Adjust the colour, tone and alpha channels with the curves adjustment, either on individual channels or by adjusting the master curve.
Settings
The following settings can be adjusted:
Select a colour mode (GRAY/RGB/CMYK/LAB) from the first pop-up menu.
Specify a single colour channel to apply the adjustment to, including the layer's alpha channel. Master (the default choice) applies the adjustment to all channels. Select from the second pop-up menu.
Picker—allows you to drag on the image to modify the adjustment. The initial click will place a node on the curve in relation to the pixel selected. Dragging up will lighten the image while dragging down will darken it. The curve graph will update accordingly.
Input minimum and Input maximum are useful for 32-bit documents with out of range tonal values, but can also be used to fine tune the manipulated tonal range in 8-bit and 16-bit documents:
Input minimum—specify, in float, a minimum value to use as tonal input.
Input maximum—specify, in float, a maximum value to use as tonal output.
To adjust a curves graph:
On the curves graph, do any of the following:
In the dialog, click Picker and then drag up or down on the page.
Drag the curve to adjust the tonal range of the image.
Click on the curve to add additional nodes.
Click to select a node and then press the to remove it.