Working with Black-and-White and Color Images - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 316

4.1.4 The Graphical Library GEGL—Developing
Black-and-White Images with GEGL Operations

The Generic Graphics Library (GEGL) has been integrated
in GIMP with version 2.6. It is supposed to be expanded to
ensure image editing with more than 8 bits per channel
with GIMP. Some of the computation functions based
on GEGL are already assembled in the GEGL Operation
window , which can be found by choosing Tools > GEGL
Operation.
In the Operation drop-down menu, there are three
operations especially for converting images into black
and white.
The mono-mixer is a simple function that
automatically converts a color image into a grayscale
image. It also offers the possibility of changing the
brightness and contrast by modifying the RGB color
values. With a little experience, the mono-mixer offers
rather sophisticated results.
The results of the contrast-curve operation, on
the other hand, are quite puzzling. This function also
converts color images to grayscale, but modifying the
values using the sliders doesn't change the rendering
of the image. The question remains as to what can be
adjusted with the sliders. The help function doesn't have
any information either.
A far more sophisticated option is the c2g ( color-
to-gray ) operation. It should be noted that this setting
is very demanding on your hardware, as are most of the
GEGL operations. My computer, mind you, an AMD Dual-
Core Athlon 64 X2 3800 with 2 GB RAM, regularly took
some time to process a 2 megapixel image. Nevertheless,
the application is stable; you do get a result after quite
some time. It is, however, difficult to handle, and every little adjustment leads
the program to compute without giving you the option to adjust the value.
At least you can be quite specific by entering the values on the right side.
To some degree, you can edit low-resolution images with few pixels in real
time. Still, every little step takes time. Overall, c2g is an interesting tool that in
addition to controlling brightness and contrast can simulate film grain. As can
be seen in figure 4.10 and figure 4.11, it is an interesting toy for those who like
to experiment.

Figure 4.9
Developing black-and-white images using the GEGL
Operation mono-mixer