Source: 5.8. Curves
The Curves tool is the most sophisticated tool for changing the color,
brightness, contrast or transparency of the active layer or a selection.
While the Levels tool allows you to work on Shadows and Highlights, the
Curves tool allows you to work on any tonal range. It works on RGB
You can get to this tool in several ways:
- In the image menu through
→ → or
- By clicking on the tool icon
in Toolbox, if this tool has been installed there. For this, please
refer to Section 1.7, “Toolbox”.
Figure 14.168. The “Adjust Color Curves” dialog
- You can save the color settings of your image by clicking 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
There are five options:
- 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.
- The curve represents the opacity of the pixels.
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
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.
- The horizontal gradient: it represents
- Curve type
- 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
- 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
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.
We create anchors and segments on the curve and we move them to
shape the curve. This curve maps “input” tones of
the active layer or selection to “output” tones.
Making the curve more horizontal forces all the input tonal range to
occupy a shrunk output tonal range.
The histogram shows the compression of pixels into the output range.
Darkest and brightest pixels disappeared: contrast decreases.
Figure 14.171. Making the curve more horizontal
Moving the upper end point to the left and the lower end point to
the right is the same as moving the white slider to the left and the
black slider to the right in the Levels tool: all pixels whose value
is more than the white point (the flat part of the curve) are made
white (more colored / more opaque according to the selected
channel). All pixels whose value is less than the black point (the
lower flattened curve) are made black (black / completely
transparent). Pixels corresponding to points of the curve that have
moved up are made lighter. Pixels corresponding to points of the
curve that have moved down are made darker (green arrows). All
these pixels will be extended to the whole output tonal range.
The histogram shows the extension of values, from black (0) to White
(255): contrast is increased. Since the Value channel is selected,
changes affect all color channels and colors increase.
Figure 14.172. Making the curve more vertical
Black is made White (fully colored / fully opaque). White is made
black (black, fully transparent). All pixels adopt the complementary
color. Why that? Because subtracting the channel values from 255
gives the complementary color. For example: 19;197;248 a sky blue
gives 255-19; 255-197; 255-248 = 236;58;7, a bright red.
Contrast is increased in mid tones because the curve is steeper
there. Highlights and Shadows are increased but contrast is slightly
less in these areas because the curve is flatter.