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.
There are different possibilities to activate the tool:
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From the main menu: → .
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By clicking the tool icon
in the Toolbox if this tool has been installed there. For this, please
refer to Secção 6.13, “Caixa de ferramentas”.
- Presets, Blending Options, Preview, Merge filter, Split view
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![[Nota]](images/note.png)
Nota These options are described in Secção 8.1.2, “Colors Common Features”.
- Channel
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There are five options:
- Value
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The curve represents the Value, i.e. the brightness of pixels as you can see them in the composite image.
- Red, Green, Blue
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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
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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
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This button deletes all changes made to the selected channel and returns to default values.
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Linear
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Non-Linear
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Perceptual
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These three buttons determine whether the tone reproduction (TRC) will be displayed using a linear, non-linear, or perceptual X axis.
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Linear histogram
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Logarithmic histogram
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These two buttons determine whether the histogram will be displayed using a linear or logarithmic Y axis. For images taken from photographs, the linear mode is most commonly useful. For images that contain substantial areas of constant color, though, a linear histogram will often be dominated by a single bar, and a logarithmic histogram will often be more useful.
You can also use the same options in the Tool Options dialog if Curves has been enabled as a tool. This grayed out histogram is not displayed by default.
- Main Editing Area
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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 into two parts; the upper part then represents the tonal balance of the layer or selection.
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The vertical gradient: it represents the destination, the output tonal scale. It ranges from 0 (black) to 255 (white), from Shadows to Highlights.
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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 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 snaps to the original curve: this is particularly useful for adding points along the curve.
Inactive points are white. The active point is black. You can activate a point by clicking 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:
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Click-and-drag the point.
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Using Up and Down arrow keys (Holding the Shift down lets you move it by increments of 15 pixels) to move the point vertically.
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Ctrl + click-and-drag allows you to move the point along the curve in its segment, independently of the pointer position.
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Another way to move point is using the “Input” and “Output” spin buttons: see below.
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