Using Curves
by Mike Russell, exclusive to nikondigital.org, March 2006
Curves
are the most powerful way to alter the values of an image. Most image
editing programs, including Photoshop, Paint Shop Pro, and Picture
Window Pro support curves. In this article, I'll cover the basics
of curves. Then I'd like to show you some things you can do with curves that
will improve your images.
Curves are
a Filter
Think of a curve as a filter
that modifies an image by changing the pixel values that make up an
image. The numbers from 0 to 255 along the horizontal edge of the curve
in Figure 1 represent pixel values before the curve is applied, and the
numbers along the vertical axis are the image pixel values after the curve is
applied. The curve in Figure 1 shows this process with a pixel value being
mapped from 108 to 103.
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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.
Figure 2: Click on or near the curve. This will create a control
point. |
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Figure 3: Hold the mouse button down and drag the control point
to alter the shape of the curve. |
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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. |
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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. |
 
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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.. |
 
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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. |
 
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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. |
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Original Image |
After S curve
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After Inverted S
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After "Lizard Tail"
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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. |
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Original |
After Lab
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After too much Lab!
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After HSB
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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.
Photoshop Elements does not support curves, but you may use the Curvemeister demo to follow the example given in this
series of curve articles. |
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