Using Curves

Figure 1: Reading a curve: To convert an input value to its corresponding output value, start at the input value, then draw a vertical line upward until it intersects the curve.  Draw a horizontal line to the left and read the output value on the vertical axis. 

In Figure 2, the cursor is positioned over the before value 88.  Since the curve is a straight line, the after value is also 88.

Figure 3 shows a curve that darkens the image by decreasing pixel values, and the highlighted point had a before value of  114 and an after value of 59.

How to Change a Curve

To change the shape of the curve as shown in figures 2 and 3, open an image in your favorite image editing program, and click on the curve itself to create a control point.  Then drag the control point downward. This bends the curve, making the image darker.

Notice also that the right half of the curve in Figure 3. is steeper than the left half.  Steepness means contrast. The curve in Figure 3 is a fairly common curve because it removes contrast from the shadows, and adds contrast to medium and bright areas.

To remove a control point, click and drag it to the edge of the curve.

Straight Line Curves

Here's a quick demonstration  a very important curve: the straight line.  Moving the end points of a curve is the single most important curve operation.  This operation determines the overall contrast of the image, and defines the darkest and brightest areas that can contain detail.

Figure 4. The original image is lacking in contrast.

This is an image of a brawny male California newt,  taken just a few days before this article was written, in the newt pond at the Berkeley Botanical Garden.

As you can see, this image needs some help in the area of contrast and color.

Figure 4a.  Figure 4, after applying a straight line correction to the RGB curve.  Notice that the colors are more saturated.  This is a characteristic of the RGB color space..

Figure 4b. Figure 4, after applying a straight line correction to the Lightness curve in Lab mode.  Notice that the colors are less saturated.

To use this adjustment, you must first convert your image to Lab mode.  Most image editing software supports the Lab color mode.

Setting a Neutral

Now lets do a little bit of color.  When and if the image contains something that should be gray, but isn't, that is our cue to setting a neutral.  After setting the end points of the curve, the second most important operation is setting a neutral point.  In RGB the procedure for setting a neutral is simple: make the red, green, and blue values the same.

Figure 5. This is Figure 4a, with a single point added to the curve for the blue channel.  The curve point is placed so that the red, green, and blue values of a point on the bottom of the pond are equal.

Two S's and a Lizard's Tail

Here are three common curve shapes that you can use immediately to improve the appearance of your images.  Check out the effect of each of the three curves on the right on the dark, medium, and light areas of fur.

Original Image

After S curve

After Inverted S

After "Lizard Tail"

The S curve is probably the most familiar curve shape.  The S curve buys contrast in the medium bright areas of the image (the midtone), and trades away detail in the shadow and highlight, without creating any completely black or white areas.  Notice that the lemur's face has more contrast, as does the fur on the right lower corner.  The S curve is steeper in the part of the curve that controls the midtones - the middle of the curve.  The white fur is less detailed, and the animal is merging with the black background.

The Inverted S has the opposite effect of the S curve.  It rescues shadow and highlight detail at the expense of contrast in the midtone.

The Lizard Tail is really a half of an inverted S, but typically has a very small "tail" intended to rescue shadow detail, while having a minimal effect on the rest of the image.  Notice that the lemur stands out even more from the background, and there is additional texture in the darker patch of fur to the right.  You can also use this shape to emphasize highlight detail.

If your software supports the Lab color space, you may use these shapes in the Lightness curve.  The Lab color space allows you to adjust Lightness, and avoid color side effects that occur with RGB.

Color

One of the more interesting capabilities of curves is the ability to bump color saturation and achieve a "Digital Velvia" effect.  RGB is not the color space of choice for manipulating color saturation with curves.  Although most editing programs support increasing color saturation in RGB mode, color saturation is easy to control using Lab curves or HSB mode.

Original

After Lab

After too much Lab!

After HSB

The first Lab example uses a simple steepening of the a and b curves (only the a curve is shown above) to achieve an increase in saturation.  The big win with Lab is that color saturation may be altered without changing the overall image contrast or brightness. When making this move be sure to keep the curves centered to avoid a color cast.  Although I would have liked to make the fur even more colorful, I had to limit the steepness to prevent the lemur's eyes from taking on an unnatural glow.

The second Lab example shows what can happen if you go too far.  The fur has a nice, rich color but the eyes look like a demon from the lemur version of the Exorcist movie.

HSB is still a less popular color space than Lab or RGB, but it has the advantage that you may increase colors in less saturated parts of the image and preserve areas that already have plenty of color.  The fur color is even more intense than with the Lab example, yet there is no sign of the demon eyes from the previous example.

Summing Up

Curves are the simplest, most universal, and most powerful tool for controlling contrast and color.  Not only image editing programs, but scanner software, and raw file conversion software uses curves.  If you have not used curves regularly, I hope that the examples this article have intrigued you enough that you will start using some of the curve moves shown here.

For additional reading, see the Curvemeister's Curves Anthology,  and Dan Margulis's Professional Photoshop and Photoshop LAB Color books.

Mike Russell is the author of Curvemeister, a Photoshop Plugin for Windows that makes curves easier to use, more powerful, and fun.