What Is A Color Key In Animation?

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This reserved commodity originally appeared in CHROMiX ColorNews Issue 21 on October 18, 2005.

Click here to meet the original in its original context.
E-mail colornews(at)chromix.com to subscribe to the ColorNews newsletter.

by Steve Upton

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Contents

• i The color Central
  • ane.1 Black starting time / max / width
  • one.2 Dot Gain vs color stability
  • i.iii Print chore stability on printing VS adjustment on printing
  • 1.4 Ink Speckling
  • ane.v Ink costs & drying fourth dimension
  • 1.6 Gamut
  • ane.vii Muddying saturated colors
  • 1.8 UCR vs GCR
  • ane.9 Rich black
  • ane.10 Standard vs Custom separation profiles

The color Key

If you work long enough in or near the print industry you start to take CMYK for granted. Particularly K.

In looking over my previous articles I noticed that I had yet to comprehend the topic of K and then it seemed like it was time.

The K in CMYK stands for "Key", Non black as many might have you believe. The Central plate, in traditional color separations, is the plate that holds the detail in the paradigm. In CMYK this is usually done with black ink.

In the modernistic color-managed workflow, an RGB image has an associated profile so each RGB number combination can exist converted to a defined Lab color. This is fairly direct-forward and repeatable. When creating a CMYK combination to represent that color on output, things get considerably more complicated.

Lets talk a bit about how color is created using CMYK. If you employ yellow ink to paper, your colour range starts at newspaper white and so becomes more xanthous and saturated with the more ink you utilize. But once yous become to 100% yellow in that location's nothing more than you can do without adding other inks. If you are looking for a medium-nighttime yellow you now have a whole host of choices to get it. First, yous can add cyan and magenta ink. They are both required in club to showtime each ink's tendency to motility the color toward green or cherry-red. The improver of cyan and magenta does darken the yellowish but they are also, together, blue - which is anti-yellow. So this add-on of bluish desaturates the xanthous ink quickly, limiting the range of dark yellow colors available. A second option is to add black ink. As black is added, the xanthous darkens but is not desaturated nearly every bit quickly. This tin consequence in a greater gamut of dark yellows.

When CMYK colors are created in normal workflows, either or both of these techniques are used. In fact, for a single original RGB color, many different combinations of CMYK can exist used to (theoretically) create the aforementioned colour on press.

And so how practise nosotros decide which one to use? How much blackness should be used instead of CMY? The answer, as yous should already guess, is the classic color management answer: "It depends".

Lets cover a bit more technique and terminology offset.

Cyan, magenta and xanthous inks used for offset printing are not pure enough colorants to exist mixable in equal amounts for grayness. The SWOP standard expects 50% cyan and 40% each magenta and yellow inks to produce a neutral grey. And so let'southward say we had a CMY color of sixty, fifty, 40. In theory, if we removed the components of the colour that produced gray (50,forty,forty) nosotros would and then exist left with (x,x,0). If we then add enough blackness ink to bring united states of america down to the aforementioned darkness (about 54% K), we have the CMYK combination of 10,ten,0,54 that should appear very shut to the original colour nevertheless is composed of VERY different amounts of ink! The total ink used drops from 150% to 74% AND changes from the more expensive colour inks to black ink.

This kind of colour replacement is called Gray Component Replacement (GCR). If the color range afflicted by this replacement is limited to dark, virtually-neutral colors, and then information technology is called Under Colour Removal (UCR). GCR, on the other hand, can exist applied to neutral and non-neutral colors that are either light or nighttime. That brings us back to the "how much?" and "when?" questions.

It's probably best to cover this adjacent section in bespeak grade:

Black start / max / width

In ICC profiles, the black generation method and amount is chosen at the time the contour is calculated and "baked" into the profile. For flexibility, we suggest calculating several profiles from the same measurements; each with dissimilar black generation settings (this is what we do with our ColorValet press profiling service). Blackness Start is the amount of cyan ink where black starts replacing other inks. For instance, a blackness start of 10 means that when cyan ink is less than x%, CMY will be the just inks used to create colors, but for any colors where cyan is greater than 10%, K will replace some component of the colour. Black Max is how high you want the 1000 level to get in the resulting color. If you lot discover your shadows are plugging, reducing Max K can assistance. Finally, black width is how far "out" into the saturated colors y'all want black to be substituted. A depression black width volition limit K substitution to almost-neutral colors (similar to UCR). A high black width volition allow substitution much farther out into the saturated colors. If you observe your saturated colors look dirty, endeavour lowering your blackness width.

Dot Gain vs colour stability

Here are a few bones facts almost printing (unsubstantiated, I admit, there's just not enough space here). Color casts, especially in neutrals, are the single most common (color) reason customers decline print jobs. Dot gain varies due to press fluctuation the almost in the mid tones of each channel - ranging from xl-lx% - and the variance can differ from channel to aqueduct. This means that when neutral colors have CMY inks that are in the range of twoscore-threescore%, they take a fairly good risk of shifting on press. Our example color of lx,fifty,40 has all three inks in the "danger zone" and then could be quite unstable on press. The converted color of x,ten,0,54 however, has only the blackness ink in the danger zone and changes in black dot gain won't create a color shift. (Thanks to Dan Remaley of the PIA/GATF for turning the light seedling on for me with this issue.)

Print task stability on press VS aligning on press

With the above explanation in mind, you might think that GCR should be used heavily in all images, and yet many printers recommend light GCR or UCR. Why? Well, the very aforementioned reason you would use GCR - stability of color on press - makes it very hard to change color on press. Many printers don't trust their customers' color direction or separations and and so they want the ability to accommodate the task on press. GCR IS a very good thing. Simply call up that if you lot utilise it, ensure that the color in the file is skillful (verified with an effective soft / hard proofing system) and that the printer can impress according to the specs that defined your separation. If they are nowhere near SWOP, or your files are non well colour managed, sending them high GCR files is going to cause problems. Or, more correctly, volition limit their ability to correct issues. (of course, exchange the "you"south and "them"southward if you are the printer)

Ink Speckling

In kickoff printing, the line screens for each channel tend to be the same or very close. When black is used in highlight colors, the dot size and shape is similar to the other inks and doesn't typically cause whatever problems. Inkjet printers using low-cal cyan and light magenta are a different story. Highlight colors on inkjets are often built from CMY inks but. Inkjets that don't use Epson's UltraChrome inkset accept no light blackness (gray) ink. If GCR is used in the highlight colors, the night black ink exposes the true resolution of the printer, creating speckles that people often mistake for lowering of the printer's resolution. On these systems, employ a black start value of 40-50% to keep G out of the highlights. This ways that the conscientious balance of CMY is required for light colors - beware that color balance failure (often mistakenly called metamerism) might cause problems. As well, your profiling target had improve finer sample CMY-only near-neutral colors or your profiling software has picayune hope in finding how to print neutral highlights. I bet y'all've seen this trouble equally well.

Ink costs & drying fourth dimension

This 1 is pretty simple. Less ink = less coin AND less dry time. GCR = less ink so information technology saves both time and money.

Gamut

If you think back to the dark yellow example higher up, recall that to build dark colors we could either add cyan / magenta to yellow OR add blackness. The difference to the size of the shadow gamut (the number and saturation of dark colors bachelor) is significant. Blackness slowly desaturates yellow as information technology darkens information technology. In comparison, the blueish cyan/magenta mixture darkens and quickly desaturates yellow because information technology cancels information technology out. The result is many more saturated dark yellows when blackness is used rather than cyan/magenta. If you haven't realized it notwithstanding, night-yellowish is dark-brown, and having full saturation in browns means wood, leather, and hair or flesh tones as they fall into shadow wait MUCH better. If y'all are missing your browns, the profile will desaturate them into becoming more than gray and you will be disappointed in your wood and leather and often see sharp transitions as faces and pilus autumn into shadow... audio familiar?

ColorThink Pro has a new graphing feature, Constrain Channels, which features sliders that allow you to "pull back" color channels individually. Pulling the K channel dorsum to 0 is an interesting sit-in of how important blackness is to the gamut of a press. The gamut size is reduced by 20 to 30%.

Muddying saturated colors

The use of also much black in saturated colors tin "dingy them up" on some printing systems. Come across Black Width above.

UCR vs GCR

Restricting black replacement to neutrals is what UCR is all about. GCR came along later and extended this technique beyond the neutrals and into the colors. If done correctly, GCR can be very effective and improve image quality. Early on GCR didn't always work as expected and many fell dorsum to UCR for safety. GCR is at present at the indicate where it is reliable and effective, and UCR is falling out of use. As the function of UCR is a special case of GCR (dark neutrals just), I await the term UCR will autumn out of employ and we will refer to all black replacement as GCR.

Rich blackness

100% One thousand is non typically the darkest color you can go from a printer or press. When Cyan and other inks are added, the darkness can oft drop noticeably (from an L* value of 19 down to ix in SWOP TR001 for instance). The combined colour can either be neutral or have an intended hint of color. Blue blacks are popular as well as warm blacks and tend to be used differently in different cultures and parts of the earth. Some profiling packages allow y'all to select the combination of inks for "max black".

Standard vs Custom separation profiles

As I mentioned in a higher place, the black generation method and amounts are ready at the time of profile generation. They are not changeable on-the-fly as many would like. This is a limitation of the current ICC profile architecture. The profiles that send with Adobe publishing products are high-quality and some are based on published standards, such equally US Sheetfed Coated v2, but there is but ane profile for each printing status. This means Adobe chose a middle-of-the-road group of GCR settings when the profile was originally created. Color images vary widely and different levels of black generation should be used for each image type. Images that are predominantly neutral should typically accept more GCR than those with many saturated colors. Flesh tones benefit from GCR but too much makes them appear muddy on press. Because of the baked-in limitation of ICC profiles it's best to keep a group of press profiles on hand so you can vary your blackness generation optimally.

As you tin can see, the subject of black is big and I oasis't really scratched the surface. Suffice to say that "that actress channel" adds a lot of depth to color printing both in the color sense and in the number of choices nosotros have and the number of topics there are to study. I hope I've at least opened your eyes to some of the furnishings of blackness and black generation. As always, I'm open to comments and more information if you have it.

Thanks for reading,
Steve Upton

Source: http://www.colorwiki.com/wiki/The_Color_Key

Posted by: johnsonyoustion.blogspot.com

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