[cairo] Subtractive API, part 0

[Chris Murphy]

On Jan 30, 2010, at 1:44 PM, ecir hana wrote:
> 
> I'm definitely not a good person to ask this, maybe others will tell
> you more. All I can say is how I understand CMS, corrections are very
> welcomed:
> 
> CMS is basically two things, one simple and one really difficult.
> There actually is one and only, true color space - LAB.

There are many color spaces. There are many device independent color spaces, including LAB which is a derivative of XYZ. Also derivatives are Jch, Qmh from CIECAM02, for better dealing with color appearance issues.


> When you also
> define the white balance what you get is 4-dimensional space and if
> you pick a point there everyone in the whole world would precisely
> understand what color you are talking about. That's the easy one. The
> problem is that for purely technological reasons there is no device
> which can show LAB. All monitors, printers, papers, scanners, cameras,
> ... are different. So what you do is you come up with a mapping which
> says this color on this device should be that color in LAB. This
> mapping is called ICC. You go from your monitor to LAB and from LAB to
> your printer (i.e. two ICCs). And you know what? In one instance it in
> fact works - if you are able to control all the devices in your
> workflow. Say you are a huge magazine publisher. You know what
> monitors you use, what lamps there shine and what cameras you take
> picture with, you print all the time on the same paper with the same
> printer with the same dyes. You print a few test sheets, you measure
> the output and tweak a bit some of your profiles until it looks right.
> And then you don't touch it! The problem is, that you and I are not
> huge publishers. You come to some random printing house which gave you
> the best price, you demand some random paper which they never heard of
> and then you glue a protective foil over the final prints. Obviously,
> there is most certainly no right ICC because the numbers of possible
> ICCs grows exponentially so no one cares.

I follow this up to this point. There is always a "right" ICC profile for the source space. And there is a rather constrained range of "right" ICC profiles for the destination output process. I don't understand the characterization "that the numbers of possible ICCs grows exponentially". That statement doesn't make sense.


> So what you do then is you
> make an assumption here, a generalization there.

It's an objective process, even if there are multiple options. It might be an educated guess, even. It is not an arbitrary assumption. If you are making such assumptions, then it's any wonder why you state the following results:

> By the time your
> prints get out, the colors are so far from your original intend, that
> the portion which CMS contributes to the correctness of result is
> smaller that say, if you relayed on dot gain compensation or
> experience, for example. Not to mention that some printers simply
> refuse to take tagged sources, probably as they had bad experience
> with it.

Generic conversions to output space will result in generic output that will often require an iterative proof-correction cycle to fine tune the result. Had you had the correct source (likely) and correct destination (less likely) profiles, then the color separation would have been made for the output condition, and you'd have extremely good correlation to what you wanted in the first place.

"dot gain compensation" is built into ICC output device profiles.

"experience" with output processes with which you're unfamiliar? What is the context here? You seem to be taking a printer's perspective as though you're intimately familiar with the output process, and thus have experience. If you're a print buyer or content creator, you don't have such specific experience. You will need a combination of an appropriate output device profile, and some proofing, in order to get what you want.

Anyone producing content with Photoshop 5 (current version is 11) and greater, any version of InDesign, most any modern web browser (not including IE), has applications that understand and use ICC profiles. Photoshop won't let you NOT use them when making conversions to and from CMYK and RGB of various flavors. It's built-in, you can't turn it off. The vast majority of workflows, no matter how basic, are making some use of ICC profiles.

The "experience" workflow is a relic of a by-gone era. Modern printers understand they need to adhere to established print standards so that people can use standard output profiles for standard printing conditions, because it enables modern manufacturing principles. The custom workflow that requires "experience" is the antithesis of a modern manufacturing process because every job is treated as custom. It's expensive. It's slow. And while such workflows exist, it is not a growth industry, just the opposite.



Chris Murphy
Color Remedies (TM)
New York, NY
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Co-author "Real World Color Management, 2nd Ed"