Re: Photoshop Gamut warning vs ColorThink
Re: Photoshop Gamut warning vs ColorThink
- Subject: Re: Photoshop Gamut warning vs ColorThink
- From: Graeme Gill <email@hidden>
- Date: Fri, 22 Feb 2008 14:16:32 +1100
Chris Cox wrote:
Without a useable gamut table, round trip deltas are the only way you can
determine the gamut of the profile. Yes, it's not perfect and depends on
the accuracy of the forward and reverse colorimetric tables in the profile
-- but with the information available, that's the best you can get (AFAIK).
If someone has a better method, please let us know.
There is a more accurate method, but it isn't "better" for checking
a raster image, since it's too slow and needs extra information.
The method is to directly invert the colorimetric A2B table, since
it is the closest definition of the device response.
("invert" meaning "locate the CMYK color in the A2B table that
produces the target PCS value. If no combination of CMYK can
create it, it's out of gamut.")
For instance you can use Argyll xicclu to do this for individual
colors:
xicclu -fif -ip -l330 -a 3dap.icm
50.000000 0.000000 0.000000 [Lab] -> Lut -> 0.030272 0.000000 0.049075 0.600702 [CMYK]
50.000000 0.000000 20.000000 [Lab] -> Lut -> 0.000000 0.065972 0.526734 0.585560 [CMYK]
50.000000 0.000000 40.000000 [Lab] -> Lut -> 0.000000 0.088656 0.903911 0.561584 [CMYK]
50.000000 0.000000 50.000000 [Lab] -> Lut -> 0.000000 0.099579 1.000000 0.526780 [CMYK] (clip)
[Actual 51.848981 0.123139 47.922833, deltaE 2.783616]
50.000000 0.000000 60.000000 [Lab] -> Lut -> 0.000000 0.111158 1.000000 0.462910 [CMYK] (clip)
[Actual 55.950714 0.388107 53.535568, deltaE 8.794914]
A barrier in the way of an accurate out of gamut indication using this
method is that you need to know the TAC for the device, and this isn't
recorded in the profile. The round-trip method doesn't suffer from this
problem.
BTW - I don't believe Photoshop ever used the gamut warning tag, because we
never found one that was accurate.
I've made the contents of this tag as accurately as I could
in my profiles, but it is still not terribly accurate -
typically I've seen errors in the range 1-10 delta E.
What I'd like to do is to store the 3D gamut surface in a tag
in the profile, since this is a better form for gamut mapping,
and will give somewhat better accuracy for out of gamut detection.
Of course this is not a general solution unless it was made a standard.
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