Re: A New Topic!
Re: A New Topic!
- Subject: Re: A New Topic!
- From: Graeme Gill <email@hidden>
- Date: Thu, 07 Feb 2008 14:47:37 +1100
Mark Rice wrote:
I am a fan of color management, but I have one major problem in dealing with
in the context of most inkjet printers: most inkjet RIPS are not consistent
or repeatable in calibration procedure. That flaw means that color
management is larger doomed on these RIPS for exacting users. My experience
came from the photographic digital imaging field, where extreme precision is
a must. All photographic photo printers and film recorders use "iterative
linearization" to obtain precisely controlled grayscales that match target
densities very precisely. I have been begging RIP makers Onyx and SAI to
produce such a RIP, but they don't really see the need for it. My problem is
that if one achieves a linearization that is close, but not dead on, one can
re-linearize, but it is just as likely to make the linearization worse as
better. And, of course, if the linearization changes, the ICC profiles
become nearly useless.
It doesn't follow that iterative calibration is either necessary
or desirable. It doesn't help at all if a one pass calibration
is already at the limit of instrument and machine repeatability.
Calibration and the use of output profiles boils down to
an inversion problem. The device operates in one direction
(forward), and you'd like to invert that behaviour in order
to know what to feed into it to get a certain output.
If the device behaviour is complicated, or the domain
is so large that the forward behaviour can't in practice
be sampled in enough detail or modelled accurately enough
(ie. the 3 or 4 D color behaviour), then an iterative
approach may well be the best one to locate the input
values that produce a target output value. [But note
that you still have a sampling density issue with regard
to how many samples you solve for in the output space!]
For per channel calibration this is probably not the case though.
It's quite practical to sample the channel behaviour in fine detail
and a 1 dimensional (usually monotonic) curve can be very accurately
modelled an inverted. Given a certain patch budget, a better result
and simpler process is probably that of printing a single
detailed test chart that can combine a high sampling density with
some level of noise filtering. Keeping a history and pre-conditioning
the test chart may give a slight increase in accuracy too.
An iterative approach may give less detail between the
target output values, because it wastes it patch budget
testing the same area over and over.
I don't doubt that some (many ?) RIPS don't have a very accurate
calibration system. I suspect the explanation is more likely to
lie in how they set the output targets (are they absolute or relative ?),
how detailed is are the test charts (ie. are they at least 30
patches per channel or more ?), and what sort of algorithms they
are using to model the device behaviour (do they average out
noise at all ?), plus the post calibration device precision used
(if it's only 8 bits per component, the calibration system may
struggle to have enough control).
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
| References: | |
| | >A New Topic! (From: "Mark Rice" <email@hidden>) |