8.11. 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.

8.11.1. Activating the Tool

You can get to this tool in two ways:

  • In the image menu through ColorsCurves….

  • By clicking on the tool icon in Toolbox, if this tool has been installed there. For this, please refer to 1.11절. “도구상자”.

8.11.2. Curves options

그림 16.157. The Curves dialog

The Curves dialog

Presets, Preview, Split view

These options are common to GEGL-based filters. Please refer to 8.1절. “Introduction to the Colors Menu”.

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.

Adjust curves in linear light and Adjust curves perceptually

These new buttons in GIMP-2.10 allow you to switch between linear and perceptual (non-linear) modes.

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 output tone. GIMP automatically places a point at both ends of the curve, for black (0) and white (255).

    If you click on the curve, a new point is created. When the mouse pointer goes over a point, it takes the form of a small hand. You can click-and-drag the point to bend the curve.

    If you click outside the curve, a point is also created, and the curve includes it automatically. If you Ctrl-click outside the curve, the Y-coordinate will snap to the original curve: this is particularly useful for adding points along the curve.

    Unactive points are white. The active point is black. You can activate a point by clicking on it. You can also swap the point activation by using the Left and Right arrow keys of your keyboard.

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

    To move a point, there are several possibilities to fine tune the point position:

    • Click-and-drag the point.

    • Using Up and Down arrow keys (Holding the Shift down lets you move it by increments of 15 pixels) to move the point vertically.

    • Ctrl + click-and-drag allows you to move the point along the curve in its segment, independently of the pointer position.

    • Another way to move point is using the Input and Output spin buttons, new in GIMP-2.10.12: see below.

    To delete all points (apart from both ends that can't be deleted), click on the Reset Channel button. To delete only one point, move it beyond any adjacent point horizontally.

    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 a point in the selected channel. If you Ctrl-click, you create a point in all channels, possibly including the Alpha channel. You can also Shift-drag and Ctrl-drag: this will move the vertical line and the point 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 and is not updated during treatment.

Input, Output

These spin buttons allow setting the selected point coordinates numerically and accurately if needed.

Type

Control points can be either Smooth (default: all points are smooth) or Corner points. These Corner points result in sharp angle. They are displayed using a diamond shape. You can toggle between Smooth and Corner for the selected point.

Curve type
Smooth

This Smooth option concerns the whole curve, not only the selected point as above.

Free Hand

With this mode, you can draw a free hand line that you can smooth by clicking the Smooth Curve type.

8.11.3. Using Curves

8.11.3.1. Summary and basic shapes

We create points 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.

8.11.3.1.1. How the Curves tool works

Moving the point a pixel upwards makes this pixel brighter.

Moving the point upwards

8.11.3.1.2. 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.

그림 16.158. Making the curve more horizontal

Making the curve more horizontal

Make the curve more horizontal

Making the curve more horizontal

The resulting histogram


8.11.3.1.3. 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.

그림 16.159. Making the curve more vertical

Making the curve more vertical

Make the curve more vertical

Making the curve more vertical

Result and its histogram


8.11.3.2. Practical cases
8.11.3.2.1. 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.

8.11.3.2.2. 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.

8.11.3.2.3. 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