Curve Extraction is a new process of creating a curve that converts image A to image B, effectively "extracting" a curve from the data from two images.

There are several situations in image color correction where curve extraction is interesting. One common situation is rescuing one or more images that were captured using the wrong curve settings.

It is possible to accomplish this in Photoshop manually. As far as I know Curvemeister is the only program that uses curves to automate this procedure.

Good results may be achieved with very little work, and the automated procedure documented here is much faster than creating the curves manually.

As of June 2004, version 1.1.14 of the Curvemeister demo supports all the functionality that is required for this tutorial.

Repairing White Balance with Curve Extraction

The actual curve extraction may be most conveniently done by capturing two images after the fact, with the camera set to the color balance setting you used and the color balance setting you should have used.

The subject matter for your test image should have a lots of colors and neutral grays. The exact colors do not matter; variety does. Three or four sample points, spaced evenly throughout the brightness range of the image are sufficient. Adding more sample points does not necessarily improve accuracy.

A Macbeth Color Checker works well. These are expensive, though, and you may go to your local hardware store and get a free paint chip card and you just may accomplish the same thing.

2 Select RGB mode, and make all the curves visible. This will make it easier to verify that the sample points are giving us good curve coverage.

Use the Curve option menu to set the default sample size to 11x11. This will minimize the effects of any noise in your image.

3 Alt-click on the image to create your first sample point on an area of approximate medium gray. The cursor is pointing to the sample's menu button.

Click on the menu button, and pin the sample point to the current color, as shown.

Note: version 1.1.14 has a bug that may require clicking on the image window before this menu item is activated. When this bug is fixed, you may use ctrl-A to select all the curve points, and pin all the sample points with one menu click.

4 Repeat this operation and lay down a variety of sample points. Do a Pin to Current command for each new sample point, as you did before .

Add three or four sample points, taking care that the curve control points are spread fairly evenly along each of the red, green, and blue curves.

Don't overdo it - adding too many points will cause kinks in the curve, and you will need to remove them later.

Note: The term sample point refers to the eyedropper points you lay down on the image. Curve point refers to the points that appear on the actual curve..

5 After you have laid down your sample points, examine each curve individually to make sure you have good coverage - at least four well-spaced points.

This is what the blue curve looks like for our example. Notice the pretty good "point spread", although a fourth point at about the 1/3 tone would not be a bad idea.

This curve is always a straight line, because we are pinning image samples to their original values..

7 Start Curvemeister once again, and load the sample points from the .edr file you created in the previous step.

Look carefully at each curve in turn. If you see kinks in the curve, this is an indication that you have added too many sample points. Simply delete the sample points until the kinks go away, and re-save the sample points as you did in the previous step.

8 Here is the image after loading the sample points. Click the Save button to tentatively save the curve file. This is the curve that you would use to convert your images that were originally captured using the wrong white balance setting.

But our job is not done. Let's double-check our work, and see if there are any refinements we can do.

9 Compare the original, correctly captured image with the curved version of the incorrect file.

Some of the brighter reds have changed, and the background wall is peach colored (matching the real color more closely).

Just try doing the same process by hand!

Download the curve file

10 Another way to check our work is to immediately reload the sample points into the curved version of the image.

If I were unhappy with the image, I would use this curve as a guide, then go back to step 4 and either move or add more sample points.

In this case, looking at the curves, the sample points are spot on.