The "Curves" widget is a built-in feature in Blender's UI, and can be used anywhere, provided the curve data itself is being delivered to this widget.
Currently it is in use in the Node Editor and in the UV Window.
This widget will map an input value horizontally and return the new value as indicated by the height of the curve.
Multiple curves can be edited in a single widget. The typical use, RGB curves, has "Combined" result or "Color" ("C") as the first curve, and provides curves for the individual R, G, and B components. All four curves are active together, the "C" curve gets evaluated first.
Selecting curve points
- LeftMouse click always selects 1 point and deselects the rest.
- Hold Shift while clicking to extend the selection or select fewer points.
Editing curves
- LeftMouse click-drag on a point will move points.
- A LeftMouse click on a curve will add a new point.
- Dragging a point exactly on top of another will merge them.
- Holding SHIFT while dragging snaps to grid units.
- CTRL+click adds a point.
- Use the X icon to remove selected points.
Editing the view
The default view is locked to a 0.0-1.0 area. If clipping is set, which is default, you cannot zoom out or drag the view. Disable clipping with the icon resembling a #.
- Leftmouse click-drag outside of curve moves the view
- Use the + and - icons to zoom in or out.
Special tools
The wrench icon gives a menu with choices to reset a view, to define interpolation of points, or to reset the curve.
The Image window now can display float images as well as regular 32 bits images. The Curves tool allows you to create a mapping from the float range to a displayable result in OpenGL.
You can find the Curves Panel in the first View pulldown menu. Apart from the curves itself, you can also define a minumum and maximum value to map to 0 and 255 respectively.
A quick - and interactively updating - method to set these values is holding down SHIFT (min) or CTRL (max) while sampling an image with LMB.
Note that the original float colors don't change when using Curves. However, if you save the image as 32 bits, it will save the adjusted version.
Below three Curve Panel settings from a HD demo image from www.openexr.com. (Click on image for a larger view).
