCMS xicclu tool & gamut check [formerly: Affinity Photo and colorsync issue, maybe ]
Re: ArgyllCMS xicclu tool & gamut check [formerly: Affinity Photo and colorsync issue, maybe ]
- Subject: Re: ArgyllCMS xicclu tool & gamut check [formerly: Affinity Photo and colorsync issue, maybe ]
- From: edward taffel <email@hidden>
- Date: Wed, 30 Mar 2016 13:13:36 -0400
> On Mar 29, 2016, at 9:57 PM, Graeme Gill <email@hidden> wrote:
>
>>>> 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.)
thanks for your detailed reply, & making us aware of gamut-check w/ xicclu .
the scenario i mentioned occurs in the context of user specified/edited synthetic color, & this incurs frequent gamut-check calls from a ui control;
but i’ll [try to] set up a batch test on sample data for evaluation of xicclu ‘-fif’, & look forward to seeing the results.
> Graeme Gill.
> _______________________________________________
> Do not post admin requests to the list. They will be ignored.
> Colorsync-users mailing list (email@hidden)
> Help/Unsubscribe/Update your Subscription:
>
> This email sent to email@hidden
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Colorsync-users mailing list (email@hidden)
Help/Unsubscribe/Update your Subscription:
This email sent to email@hidden