Presets

You can save the color settings of your image by clicking the
Add settings to favourites button

The

button opens a menu:

 

Figure 14.169. Preset Menu

 

which lets you Import Settings from File or
Export Settings to File,
and gives you access to the Manage Save Settings dialog:

 

Figure 14.170. Manage saved Settings Dialog

 

Channel

There are five options:

Value

The curve represents the Value, i.e. the brightness of
pixels as you can see them in the composite image.

Red; Green; Blue

The curve represents the quantity of color in each of the
three RGB channels. Here, dark means
little of the color.
Light means a lot
of the color.

Alpha

The curve represents the opacity of the pixels.
Dark means
very transparent.
Light means very
opaque. Your image or active layer must have
an Alpha channel for this option to be enabled.

Reset Channel

This button deletes all changes made to the selected channel and
returns to default values.

Linear and Logarithmic buttons

These buttons allow to choose the Linear or Logarithmic type of
the histogram. You can also use the same options in Tool Options
dialog. This grayed out histogram is not displayed by default.

Main Editing Area

• The horizontal gradient: it represents
  the input tonal scale. It, too, ranges from 0 (black) to 255
  (white), from Shadows to Highlights. When you
  adjust the curve, it splits up into two parts; the upper part
  then represents the tonal balance of the
  layer or selection.
• The vertical gradient: it represents the
  destination, the output tonal scale. It ranges from 0 (black)
  to 255 (white), from Shadows to Highlights.
• The chart: the curve is drawn on a grid
  and goes from the bottom left corner to the top right corner.
  The pointer x/y position is permanently displayed in the top
  left part of the grid. By default, this curve is straight,
  because every input level corresponds to the same
  ouput tone. GIMP automatically places an anchor at both
  ends of the curve, for black (0) and white (255).If you click on the curve, a new anchor
  is created. When the mouse pointer goes over an anchor, it
  takes the form of a small hand. You can
  click-and-drag the anchor to bend the curve. If you click
  outside of the curve, an anchor is also created, and the
  curve includes it automatically.

  Unactive anchors are black. The active anchor is white. You
  can activate an anchor by clicking on it. You can also swap
  the anchor activation by using the Left and Right arrow keys
  of your keyboard. You can move the anchor vertically with the
  Up and Down arrow keys. This allows you to fine tune the
  anchor position. Holding the Shift down lets
  you move it by increments of 15 pixels.

  Two anchors define a curve segment which
  represents a tonal range in the layer. You can click-and-drag
  this segment (this creates a new anchor). Of course, you
  can’t drag it beyond the end anchors.

  To delete all anchors (apart from both ends), click on the
  Reset Channel button. To delete only one
  anchor, move beyond any adjacent anchor on horizontal axis.

  Meanwhile, on the canvas, the mouse pointer has the form of
  an eye-dropper. If you click on a pixel, a vertical line
  appears on the chart, positioned to the source value of this
  pixel in the selected channel. If you
  Shift-click, you create an anchor in the
  selected channel. If you Ctrl-click, you
  create an anchor in all channels, possibly including the
  Alpha channel. You can also Shift-drag and
  Ctrl-drag: this
  will move the vertical line and the anchor will show up when
  releasing the mouse left button.

  The histogram of the active layer or selection for the
  selected channel is represented grayed out in the chart. It’s
  only a reference.

Curve type

Smooth

This the default mode. It constrains the curve type to a
smooth line with tension. It provides a more realistic
render than the following.

Free Hand

With this mode, you can draw a broken line that you can
smooth by clicking the Curve Type
button again.

Preview

The Preview button makes all changes to the levels dynamically so
that the new level settings can be viewed immediately.

Tool Options dialog

Although this tool is not present in the Toolbox by default (For this,
please refer to Section 1.7, “Toolbox” if you want to add
it), nevertheless it has a Tool Option Dialog under the Toolbox. These
options are described here:

Histogram Scale

These two options have the same action as the Logarithmic

and Linear

buttons in the Curves dialog.

Sample Average

This slider sets the “radius” of the color-picking
area. This area appears as a more or less enlarged square when you
maintain the click on a pixel. Here, the eye-dropper is used to
locate a pixel: radius = 1 seems the best.