Exposure

Digital images inherent characteristics from the devices and conditions that created them. It is the goal of Color Management to automatically compensate for these characteristics, but often the compensation is flawed or inadequate.

ColorBender can quickly and easily correct most of these problems. The two most important adjustments are

• Black Point, White Point and Gray Balance
• Gamma

For Black Point, White Point and Gray Balance, ColorBender uses three buttons that activate automatic algorithms. The user just presses the buttons for the corrections.

For Gamma, the user can use the preset menu for common cases or can enter the gamma values directly.

    

There may occassionally be other exposure corrections required, but these can usually be done easily with the ColorBender Selective Adjustments.

Black Point, White Point and Gray Balance

How to Recognize Black Point, White Point and Gray Balance Problems

These problems are best analyzed by looking at the neutrals in an image. The following three simulations show, respectively, incorrect black point, incorrect gray balance and incorrect white point.

For this example, these problems can be deduced by looking at the shoe sole, the highlights on the face, and the background play structure. This table summarizes the observations.

	neutral black	mid gray	neutral white
	(bottom of shoe)	(play structure)	(face highlights)
first image	blue	some blue	white
second image	black	magenta	white
third image	black	some yellow	yellow

A characteristic of black point or white point error is that there is a partial corresponding error in the mid grays. By analyzing the table we conclude that the three images have black point, gray balance and white point errors, respectively.

  
black point is too blue                                    grays are too magenta                               white point is too yellow

In practice an image may contain all three errors.

How to fix Black Point, White Point and Gray Balance Problems

ColorBender quickly and easily corrects these three errors by clicking the Blacks, Grays or Whites buttons on the Control Panel. If the image has multiple errors, then a combination of these buttons can be selected.

  
black point auto corrected                              grays balance auto corrected                            white point auto corrected

These are actual results from auto correcting the above three example images.

Any combination of these buttons can be selected. In a few clicks you can test to see if the corrections improve the image. Depending on the problem, any of the 8 combinations may be the best solution.

Any remaining problems can be corrected using the adjustment controls. Notice in the middle image above that the brightness needs to be increased to match the other two.

Auto Black Point, White Point and Gray Balance Technology

For those interested, the ColorBender technology for these functions work as follows. A weighted histogram is calculated giving higher weight to neutrals. This minimizes the effects of brightly color objects. An analytic transfer function is calculated for each channel that gives the required balance. (Most other applications use histogram clipping that introduces burn outs, flat spots and color fringing.)

Gamma

How to recognize gamma problems

Using incorrect gamma gives the following effects

	Gamma increased	Gamma decreased
brightness	darker	brighter
color tone	more vivid	less vivid
color shift	more RGB values	more CMY values

While the brightness and tone changes dominate the visual effects, when they are corrected the color shift errors remain and are difficult to fix. The following example images show the underlying color shift. These images were given different gamma values and then the tone and brightness are set to the same values so that the color shift effects are isolated.

The original image with correct gamma, shown in the middle, has a jacket with a color halfway between blue and cyan. If the gamma is decreased, the color shift is towards cyan. If the gamma is increased, the color shift is towards blue.

Note that RGB, CMY and near neutral hues are not changed, but other hues shift their value. The effect is complicated and emphasizes the need to use correct gamma values throughout the image chain.

  
color gamma = 0.5                  color gamma = 1.0                  color gamma = 2.0

How to set gamma

Standard gamma settings are selected with 1 click/select from the Exposure Presets Popup Menu. The settings are

• Input Image is from a Mac
• Input Image is from a PC
• Input Image is sRGB
• Set everything to Mac standards

The user can enter the numerical gamma value for each of the devices listed below. The "color" gamma value is unique to ColorBender. This setting preserves brightness and color tone and shifts the hue to correspond to a gamma effect (as shown in the previous example). Thus, if image gamma has been improperly set it can be repaired by this setting.

ColorBender 1.1 is a Universal Application that runs on Mac OS X 10.4 or later

© 4G Color 2005, 2006, 2007 all rights reserved, Website updated March 7, 2007