Image Processing Reference
or rumpled instead of flat and dry when it was scanned.) The image touchup
process will come in handy when correcting this problem. Also, you'll notice
that the image is a little dull and the colors are stale. Taking the following
actions might save the day:
• Correcting the tonality (color levels and gradation curves)
• Setting the brightness or contrast
• Setting the hue or saturation
You will find options for setting color depth, brightness, contrast, and
color in the Tools > Colors menu.
But first, let's explore the two most important sets of options in detail:
• Levels (tonality correction)
• Curves (color correction).
2.5.8 Setting the Contrast and Color—
Levels (Tonality Correction)
Correcting tonality will improve the quality of almost any image. GIMP offers
several options to do this.
The Levels options (tonality correction) can be found via Tools > Color
Tools > Levels or Colors > Levels .
The most striking effect in the Levels dialog is the Input Levels curve, which
is referred to as the color histogram of the image. Initially, it is created from
the image's RGB color channel ( Channel: Value ).
The curve shows how the color lightness values are distributed in the
image. In the recently modified miami.tif image, you can see that the curve
starts a short distance from the left margin and ends before the right margin.
Roughly speaking, this means that the image does not possess “real” black
values (shadows). It also tells you that the image has only a small amount of
“real” white values (highlights).
Underneath the histogram are the numerical values for the image's
lightness ( Output Levels ). There is also a black-to-white gradient bar, which
corresponds to these values. You can move the triangles under the gradient to
change the lightness of the image. This tool works similarly to the settings in
the Brightness-Contrast dialog. However, you'll probably find that the handling
of the Brightness-Contrast dialog is more comfortable.
The Levels dialog has an Auto
button. Clicking this button will
apply a tonality correction that is
automatically calculated from the
image values. You can also use
the automatic tonality correction
under Colors > Auto > White Balance .
For many images, this function is
sufficient to optimize the image
quality. However, if you want to
develop your pictures to your own
preferences, you will have to use
the Levels or Curves function.