Re: New Topic
Re: New Topic
- Subject: Re: New Topic
- From: Dan Reid <email@hidden>
- Date: Fri, 08 Feb 2008 09:23:35 -0700
- Thread-topic: New Topic
on 2/8/08 8:50 AM, Mark Rice at email@hidden wrote:
> The RGB file needs to be converted to CMYK only if the printer is calibrated
> as a CMYK device. Many RIPS offer the choice of calibrating as RGB or CMYK. I
> know that has to be converted I let the RIP do that. That is one part of the
> RIP that seems to work well.
>
> Setting 50-40-40 is pretty crude at best. I know some experts (people that
> calibrate materials for manufacturers for a living) that edit ONYX profiles to
> make M and C equal to get good color. I was skeptical, but it worked for them.
> I just don¹t think that shooting in the dark is a good way to achieve
> repeatability.
>
I never quite understood the theory of letting a driver/RIP convert RGB to
CMYK for you if the device uses CMYK inks. But then again there are a lot of
faith based groups out there. .......
50-40-40 is starting point, not the end all. I wouldn¹t recommend making
magenta and cyan equal as you are going to get color cross overs when the
profiling package builds the black generation. Have you looked at your black
generation curves in Colorshop X? Common wisdom is you want one color to
dominant to eliminate any color bias across the tonal scale. I can
understand why you are thinking equal CMY because of the RGB output devices
you have worked with in the past but CMYK devices behave quite a bit
differently from Lightjets and Lambdas as you have found out.
Thanks.
--
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
>
>
> From: Dan Reid [mailto:email@hidden]
> Sent: Thursday, February 07, 2008 7:57 PM
> To: Mark Rice
> Cc: colorsync
> Subject: Re: 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)
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