Image Processing Reference
4.1.3 Developing Black-and-White Images with
the Channel Mixer
The results of the Desaturate function in turning images into black and white
are not very satisfying. Actually, we don't have much influence in the way an
image is altered. You can only change the brightness and contrast afterwards
by using the tonality correction (Levels) and gradation curves (Curves) or the
Brightness-Contrast function. These can be found on the Colors menu.
More options for the black-and-white development can be found in the
Channel Mixer ( Colors > Components > Channel Mixer ). This function lets you
adjust the amount, or rather the brightness, of the red, blue, and green color
components in an image. Therefore, you are also able to desaturate the image
as long as the image is available in the RGB mode. The Channel Mixer also
allows you to reposition the color channels to modify and shift the colors of
your image (figure 4.5).
As soon as you check the Monochrome box, the image is turned into
a grayscale image. At the same time the Output channel is deactivated.
Essentially, you can only adjust the brightness of the grayscale image. Thus,
the function does not enable you to develop the image any further.
When you select Preserve luminosity , it starts getting interesting. This
setting causes a modification of the color components in brightness and
contrast in the individual color channels. Therefore, many variations for
developing the image from color to grayscale are possible.
The performance of the function is somewhat different with grayscale
photographs, even those taken in RGB mode. When you select a color channel
in the Output channel , you can colorize the image with the red, green, and
blue sliders. By selecting another color channel, you can further modify your
image. Once you select the Monochrome check box, the Output channel is
deactivated and the image is turned into a grayscale image again. By using
the three color sliders, you can then adjust the brightness of your grayscale
image. However, selecting Preserve luminosity will not modify your image
any further. On the contrary, the image will be reset to grayscale without any
further option for editing.
Figures 4.7 and 4.8 exemplify the differences in black-and-white
developing when using the Desaturate function and the Channel Mixer .
Modifications of color in an image using
the Channel Mixer. The role of the default
setting in the Output channel is secondary;
the sliders are more important for chang-
ing the color components of your image.
Developing a grayscale image using the