Adjusting Brightness Curves

You can adjust the tonal range of an image with the Curves command. Unlike the Levels command, which can set the minimum, maximum, and median values, Curves adjusts the entire range of values. It lets you map input values to output values according to a line (“curve”) on a graph. Curves provides the most control over the tonal range of an image.

This command is not available when a paint object in Black & White mode or Indexed mode is selected.

In the Curves dialog box, brightness values range from 0 (black) to 255 (white), or 0 percent (white) to 100 percent (black). To switch between these scales, click the grayscale bar under the graph.

A typical setting is a gentle S-curve (or inverted S-curve, depending on the scale you use), which adds contrast to an image without appearing too harsh.

Curves Dialog Box

The graph shows how Canvas X Draw maps input brightness values to output values. Select Preview to see the effect on the image.

To Adjust Brightness Curves:

1. Select one paint object to adjust. The paint object can be in Edit mode. You can select an area in the image. If you don’t make a selection, the entire image is affected.
2. Choose Image | Adjust | Curves.

To redraw the curve completely, or to create sharp changes in brightness for a tonal range, click the pencil and draw a new curve or segment.

3. In the Channel pop-up menu, select the composite channel or an individual channel to adjust. To adjust multiple channels, select the channels in the Channels palette first.
4. To change the shape of the existing curve, make sure the curve button at the bottom-left is selected.
   • Click points that you want to keep the same.
   • Drag points on the curve that you want to change. Or, enter values in the Input and Output boxes. For example, to keep midtones the same, click the center of the curve, then drag other areas of the curve. To adjust midtones without affecting highlights and shadows, click the quarter and three-quarter points of the curve, and drag the middle.
5. If you draw disjointed segments with the pencil, you can click Smooth to create one continuous curve.
6. Click OK to apply the current settings to the image.

Saving and Loading Curves Dialog Box Settings

You can save Curves dialog box settings to use again; e.g., after correcting the brightness curve for a particular Photo CD image, you can save these settings and later apply them to other images from the same source.

To Save Curves Settings:

In the Curves dialog box, click Save. In the directory dialog box, type a name for the settings file, select a location, and click Save.

To Load Curves Settings:

In the Curves dialog box, click Load. In the directory dialog box, locate the settings file and click Open.

See also:

• Levels