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Saturday, April 13, 2013

Colour Correction (and Eyeballs)

When you look at a picture, you often see many small problems.  In my happy retirement I've been monkeying a lot with things.  Out of curiosity mostly.

Colour correction is something to struggle with.  SL is totally unreal, and wind lights don't help this.  Whites look off white, blacks look gray or brownish.  Windlights end up making things yellow, gray, brown or pink, or everything just shadowy or bright like the sun.  To compound the problem you need a good video card to do many things.

People go through all sorts of photoshop tricks to try to get those colours just right.  And most don't succeed -- it's hard to get right so they do things in like sepia the photos. Of course everyone puts in their artistic flavours to establish atmosphere so much of it is on purpose.

There are lots of techniques but I came across an interesting one called "Color Correction By The Numbers" Chapter 2.  So I list it here for mostly my personal reference and because my blog needs something new, but maybe people out there might also be interested.

For RGB techniques, maybe try http://digital-photography-school.com/color-correction-photoshop.
I haven't tried it myself.


Use the full range of available tones every time and don't give the viewers any colours that they will know better than to believe.  

For example,
A white horse should look white.  A white horse should not look pink.  If you adjust the histograms to make the white horse look white then everything else should remain looking normal.  No pinkish blue skies!

Our visual system is self-calibrating.  The longer we stare at a monitor that is flooding our receptors the more our eyes will adjust and "play tricks" on us.

Optical Illusions are simple examples of how you can trick eyeballs into deceiving you.  Check out http://www.personal.kent.edu/~rmuhamma/OpticalIllusion/illusion.html for excellent examples AND tips.  You can use these in fashions or in choosing things like ... eyeballs!

For example
"Colors often appear brighter and more vibrant when they are bordered by frames. Black lines are commonly used to enhance colors in applications like stained glass. This tactic creates a certain effect, as shown below, and prevents color clashing. Notice that the drawing on the left colors appear significantly brighter and pure."

An eyeball that is shadowed with black (bordered by a black frame) enhance the colours causing them to seem brighter and more vibrant.  My favourite eyes all have this shadowy black border and I specifically look for the depth of that detail when I shop as opposed to just how glassy it looks.  I don't like the border too thick or too dark but just a bit more than subtle.

Look into my eyes!  And I so need to colour correct this snapshot.

In order to take the guess work out of Colour Correction we have Magic Numbers.  Math doesn't lie!

Other tips use RGB, which is good for programs like Gimp or Paintshop.  But we look at CMYK instead.

This is the colour method based upon pigments. "CMYK" stands for Cyan, Magenta, Yellow, and the K in CMYK stands for 'Key', NOT black as many might have you believe (though it's commonly black).

The Key plate, in traditional colour separations, is the plate that holds the detail in the image. In CMYK this is usually done with black ink.

In CMYK a neutral colour is described as C being higher than M which is equal to Y.

  • Shadow is the Darkest significant neutral area of an image.
  • Most images have a dark part we can use as shadow.
  • Shadow should be the heaviest value that can use.
  • An unbalanced shadow maybe a symptom of colour cast that may be hurting other parts of the image.
When in doubt use: C=80, M=70, Y=70 and K=70, though one or more of these values maybe be higher in a deep colour.

SWOP (Specifications for Web Offset Publications) mandates a 300 maximum (C+M+Y+K).
Magazines usually tweak this down to 280.  Newspapers ask for 240.

80+70+70+70 = 290.. close enough.

  • Hilight is the lightest significant part of the image.
  • With two stipulations:
    • 1. Hilight CANNOT be a reflection of a light source (called speculars or catchlights)
    • 2. Hlight must be something that we are willing to represent to the viewer as white.
Use C=5 M=2 Y=2
Some experts say C=4 M=2 Y=2 and C=3 M=1 Y=1 and C=5, M=3 Y=3 or C=6 M=3 Y=3
But magenta=yellow and cyan are a couple of points higher than those two.

What about Photoshops AUTO LEVELS?
This is the simplest form of automated colour correction.  It maximizes contrast by forcing every channel to it's full range.  Resulting in a distinct white point and black point.  Buuuuuut the whitest points or darkest points might not be where you want them to be.

If in Doubt use SWAG (Stupid Wild-Ass Guess)

An area that is supposed to be neutral is something that should be black, white or gray.

C is higher than M and Y.  M should equal Y.

C 2 or 3 pts. higher should do for Hilights
C 6 or 7 pts. higher should do for Midtones
C 9 or 10 pts. higher should do for Shadows

Fleshtones should have as much yellow as magenta and up to a third again as much in extreme cases.
Measure only the areas in normal light, not in a shadow or semi-reflection.  Avoid adjusting based on makeup areas.

Measure by:
1. Take small selection in appropriate area
2. Filter: Blue > Guassian Blur at a high value
3. Measure
4. Cancel Blur

Y = M or Y = 1.3333M

(I'll crack open my book on Skin by Lee Varis for this section but anyways)

Caucasians  C = 1.2 to 1.333 M (Y = M or a bit higher)
Dark skinned Caucasian: 15C 50M 65Y
Light skinned Caucasian: 6C 30M 35Y

Hispanic or Asian  Y = M +10 to +15 pts, C = 0.25 to 0.3M

African-American  Y=M and C and black unlimited.

The rest of the chapter goes on about histogram technique.  I taught that at CWS Advanced though I'm not sure it's still in the curriculum.  Probably not, but they have my notes ... well a few of them any ways.

Stay tuned!  Have a fashion related blog post coming up (actually it's done but debating on whether to post it or not)

Have a happy day.