Why curves? And given curves, why use them in more than one color space?

This is a fair question, and although I have to assume some of you are already sold on curves, for those of you who are not, here goes.

With so much hand waving in the world, der Curvemeister loves example images. In that spirit, each of the remaining images on the right was color corrected using Auto Levels, Auto Contrast, Levels, Brightness/Contrast, Curves RGB, Curves wg-CMYK, and Curves in the Lab color space.

Pick out your favorite, and take a peek at the answers below to see which method was used to create it.

It may just be time to take Curves out for a spin!

Note: To get the most from this example, you are encouraged access the original image data and open it in a new Photoshop document. What do you think? Are curves starting to rub off on you a little?

Answers:

1. The original image is Image A
2. Brightness and Contrast - if it works for color television, why not for your images? Image D
3. Auto Levels - this improves your image about 2/3 of the time. Image C
4. Auto Contrast - changes brightness only, and leaves your colors alone. Image B
5. Levels - manual adjustment of the beginning, end and middle of your image's brightness range. Who could ask for more? Image F
6. Curves in RGB mode. Image G
7. Curves in Lab mode. Separate control over brightness and color information. Image E
8. Curves in Curves wg-CMYK mode. Image H

2

Brightness and contrast. Everyone who has had to adjust a TV picture is familiar with these controls. It's easy, just by fiddling with the two sliders, to come up with an image that looks better than what you started with.

Most people quickly graduate from here to Levels, and with good reason. It's impossible to increase the contrast without simultaneously losing a certain amount of detail in both shadows and highlights.

Brightness operates by removing shadow or highlight detail, making blacks tend toward gray. If you compensate for this by increasing the contrast sacrifices more highlight detail. In spite of all of this, Brightness is easy to use, and some people are happy with it for quite some time, that is, until they discover levels.

3

Auto Levels and Auto Contrast. These are completely automated processes, based on the levels and contrast controls. Auto Levels adjusts everything in sight to cover a maximum numerical range, but it makes some simplistic assumptions, and may change colors in ways you do not want, adding a large dose of color cast. In this case the yellow of the balloon is brightened up, but the sky takes on a green cast.

Auto Contrast is a refinement of Auto Levels that changes brightness only, leaving colors alone. Even this refinement has its limitations, because Auto Contrast has no concept of drama, or which bright and dark areas of the image are important.

4

Levels is a substantial improvement over Brightness and Contrast, because it provides control of overall shadow and highlight as well as separate adjustments for each channel. In addition, the overall brightness of the image may be changed by moving the middle gray triangle, called the gamma adjustment.

Add to that the fact that levels displays a histogram representing the range of values used by the image, and levels is a powerful accessory to curves.

There is more that could be done with levels than I have shown here, however all of those advantages are shared by curves. Levels is really a curve with three points. This means, for example that levels has no way to represent an S-shaped bend in a curve.

5

Curves in RGB mode. The RGB Master Curve is not really needed, but is convenient for quickly setting the shadow and highlight, provided you do not introduce too much of a color cast.

RGB is prone to creating color casts, and by the same token is also excellent for getting rid of color casts that affect particular brightness ranges. In this example, the yellow baloon and blue sky occupy different parts of the red curve, and because of the flexibility of curves, some interesting adjustments are possible..

In this example, I used Curvemeister's "pin lower left" command to lock down the dark end of the Red curve. This preserved the color of the sky, preventing any additional red from turning the sky purple. I then bumped up the light end to jazz the yellow balloon up a bit, at the cost of making the balloon more yellow.

This results in a satisfactory improvement in the image, with more color variation in both the baloon and sky, and increased saturation in the yellow baloon, making it stand out better from the compimentary blue sky. This adjustment cannot be done in levels because levels is limited to a single interior adjustment point.

6

Curves in Lab mode. Lab provides a single Lightness curve, which controls how bright or dark the image is, as if the image were in black and white.. The a and b curves control color. The result is independent control of brightness and color.

I used a very common sequence for Lab mode, bumping the L channel to increase contrast, and steepening the a and b curves to increase the overall saturation of color in the image without introducing any color casts. This is done by ensuring that the a and b curves cross the center point of their respective curves.

The end result is like the RGB correction, but even more so. The lighter areas of the balloon in particular get additional kick, in the form of contrast, from this treatment. At the same time the balloon retainins more detail, without compromising the saturation of the yellow color.

7

Curves in wg-CMYK mode. This mode is similar to Photoshop's CMYK mode, but does not suffer from the color fidelity problems arising from converting to and from CMYK.

wg-CMYK's (and CMYK's) trump card is the ability to control shadow detail very precisely, and to retain stable neutral values while making relatively large changes to color values.

For this image, I set shadow and highlight via the master curve, and greatly bumped the amount of yellow in the balloon, and making the basket much more saturated brown.

I also decreased yellow in the sky, yielding a purer blue to make the balloon stand out more.

A small increase in the amount of black channel made the printing on the balloon clearer, without affecting the detail visible in the darkest area of the balloon.

Overall, this image has the most saturated colors and dramatically more detail in the vertical black bands of the baloon. The yellow curve can be more bold because so much of the detail is held in the black channel. These are the hallmark of wg-CMYK.

8

Curvemeister deliver on the promise of quickly achieving better images. Compared to the other color adjustment tools discussed here, Curvemeister offers tremendous potential for adjusting the color of an image.

Combined with the various color modes, and using the techniques touched on here, you will be prepared to deal with a very large assortment of unique color correction puzzles.

Curvemeister leverages this potential by adding the wg-CMYK, or "Wide Gamut CMYK", color space, and by making it easy to access different color modes while you make your color correction decisions.

I hope I've convinced, or at least intrugued with this demonstration.