[argyllcms] Re: Absolute colorimetric - dark saturated colours excessively light

Alastair M. Robinson wrote:

.ti3 file, profile and makefile here:
http://www.blackfiveservices.co.uk/R300_Plain_20060122.zip


OK, taking the original sRGB IT8 chart as an example, the
most saturated color in column 4 is J4, and this seems to
be a target color of about Lab = 13, 32, -58.

Taking a look at the profile, the black (0,0,0) value
seems to be Lab 39.388774 3.631250 -2.948365, so the
black points are rather different.

Running xicclu on the profile:

  xicclu -fif -ir -a R300_Plain_20060122.icm

and feeding in the target 13 32 -58 in in relative colorimetric we get:

13.000000 32.000000 -58.000000 [Lab] -> Lut -> 0.083257 0.000000 1.000000 [RGB] 
(clip)
[Actual 47.500649 12.190514 -34.425555, deltaE 46.243540]

which is certainly a very large delta E, but notice that it's almost all
blue, nothing else (8% Red).

Checking the B2A table:

  xicclu -pl -fb -ir R300_Plain_20060122.icm

13.000000 32.000000 -58.000000 [Lab] -> Lut -> 0.090672 0.000000 1.000000 [RGB]
so the same sort of value.

I'm not sure how any profile will be able to get a more saturated blue
out of it than that really. Even 0 0 1 gives:

46.642085 8.602886 -35.679842 [Lab] which is a delta E of 46.6, which is
greater.

Now your softproof indicates J4 at about 44 11 -22, which is a little
different to 47.500649 12.190514 -34.425555, but it is in the ballpark,
and it's a different exercise to figure out where these inaccuracies come from.
(If I use your IT8 jpeg and do a relative colorimetric 
sRGB->R300_Plain_20060122->sRGB
 using cctiff, then J4 winds up with Lab 47 13 -34 which is a bit closer to
 what I'd expect.)

[Well, it's got rather late, so that's enough for one day!]

cheers,

Graeme Gill.


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