Adjusting image curves
This topic explains how to make curve adjustments in the image editor.
Click on the Curves button on the toolbar to open the Curves dialog. Essentially, both Curves and Levels can be used to adjust the image contrast and set black and white points; which method you choose is largely a matter of preference. The X axis in the Curves diagram represents range of image brightness from black to white, while the Y axis shows how bright the corresponding pixel in the X axis will be in the resulting image. To adjust the black point in the image, click and drag the dot in the lower left corner along to the right, leaving its Y axis value unchanged. Similarly, you can set the image white point by dragging the dot in the upper right corner to the left along the X axis without changing its output value along the Y axis. You can click anywhere along the diagonal line and drag it to change the mapping of pixel values, or you can enter an input value and its corresponding output mapping. By default the Curves dialog displays the Master color channel, letting you adjust the overall image levels, but you can choose the red, green or blue color channel from the drop-down list for individual adjustment. To remove a point on the curve, click it and then drag it outside the Curves dialog. If you are used to working with CMYK values where a value of zero corresponds to white, you can toggle the white/black setting by clicking on the gradient grayscale bar along the X axis.
By clicking on the white and black arrows in the middle of the gradient field along the X axis you can reverse the black/white point on the scale. Also, the Curves dialog has four buttons:
Reset: Resets the adjustments that you have made to curves.
Load: load a predefined curve file from a file.
Save: Save the current curve settings to a file.
Predefined: Click this button to choose from four predefined curve settings.