Re: New Topic
Re: New Topic
- Subject: Re: New Topic
- From: Dan Reid <email@hidden>
- Date: Thu, 07 Feb 2008 17:56:41 -0700
- Thread-topic: New Topic
on 2/7/08 11:56 AM, Mark Rice at email@hidden wrote:
> No, I am not having a laugh. I have used the LVT film recorder, the Durst
> Lambda, and the Oce Lightjet. All can deliver precisely calibrated
> grayscales for months on end, even though the chemical developing process
> may drift. Each value of the gray scale has target densities in the
> computer, and the output densities are extremely close to the target
> densities.
>
> The Onyx and SAI RIPS, the two most prevalent DO NOT even have any target
> densities. The curve shape is recalculated by some form of internal
> algorithm (an unknown one that the manufacturers will not release), and is
> recalculated each time a re-linearization is done. What the the calculation
> based on? Simply ink density limitations, which are selected by eye, by
> guess or by golly, by each user. This is the crudest possible process I can
> imagine.
Since Onyx uses density (unfortunately) they use the Yule-Nielson N-factor.
Wikipedia that to figure out how that works.
Onyx auto generates the Yule-Nielson N-factor with their graybalance
routine.
>
> To see what a "re-linearization" provides on the average RIP, try this: take
> an RGB grayscale and print it in the RIP after linearization has been done,
> but before the ICC profile is generated. Everyone I have seen is
> ridiculously bad. The reason is the each channel is "linearized"
> independently, based on ink limitations chosen via the crude method shown
> above. This process is NOT repeatable!
>
Uh, Mark the RGB file has to be converted to CMYK if it's an inkjet. The
CMYK profile used in the conversion with stipulate how to build graybalance
for better (probably worse). Your method is not going to show you how
neutral the CMY are of an inkjet before profiling. Maybe consider the
standard graybalance of 50/40/40 (CMY) instead of asking the RIP to convert
a RGB to CMYK for you to evaluate the neutrality of a CMYK device?
> Martin, I did not copyright the process - I simply copyrighted the article I
> wrote.
>
> Graeme - you raise some good points.
>
> 1. The process does not require inversion, even if the device behavior is
> complicated. It simply uses an algorithm that I will compare to herding
> sheep - one creates a set of target aim densities, and the iterative process
> "herds" the device behavior in that direction. As I mentioned, the iterative
> process has to be changed as the targets are more closely approached, or it
> will overshoot the target.
>
> 2. Again, I have to mention that the starting point for calibration is
> critical, and is done in a very haphazard fashion in most RIPS - the
> "eyeball" choosing of ink limitations. There is not method of determining
> what set of ink limitations produces neutral values on paper.
>
True, not by the RIP manufacturers. But there are ways of evaluating ink
restrictions and lins and neutrality using a spectro. You won't be able to
evaluate any of these things with density. You need to be measuring and
collecting LAB data to evaluate these things.
> My frustration is showing because of this problem - I get a calibration that
> is nearly neutral, and nearly color appropriate (I was going to say
> "accurate", but I knew that would stir up another argument), and when I try
> to make it better, it is much more likely to get worse.
>
I know you want it to be perfect but that's not always possible, as you have
found. If you can achieve a good neutral output of 50/40/40 on an inkjet
(measuring with a spectro, and evaluating with LCH) you will be amazed at
how well the profiling package can take care of fine tuning the rest of the
graybalance from there. Good luck in your quest for neutrality! (he, he,
hah, ha)
--
Dan B. Reid
RPimaging, INC
Color Management Products and Training for Print, Internet, and Motion
Graphics
http://www.rpimaging.com | Toll Free: (866) RGB-CMYK
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