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Tag Archives: L*A*B

The Color Rhyme

With Lewis Carroll being in my mind, as well as Dr. Seuss, several years ago in 2003, I wrote my little “Color Rhyme” below hoping to assist my students. I hope you enjoy it, and feel free to share it! (This is the latest version.)

Cyan, Magenta and Yellow are the three primary colors of pigments, used in printing everything in color, just about. Red, Green and Blue are the primary colors of light that your RGB computer monitor and TV set use to create all the colors you see there.

How pigments work with “white” light to produce color is what this rhyme is about!

The Color Rhyme
(Starring Cyan, Magenta, Yellow, Red, Green and Blue!)

By Terry Leigh Britton

Red light is all right
And Green light can be bright
And Blue light sure is Blue
But what’s that mean to you?

Well, if covered with ink
It just might make you think
About Red Green and Blue
And what’s got onto you!

I mean C-M-Y-K
In the light of the day
Reflects Red, Green or Blue
But they soaks in some, too!

That is, talk of Cyan:
Soaks as much as it can
Of Red light but yo-ho
Green and Blue light must go!

Green and Blue light will mix
And among their “cool” tricks:
Work together they can
So that we see Cyan!

Green and Blue strike the eye
And the rods and cones try
To explain to the brain
“That’s Cyan, ain’t that plain?!”

Yellow ink, it eats Blue,
Then reflected to you
Is pure light red and green
And so that’s what you’d seen!

Red and Green light makes Yellow!
Green and Blue light, we see, Oh!
That they make Cyan,
Just as well as they can!

And Magenta, it too
Will eat Green, but not Blue
And not Red, so those two
Are reflected to you.

So Magenta, you see,
Blue and Red light will be
Via eye and the brain
(Though that sounds quite insane!)

Now let’s talk about Green
Which we all here have seen:
How’d that get here alone
To excite rod and cone?

Well, some Yellow ink took
All Blue light like a crook
And sent out Green and Red…
But it slept in a bed

With Cyan, now you see –
As it happened to be,
Which ate up all the Red.
They’ve reflected instead

What was left, which was Green
Which is what you had seen:
Red and Blue both were et!
And so green’s what you get.

Ma-genta with Cyan
Soaks as much as they can
Of both Red light and Green
And Blue light is what’s seen.

And Magenta with Yellow
Will eat up every fellow
Except Red light, you see.
That’s reflected to thee.

“So the Blue and the Red
Made Magenta!” I’d said,
Though ‘twas in the mind’s eye
In the brain, by and by
Where these colors occurred –
But of this be assured:

– With light you see a lot
Yet in darkness, you’re not!
(… Definitely not!)

Fundamentals of Light, and the Photoshop L*a*b Color Space

Way back in 2005, rutt posted the best summation of how colors work as light I’ve ever seen, at DigitalGrin.com.

Rather than force you over there to read that post, I am replicating it right here, as it required a few edits for clarity.

“In order to understand the relationship of LAB, RGB, and CMYK, we have to understand that the colors are actually defined in terms of one another.

  1. Red is a primary in light.
  2. Cyan is the pigment opponent of red. It is defined as the pigment that reflects green and blue perfectly but no red at all. So in light, cyan is composed of equal parts green and blue, but no red.
  3. Green is a primary in light.
  4. Magenta is the pigment opponent of green. It is defined as the pigment that reflects red and blue equally, but no green at all. So in light, magenta is composed of equal parts red and blue, but no green at all.
  5. Blue is a primary in light.
  6. Yellow is the pigment opponent of blue. It is defined as the pigment that reflects red and green equally, but no blue at all. So in light, yellow is composed of equal parts red and green, but no blue at all.

Given these definitions, we can see why the pairs green, magenta and blue, yellow are called opponents. There can be no green at all where there is magenta, by definition. Shine a green light on a magenta surface and you see black; nothing is reflected. Magenta is defined in terms of what it doesn’t have, namely green.

[I edited the following part, as he was muddy in this section, contradicting later assertions – Terry]

To the extent that a color is created by combining magenta light [he is referring to the combination of blue light and red light] and green light, the color just gets grayer and lighter.

You can experiment with this in the RGB color space. To get yellow, set the red and green values to 255 and blue to 0. [You’ll see a very rich and bright yellow.] Now increase the blue. As you do so, the light does not actually become more blue, it becomes less yellow, paler and brighter. You cannot add enough blue to get a bluish yellow light. If you increase the blue all the way to 255, you get white light.”

[Note: In L*a*b mode, Lightness is expressed in the L channel, while in RGB lightness is expressed as separate values of red, green and blue — higher values meaning both brighter as well as more saturated color in RGB.]

I love the thought of there being a bluish-yellow light, as he suggests, or a greenish-magenta, or a cyanish-red! But then again, Alice Through the Looking Glass is one of my favorite books as well!! Such colors are impossibilities according to the definitions above.

For Further Reading:

See previous post, “Some Color Theory and Photoshop LAB mode – Warm-up for Full Article

Some Color Theory and Photoshop LAB mode – Warm-up for Full Article

This is an open-letter to a co-worker that I thought might make a nice warm-up to the full-blown Color Science and Photoshop LAB Mode article that is upcoming. Enjoy and watch for the article!

(Join the newsletter to be notified when I release those big things – this Color Theory one as well as an Actions tutorial video series and a full-on L*a*b series of videos are forthcoming!)

———————————————————————————->>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
Dear Libby,

The other day I mentioned colors used in the
L*a*b color mode and which were opposites.

Naturally, being an ART student, you countered me (a former graphic arts college
professor, yet!) in my definition of what opposite colors were. Well, you were right…
historically speaking at least. I didn’t have time to go into it then,
recognizing it as a near universal problem when addressing color science to
those trained in the traditional ways that Art schools portray the issue.

Check this article at Wikipedia to understand what I’m referring to!

http://en.wikipedia.org/wiki/Complementary_color

Art and design

Because of the limited range of colors that was available throughout most of the history of art, many artists still use a traditional set of complementary pairs,
including:
white and black
red and green
blue and orange
yellow and violet
The complement of each primary color (red, blue, or yellow) is roughly
the color made by mixing the other two in a subtractive system:
red complements (blue + yellow) = green
blue complements (red + yellow) = orange
yellow complements (red + blue) = violet
When two complements are mixed they produce a brown,
or, in the case of black and white, a gray.

Blue-Yellow-Red color wheel. Opposite colors are called complementary. ____________________________

What I described as opposites were the scientifically opposite colors according to how the eye sees color. Conveniently, these break down into the primary colors of light vs pigments. Those are called “additive” vs “subtractive”, as light “adds together” to make colors, while pigments “subtract out” to reflect only the remaining colors of light.
Light <=opposite=> Pigment
Red <==> Cyan
Green <==> Magenta
Blue <==> Yellow

Pigments add a “K” or blacK pigment for printing to save on color ink (not having to use three colors always to produce every shade of gray).

Light mixes so that equal percentages of Red, Green and Blue equals a gray. This handy fact is used in color correcting in RGB mode, making it easy when we KNOW that a certain color in a photo is supposed to be a shade of gray.

Ideal pigments would mix 33/33/33 to produce gray, but in reality, because it is so difficult to produce a “strong” enough Cyan, the breakdown is more like 36%-C/27%-M/27%-Y to produce a neutral gray.

A *very *simplified way of looking at this is:
Cyan pigment looks “Cyan” because it soaks up all the Red light and reflects Blue
and Green light, which mix in the eye to make Cyan to the brain.

Pigments are “light sponges” this way, soaking up certain colors of light and
reflecting whatever is not soaked up.

So, similarly, a “Red” pigment is soaking up all the Blue and Green light
and only is reflecting the Red light.

So, if you are following this as to pigments, Red, Green and Blue PIGMENTS (RGB being also the light primaries) soak up two light primaries each and reflect only one.

Cyan, Magenta and Yellow soak up ONE light primary and reflect TWO! Those
are mixed in the eye to produce the color we “see”.

As I say, this is *very* simplified, as I put it above, because the white
light spectrum is comprised of more than ONLY red, green and blue light when
viewed using a prism, but the subtractive principles apply because pigments
soak up a wider part of that spectrum than only red, green and blue light…

There *IS* an orange light *frequency*, etc., but in color science, which
has to do mostly with how the eye and brain SEES colors, the RGB light
primaries apply.

Your RGB emitting computer monitor produces a yellow LOOKING color, not light at a frequency corresponding to pure yellow light! When our eye sees an equal mixture of Red and Green, it “perceives” that as Yellow (ie., Not-Blue!!!! See below!)

Your monitor does not have Yellow phosphors, and emits no Yellow frequency light, but we “see” Yellow from the simultaneous Red and Green light that the monitor DOES emit. Wild, eh?

Technically, according to the “opponent process theory” the eye sees
Green/Not Green(magenta) and Blue/Not Blue(yellow)!!!) This is exactly what
the L*a*b color mode in Photoshop allows you to adjust. This is usually
stated as “yellow as opposed to blue” and as “green as opposed to magenta”
in L*a*b terms.

Chew that over! 🙂

However, that is only a way of abstracting things, so don’t fret too much! An increase in the a and b channel’s “warm tones” together in L*a*b produces that Red (stronger Magenta in a-channel mixed with stronger Yellow in the b-channel), and a decrease in both a and b channels (the cool tones) produces the Cyan (“Greener” in a-channel mixed with “Bluer” in b). But it’s good that you are starting to “get” the way the eye/brain actually sees via a “less or more” relationship than as literal detecting of specific frequencies like some kind of electronic sensor would!

In short, in L*a*b mode, positive numbers in the a-channel and b-channel designate the warmer colors of Magenta, Yellow and Red (both a & b positive), while negative numbers in the a-channel and b-channel designate the cooler colors of Green, Blue and Cyan (both a & b negative). A negative a-channel with a positive-b would mix Green with Yellow for that Yellowish-Green that we see most often in nature, while a positive a-channel with a negative-b would mix Magenta with Blue to produce Purple. So, that’s where the intermediaries come from, at least in the L*a*b model.

The eye actually does have cones that are preferentially sensitive to Red
light (the L, or “Long-wave” ones), as there are cones that are
preferentially sensitive to Blue light (the S, or “Short-wave” ones) and to
Yellowish-Green light (the M, or “Medium-wave” ones). These are referred to
as the “trichromatic theory<http://en.wikipedia.org/wiki/Trichromatic_theory>”.

We perceive the brightness of a color by the “spill-over” into at least one

other cone by that specific frequency exciting it as well. Wow! As it says
in the trichromatic theory<http://en.wikipedia.org/wiki/Trichromatic_theory>
page:
“For example, moderate stimulation of a medium-wavelength cone cell could
mean that it is being stimulated by very bright red (long-wavelength) light,
or by not very intense yellowish-green light. But very bright red light
would produce a stronger response from L (red-sensitive) cones than from M
(yellowish-green sensitive) cones, while not very intense yellowish light
would produce a stronger response from M cones than from other cones.

So, a combination of stimulated cones results in there being brightness and
intensity info. Cool.

Here’s the whole story as to the physiology of color perception:
http://en.wikipedia.org/wiki/Color_vision.
*
*
I love the L*a*b mode in Photoshop. As Dan Margulis puts in his opening to
Chapter 2 of The Canyon Conundrum, “The structure of LAB is frightening:
opponent-color channels; a zero in the middle of a curve; negative numbers
for cool colors and positive numbers for warm ones; colors that are well
outside the gamut of any output device. And outright imaginary colors, ones
that don’t and couldn’t possibly exist anywhere but in the mind. But there’s
logic behind the lunacy, and with practice the system is easy to use.

I agree!

Good further reading:

The “Don’t Panic!” thread at DigitalGrin.com, where rutt summarizes chapter 2 of Dan’s book and there is discussion afterwards. Excellent!

Sneak Peek – L*A*B Color Mode
Image Improvement Techniques

I’m busy working on a series of articles and videos I’ll release here soon on using the LAB mode for improving images (or L*A*B mode as some prefer to write it, myself included!)

A friend, Fred Vaughan, has agreed to allow me to use his beautiful photographs taken in Colorado and elsewhere in the western U.S. as my subjects.

Below is a sample done using just some fairly simple curves – all work being performed in the L*A*B color space!

By increasing color contrast (not merely by increasing saturation), we can bring out the natural coloration that the light presented to our eyes, and restore that which is lost by the static interpretation of the camera lens.

Before (Click image for full-screen versions – you can load both into separate tabs to A/B compare them):

Fred Vaughan Image - original

And after having the curves shown beneath the image applied (Click image for full-screen version):

Note how the vibrancy of the full daylight is restored from the above version where the camera had “flattened out” the color’s dynamic range.

Watch this space for some nice full tutorials soon! But in the meantime, please try some L*A*B mode moves on your own!

Fred Vaughan image - with LAB Curves

Lightness Channel – I added some brightness to the lower-mids to open up the dark trees in the middle background, adjusting the top to allow the light to appear to “shine through” the leaves
“A” channel – the lower part is made “steeper” to bring out the darker greens in the mountainside
“B” channel – the odd “hook” in the lower section removes some blue haze obscuring the mountain’s features

Rutt at Dgrin offers another gem – Dan Margulis Portrait Action

I mentioned a very prolific poster at DigitalGrin.com naming himself “rutt” (I believe John is his real name). He is a follower, as I am, of Dan Margulis. rutt has offered up his own “DanMargulisPortrait.atn” (link is often dead – see below) — a Photoshop action — in the thread  of the Chapter 16 of the Photoshop Lab Color book discussion thread on page 12 (!) of the thread posts. That chapter was the final chapter in Dan’s excellent book, and gave a neat “recipe” for optimizing portraits and any face shots you might have. Rutt’s photoshop action automates the process by stepping you through that same recipe’s moves.

The above is his link to the action from that thread. I’ve also put up a safety backup link on my server – “backup of DanMargulisPortrait.atn“, mostly in response to the fact that this link has died in the past!

As that link is pretty buried in there (I mean, page 12 – yikes!), I wanted to create at least a couple back-links pointing to the post  so Google might find it easier!

Read the thread and you will see some pretty nice results demonstrated there from using this technique!

That’s all – I really just wanted to share this with you quickly! Enjoy!