[argyllcms] Re: Verifying profile quality of LUT-based scanner and printer profiles
From: Milton Taylor <milton.taylor@xxxxxxxxxxx>
To: argyllcms@xxxxxxxxxxxxx
Date: Mon, 20 Mar 2006 16:57:15 +1100
The more you learn the more questions it raises...!
OK I think I'm getting closer to understanding what's happening here.
Certainly the banding and posterisation that is happening on the Hue/Sat
rainbow has a lot to do with gamut clipping. I can see that this is
indeed a savage test, but a fair amount of colours [mainly the greens
and blues] even down to Saturation=50% are out of gamut. (At L=0.5) I
don't believe this is peculiar to this printer though.
But this has exposed what I think may be the real underlying problem:
the 'smoothness' of the contours that define the device's gamut. It
seems to me that at the moment, the posterisation and banding I am
seeing in the rainbow could be caused by sharpish 'angles' in the
contours of the gamut's skin (thinking in a 3D sense), leading to weird
effects by the gamut clipping routines when rel col is used. I gather
this is what you referred to as something you were working on?
The smoothest result is achieved on the sRGB gamut being printed as
Perceptual rendering, although some posterisation is still evident.
Also, the color range is now woeful... all the deep blues and greens
have gone! Clearly the shape of this printer's gamut (or what the
profile think's is the gamut) must be cigar shaped, and perceptual is
sacrificing one color dimension far more than the other in order to make
the whole thing fit, and still preserve color relationships.
I was a bit surprised to see that a fair swag of the color space in sRGB
falls outside the printer's gamut. For some reason I had assumed that
sRGB fitted reasonably well into the typical inkjet's gamut, but that is
apparently not the case. This is where I need to look at the 3D shape of
the gamut compared to other printers and to the various working spaces.
If I try a more demanding test...printing Ektaspace Hue/Sat rainbow
using perceptual intent, I now get a reasonably wide range of hues and
saturation preserved in the result, but some slight posterisation
remains around the deepest blues and gentle posterisation in some other
areas. Otherwise it's a bit uneven looking.
If the shape of the gamut skin has any significant non-linearities in
it, I imagine it is possible that these are not necessarily smoothed out
just because of using Perceptual rendering intent? Or, is it possible
that you can still get banding with Perceptual rendering intent for some
other reason?
Another question: all this was done at 8 bits per channel. I am
wondering if I started out with a 16 bit rainbow, would I possibly get
less errors being introduced when Photoshop does the conversion to
output space, even though the printer will only be taking it at 8 bits?
I suppose that is possible when working at the extremes of the gamut
where the RGB values are either almost zero or almost 255 on one of the
channels?
Lastly, I wonder if this is a case where perceptually, the eye is more
concerned about the posterisation bands than it is with the spatial size
of the colour flat spots caused by clipping. Perversely, a profile with
smaller numbers of entries in the LUT may actually cause less noticable
banding than a high quality LUT. Mind you, if the objective is to always
stay inside the printer gamut then this is a non-issue.
