Wednesday, November 30, 2016

Understanding White Balance on Color Wheel

RGB Color Wheel

Firstly, have a look at this color wheel. This is based on RGB Color Model. There is another color wheel called CMY Color Wheel, but they are basically the same, only somewhat rotated.

There is still another color wheel called RYB Color Wheel, which is somehow more popular on the web. And this color wheel is clearly different from the two mentioned above. In this wheel, Red, Blue and Yellow (instead of Green) are the primary colors. Do your own image searches to find out how different those color wheels are.

In this post, I only refer to RGB Color Wheel shown above because I believe that is the only one relevant to this topic. See how nicely additive (RGB) and subtractive (CMY) primary colors are arranged in a triangular, complementary manner!

White Balance

I'm sure you already know about color temperature with regard to white balance. So I skip discussion about it altogether here. It is a linear scale that has warmer (Amber) and cooler (Blue) sides on its ends. However, that's not the whole story. There is another axis called tint that is perpendicular to that temperature axis. It usually has greeny and purplish ends and often used to compensate greenish cast of fluorescent lights. So most advanced level cameras have both temperature and tint controls whereas entry level cameras may only have temperature control.

Let's see how color temperature and tint are mapped on the color wheel.
As you can see, Amber is not a primary (or even secondary) color and can be found halfway between Yellow and Red. The opposite of it is often called "Blue" in the discussion of color temperature, but it actually is not the same blue as the Blue of the RGB, because it is located halfway between Cyan and RGB Blue and so is somewhat whiter than RGB Blue.

Now let's look at tint. On this wheel, it has the Green of RGB on one end and the Magenta of CMY on the other. Yes, they are just primary or secondary colors.

You will use two controls (temperature and tint) on the white balance menu in your camera, but what it actually does is to change the hue (and saturation) in the color wheel.

Fixing Color Cast Effectively

I often find it rather difficult to correctly fix a particular color cast in my cameras. The reason is that there is no clue about which and how much of four colors (Amber-"Blue" and Green-Magenta) I should add or subtract.

Let's look at those four colors again.
Do you know how much of these strange colors you should add or subtract when you need to add some yellows on, or remove some redness from your image?

So now I'll try to remedy this situation.

First I rotated the color wheel 30 degrees counter-clockwise to make things little easier. Now temperature becomes vertical axis and tint is horizontal axis.
And I also gave better names for tertiary ("between") colors. Note that I renamed "Blue" of the color temperature as Azure.

If you wish to compensate a particular color cast, now what you should do is
  • to find a complementary color of that cast over the wheel above
  • then add that color with the two controls of temperature (A-B) and tint (G-M) on your camera menu
  • adjust the amount of fix considering how far you should shift using two controls.

Examples

I mainly use Olympus and Sony cameras. To me each of them has particular color cast under Auto White Balance setting.
  • Olympus XZ-1: AWB has reddish cast. To compensate this, I need to shift toward Cyan somewhat. So I would do, for example, A-2, G+1 (two steps toward Azure and one step toward Green).
  • Olympus E-PL2: AWB has yellow or somewhat chartreuse cast. The opposite of them are RGB Blue and violet. So, for example, I would do A-2, G-2 (two steps each toward Azure and Magenta).
  • Sony a5000: AWB sometimes looks too cool. This case is simple, I can fix by modifying only temperature. For example, I would do A+2, G0 (two steps toward Amber, leaving G-M at zero).

Notes

You may ask, exactly what RGB values I used to make the wheel? Here is the answer.

 primary (secondary) colors   tertiary colors
 Red  #FF0000  Amber  #FF7F00 
 Green  #00FF00  Rose  #FF007F
 Blue  #0000FF  Violet  #7F00FF
 Cyan  #00FFFF  Azure  #007FFF
 Magenta  #FF00FF  Spring Green   #00FF7F
 Yellow  #FFFF00  Chartreuse  #7FFF00

In some applications (eg. RAW developing software), however, the colors representing the temperature axis are not Amber and "Blue" (actually Azure) but rather Yellow and Blue (the true RGB Blue). In these cases, even though the colors representing the tint axis may be called "Green" and "Magenta", they are actually 30 degrees tilted version of them, Spring Green and Rose.



In other words, in most applications the four fundamental colors are Amber, "Blue" (actually Azure), Green and Magenta.


Whereas in others they are Yellow, (the true) Blue, "Green" (actually Spring Green) and "Magenta (or Red)" (actually Rose).

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