Using the Tone Curve filter

The Tone curve filter lets you perform color and tonal corrections by adjusting either individual color channels or the composite channel (all channels combined).

Original image (left); the image with adjusted tonal range (right)

Individual pixel values are plotted along a tone curve that appears in a graph and represents the balance between shadows (bottom of graph), midtones (middle of graph), and highlights (top of graph).

The tone curve shows the balance between the shadows, midtones, and highlights of an image. The original (x) and adjusted (y) pixel values are displayed side by side when you drag the tone curve. This example shows a small adjustment to the tonal range, in which pixel values of 152 are replaced with pixel values of 141.

The x-axis of the graph represents the tonal values of the original image; the y-axis of the graph represents the adjusted tonal values. Moving the curve up and to the left lightens the image and increases the color in any channel; moving the curve down and to the right darkens the image or decreases the color of a given channel. If you have an image with a blue cast, for example, you can decrease the blue in the image by selecting the Blue channel from the Active channel list box and moving the curve down and to the right.

You can adjust the color and tone of an image by dragging the tone curve interactively in the preview window, adding one or more nodes to the tone curve, selecting areas in the image window, or applying a preset. You can adjust multiple points along a curve, so you can lighten the shadows and darken the highlights.

You can also save tone curve settings as presets to use with other images. Response curve files are saved with the extension .CRV.

To apply the Tone curve effect

2 .	Click Effects Adjust Tone curve.

You can also
View the tone curves for all channels at once	Enable the Display all channels check box.
Adjust specific areas in an image	Click the Eyedropper tool, and click in the image. The pixels that you select appear as nodes on the tone curve.
Add a node to the tone curve	Click where you want to add a node along tone curve.
Smooth a freehand curve	Click the Smooth button. Tip: Click and hold the Smooth button down to continuously smooth the tonal curve.
Reset the tone curve for a selected channel	Click the Reset active channel button.
Reset the tone curve for all channels	Click the Reset button.
Equalize the tonal range of an image	Click Auto balance tone button. To specify the outlying pixels (clipped pixels) at each end of the tonal range, click the Settings button and specify the Black limit to and White limit to values in the Auto-adjust range dialog box.
Apply a preset	Click the Open button to the right of the Presets box and choose the folder where the presets are stored.
Save tone curve settings as a preset	Click the Save button to the right of the Presets box and locate the folder where you want to save the preset.

To flip the axis of the grid, click the Invert button .

To change the resolution of the grid, press Alt and click in the preview window.

To invert the selected tonal curve, click the Horizontal Flip or Vertical Flip buttons .

The histogram lets you view the adjusted tonal range and evaluate the results. For more information about histograms, see Using the histogram.

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