Working with Black-and-White and Color Images - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 321

4.2 Touching Up Black-and-White
Images—Levels, Brightness, Contrast
As mentioned in the introduction to this chapter, you can edit the brightness,
contrast, and (color) values of images both in the RGB mode and in the
grayscale mode.
Modifying black-and-white images is similar to modifying color images. For
this reason, this section will be limited to providing a overview of those functions
available with both modes and how they differ. The functions discussed below
can be found in the Colors menu and most of them in the Tools > Color Tools menu.

Function RGB Mode Grayscale Mode

Color Balance Yes RGB levels only

Hue-Saturation Yes RGB levels only

Colorize Yes RGB levels only

Brightness-Contrast Yes Yes

Threshold Yes Yes

Levels (tonality correction) Yes Yes, but main channel only

(no individual color channels)

Curves (gradation curves) Yes Yes, but main channel only

(no individual color channels)

Posterize Yes Yes

Desaturate Yes RGB levels only

Invert Yes Yes

Value Invert Yes RGB levels only

Auto :
Equalize

Yes Yes

White Balance Yes RGB levels only

Color Enhance Yes RGB levels only

Stretch HSV Yes RGB levels only

Normalize Yes Yes

Stretch Contrast Yes Yes

As shown in the preceding table, it is both possible and recommended
to edit black-and-white or grayscale images in RGB mode. Keep in mind that
many editing features can be utilized only in RGB mode.
Converting an image to grayscale mode is recommended in the following cases:
• To simplify the image.
• When the options and tools available in grayscale mode are suicient for
your editing needs.
• To produce certain graphic efects.
• When pure gray levels are suicient for image rendering.
• To optimize the ile size of a black-and-white representation (grayscale
reduces the number of colors to 256 values).