Color balance : surprizing removal of pure primary color

[pierrelabreche]

Using the color balance transformation to remove a pure RGB color, the result affects the other primary color planes.

For example, when balancing by removing pure red, the result becomes completely cyan, washing away any ratio of blue and green in the original image.

This does not seem to be the intended behavior. If you slightly change the removed color away from pure red, the output tints are visible again.

Scenario :

• Open image file
• Transformation Color Balance
• Click on Remove patch
• In the color picker, select pure red from the palette

Outcome : the output image is in pure cyan tones.

[jsachs]

The color balance transformation generates a set of RGB color curves as illustrated by the graph on the right side of the dialog box. It does so by trying to eliminate color casts. It is not designed to make extreme changes to images such as removing pure red. In this case, it increases green and blue to compensate, resulting in a pure cyan output image. If you just want to remove all the red from an image, use the Color Curves transformation in RGB mode and set the red channel to zero.