5.8. Curves

5.8. Curves

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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
images.

5.8.1. Activating the Tool

You can get to this tool in several ways:

  • In the image menu through
    ToolsColor ToolsCurves or
    ColorsCurves.
  • By clicking on the tool icon



    in Toolbox, if this tool has been installed there. For this, please
    refer to Section 1.7, “Toolbox”.

5.8.2. Adjust Color Curves options

Figure 14.168. The Adjust Color Curves dialog

The Adjust Color Curves dialog

 

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

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

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.

5.8.3. Using the Curves tool

5.8.3.1. Summary and basic shapes

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.

How the Curves tool works

Moving the anchor of a pixel upwards makes this pixel brighter.

Moving the anchor upwards

Making the curve more horizontal

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

Making the curve more horizontal

Make the curve more horizontal

Making the curve more horizontal

The resulting histogram

 

Making the curve more vertical

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

Making the curve more vertical

Make the curve more vertical

Making the curve more vertical

Result and its histogram

 

5.8.3.2. Practical cases
Invert colors

Inverted curve

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.

Enhance contrast

Contrast enhanced

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.

Working on color channels

For every channel, we moved the white point horizontally to the
left, to the first Highlights. This lightens Highlights up. Then we
shaped the curve to lighten Mid tons and Shadows while keeping
black.

The original image and the result

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