Image Processing Reference
2.6 Touchup Work 1—
Removing Color Cast
2.6.1 What Is Touchup Work?
So far you have learned about the basic program functions and how you can
use them to improve the quality of your images. In the previous sections, you
have used them to edit, or modify, your image as a whole.
However, an image may have additional blemishes that you wish to
• It may have signiicant color cast.
• Older, scanned photos may have scratches and spots. Scanned slides may
have dust, bits of fluff, or unwanted image elements, such as embedded
text, which can be removed.
• The subjects in the i mage could have red-eye resulting from the image
being taken with a flashbulb or an electronic flash.
• Some images may have a pale, dull, or stale sky that needs to be freshened up.
The work required to remove such blemishes is called image touchup or
retouch. Tonality correction is also considered touchup work in image editing.
The following sections explain techniques and tools for constructive
touchup . I will use examples to show you how to remove various image
blemishes. I will also cover two touchup steps in chapter 3, “Using Masks and
Layers—Painting, Filling, and Color Tools”, because they require masks.
How Does Color Cast Happen?
Color cast can be seen as discoloration throughout an image. You might
notice the blue color cast that results from taking a photo in bright light under
the sky without a skylight filter or the yellow color cast that results from taking
pictures without a filter in artificially illuminated rooms. Color cast can also
be a result of improperly developing an image in the lab or using the wrong
settings. Scanning slides often produces a color cast.
2.6.2 Color Correcting Options
Tonality correction ( Levels function) is appropriate for editing images with
color cast. In this exercise, you will edit the red, green, and blue color channels
individually. Color Balance can also be employed to remove color cast,
particularly when the discolorations are minor. The following section provides
step-by-step instructions for using these two functions.