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This chapter is from the book

This chapter is from the book

Curves

The Curves dialog box has remained nearly untouched for years, so the major surgery it got in CS3 caused quite a stir. Sometimes changes to favorite features are unnerving, but in this case I think you’ll love the new goodies in Curves. The list of changes is large and includes what most users have been asking for. In the past, I used Levels for grayscale images and simple adjustments because it offered a few features that were not found in Curves (like a histogram for instance). But since I’ve been working in CS3, I haven’t touched Levels because Curves can do everything that’s available in Levels and a whole lot more.

If you’re not already comfortable with Curves (it’s by no means a new feature since it’s been in Photoshop for over 15 years), don’t worry. I’ll give you an overview of how to use it before we plunge into the changes that were made in CS3. If you’re already comfortable using Curves, you might want to skip the next section and get right into the new stuff starting on page 67.

Introducing Curves

I consider Curves to be the most powerful and versatile adjustment in all of Photoshop. Many of those other adjustments (like Brightness/Contrast, Levels, and Color Balance) are actually using Curves behind the scenes while trying to present you with a more user-friendly interface. The problem with that is that anytime Adobe tries to simplify the interface (by turning it into a dialog box with a few sliders, like Levels), they always end up with a tool that has nowhere near the versatility of the real thing. Knowing how to use Curves can help raise the quality of your images to a level that is far beyond what the simpler adjustments can do.

Curves is on the list of features that many people are afraid of (Channels, the Pen tool and Displacement Maps come to mind). Curves is on the list because it is not easy to learn on your own and it’s easy to screw up your image if you don’t know what you’re doing. Don’t let that hold you back. I’ll walk you through the basics so you can get started using them right away with confidence that you won’t hurt your images. So if you’ve been taking the easy way out with those inferior adjustment tools, there has never been a better time to leave them behind and throw yourself whole-heartedly into Curves.

The Basics

When you open the Curves dialog box, you’ll be presented with a grid that contains a diagonal line. The straight line indicates that no adjustment has been made yet (it would be curved if an adjustment were being applied). The gradient at the bottom of the grid represents all the brightness levels in your image, and the diagonal line indicates how much light or ink would be needed to create the shades in the gradient.

Just look at the graph above where Photoshop is using light to create the image (I’ll show you how to switch from light to ink in a moment). The line is all the way at the bottom above black to indicate that no light is needed to produce black. The line is all the way to the top above white to indicate that you’d need to use as much light as possible to create white. If you look at the position of the line above any of the shades in the gradient, you’ll see exactly how much light would be needed to create them.

You can switch between working with light and ink by clicking on the Curve Display Options icon at the bottom of the Curves dialog box and then switching between the Light and Pigment/Ink settings. All that does is switch the orientation of the gradient, which causes the gradient to reflect what you’d get if you used the amount of ink represented by the diagonal line (no ink to create white and 100% ink to create black; it’s the exact opposite of how light works). Using light or ink is a personal choice and does not necessarily reflect how you plan to reproduce your image. I find most photographers are more comfortable thinking about light, while graphic designers and prepress professionals prefer thinking about ink. I’m going to use the Light setting throughout this explanation of Curves because that’s the setting I personally prefer.

Height Determines Brightness

If you click within your image while the Curves dialog box is open, you will see a circle appear on the diagonal line which will indicate how much light is being used to create that area in your image. You can also look at the part of the horizontal gradient directly below the circle to see how bright the area is that you are clicking on. If you want to adjust the brightness of an area, hold Command (Mac), or Ctrl (Win) and click within your image to add a point to the curve. After adding a point, you can move the point vertically to change how much light is used in that area of your image. If you move it up, you’ll add light to an area and therefore brighten it. If you move the point down, you’ll decrease the amount of light in the area and therefore darken it.

When adjusting the brightness, try to (at first) just concentrate on the area that you clicked on and largely ignore what’s happening to the rest of the image until the area you targeted becomes as bright as you desire. Then notice the rest of the image. Moving a point up or down will affect the majority of the curve and therefore change the brightness of the majority of your image, so it’s not unlikely that other areas could change in undesirable ways. To correct for that you can Command-click (Mac), or Ctrl-click (Win) on those areas—which adds points—and use those new points to move the curve up or down to control the brightness of those areas.

Here’s an example of how I used Curves to darken part of an image: I started by Command-clicking (Mac), or Ctrl-clicking (Win) on the bright area near the top of the image. I then moved the point that was added to the curve down until I liked the brightness of that area. But, after making that simple adjustment, I noticed that the darker areas were too dark. To fix that problem, I Command-clicked (Mac), or Ctrl-clicked (Win) on a dark area and moved the curve up to add light and brighten the area.

If you want to brighten or darken an isolated area, you may need to add additional points to the curve and adjust their locations to keep the majority of the curve in its original position while only allowing a small area of the curve to move up or down.

On the left edge of the Curves grid is a vertical gradient that shows you how bright an area will become if you move a point to a particular height. For instance, if you move a point all the way to the bottom you’ll make an area black, while moving it all the way to the top will produce white, etc. I usually ignore this gradient and simply watch the image to see when it becomes the desired brightness.

Angle Determines Contrast & Detail

The angle of the curve will determine how much contrast you end up with in an area. Making the curve steeper (more toward a vertical line) will produce a greater difference between light and dark therefore increasing contrast and exaggerating detail. Making the curve flatter (more toward a horizontal line) will make the brightness levels in an area more similar, which will decrease contrast and make it more difficult to see detail.

To change the contrast of an area, start by clicking and dragging across the area while watching the circle move across the curve. Note the area of the curve that the circle runs across and then add a point on each end of that area. Once you’ve done that, you have three choices on how to make the curve steeper or flatter. When increasing contrast:

  1. Move the upper dot further up to brighten the brightest part of the area you dragged across while leaving the darkest area unchanged.
  2. Move the lower dot further down to darken the darkest area while keeping the bright area unchanged.
  3. Move the upper dot up and the lower dot down to brighten the bright park and darken the dark part.

When decreasing contrast, you have the following three choices:

  1. Move the upper point down to make the bright areas more similar to the dark ones (while not changing the brightness of the dark areas).
  2. Move the lower point up to make the dark areas more similar to the bright ones (while not changing the brightness of the bright areas).
  3. Move the upper point down and the lower point up to both darken the bright areas and brighten the dark areas.

Here’s an example of adding contrast to an image: I started by dragging across the flat area on the left of the image to see where the circle ran in the Curves dialog box. I then added a point on each side of that area and moved one point up and the other one down (option 3 from the list for adding contrast) to boost the contrast and make more detail pop out.

After doing that, I noticed the triangular area to the right became more colorful and I wanted it to remain unchanged. I clicked and dragged across that area, added two points in the general proximity of that area and adjusted them until the curve went back to its original position, which brought that area back to normal.

Two Things to Look Out For

After you’ve adjusted the brightness or contrast of an area, look over the curve and scan for potential problems. There are two common situations that will cause your image to look terrible:

Flat Equals No Detail When adjusting the brightness or contrast of an image, you might inadvertently cause part of the curve to become perfectly flat. That will make the part of your image that used to have detail to appear as a solid color with zero detail. This is most common when the curve ends up either bottoming out or topping out. When that happens, click in the middle of the flat part of the curve to add a point, then drag the newly added point toward the point that is farthest away (of the two dots that are adjacent to the one you just added). Then move the point around in a little circle while you watch the overall shape of the curve. The idea is to find the position that prevents the flat spot while producing a smooth looking curve.

Avoid Downhill At All Costs After adjusting an image, mentally trace the resulting curve starting from the lower left corner and moving toward the right. Ideally, the curve should always go “uphill” as you move toward the right. If any part of the curve goes downhill instead of up, you’re going to have problems on your hands. A downhill curve will cause part of the image to be inverted (dark areas becoming bright while formerly bright areas become dark). When this happens, try to be less aggressive with the adjustment you are performing to see if it will prevent the downhill issue. I guarantee you that every time you let the curve go downhill you will not like the look of the image.

Channels for Color Shifts

Behind the scenes, a color image is made out of three or four colors which reflect the color mode of the image (Red, Green & Blue or Cyan, Magenta, Yellow & Black). The Channels pop-up menu near the top of the Curves dialog box will determine how many of these colors will be affected by your adjustment. Leaving the menu at its default setting will adjust all of the colors an equal amount, which will change the brightness of the image without affecting its color. Choosing one of the colors from the Channels pop-up menu will allow you to control exactly how much of that particular color is used within your image.

Moving the Red curve up will cause the image to become brighter (because you’re using more light) and more reddish (because you’re increasing the amount of red light used to make up the image). Moving the Red curve down will cause the image to become darker and less red. If the image didn’t look overly red to begin with, this will usually cause the image to shift toward cyan since cyan ink absorbs red light and is considered the opposite of red—just as green light is absorbed by magenta ink and blue light is absorbed by yellow ink. If you have trouble remembering which color of ink absorbs each color of light, either move each color curve wildly to see how your image shifts, or open the Info palette by choosing Window>Info. This handy palette will show the colors of light on the left side and the colors of ink that absorb them on the right.

Adjusting the individual color channels can be useful when an image has a color cast or when you’d like the image to have a warmer or cooler feeling.

Now that you have an idea of how to think about Curves, let’s start to explore the features that were added in Photoshop CS3. To access most of these features, you’ll have to click on the Curve Display Options icon at the bottom of the Curves dialog box.

Histogram

If you could run your image through an X-ray machine the result would most certainly end up looking like a histogram. And there’s nothing better than a histogram to tell you if your image has any problems with color or tone. CS3 has integrated this wonderful diagnostic tool right into the Curves interface so you can make intelligent decisions while you’re making an adjustment.

Turning on the Histogram checkbox will cause a bar chart to be overlaid on the curve. The chart indicates how prevalent each of the brightness levels are that are found in the horizontal bar at the bottom of the Curves grid. One bar on the chart will always extend all the way to the top of the Curves grid to indicate which shade is the most prevalent, while the other bars show how the less prevalent shades compare to the one that is most prevalent. If no bar is shown above a particular brightness level, then that shade is nowhere to be found in the image. There are three main concepts I use when analyzing the Histogram found in the Curves dialog box:

Analyze Brightness Range

When looking at a Histogram, the width will be an indication of the brightness range present in an image. If the histogram does not cover the full width of the Curves grid, the image you are adjusting does not contain the full brightness range from black to white. A gap on the left end of the histogram indicates that the darkest area of the image is not very dark and contains no blacks. A gap on the right side of the histogram indicates that the brightest area of the image is not close to being white. Many images will look their best if they contain the full brightness range. This can be accomplished by either performing color correction—using the eyedroppers in Curves, or adjusting the new Black and White point sliders—which we’ll discuss later in this chapter.

If the histogram for an unadjusted image extends all the way across the Curves grid, you should inspect the first and last bar on the chart. If either one of those bars are tall, it indicates that your image might be lacking shadow or highlight detail because there is a large area of either solid black (indicated by a spike on the left end of the histogram) or solid white (indicated by a spike on the right). We’ll talk more about lost highlight and shadow detail (often known as clipping) when we talk about the new Clipping Display feature later in this chapter.

Check for Saturation Clipping

The histogram can also be useful when viewing the individual color curves (via the Channels pop-up menu near the top of the Curves dialog box). If a tall spike is visible on either end of the histogram for the individual color channels, areas of your image that contain saturated colors might be losing detail. This is often caused by an over adjustment of the Saturation slider in the Hue/Saturation dialog box. There’s no way to bring back the lost detail while in the Curves dialog box (you’d have to undo the Hue/Saturation adjustment and use a lower setting), but at least you can tell when it’s happening.

Add Contrast Without Compromise

Anytime you increase contrast in one brightness range (by making the curve steeper), you’ll end up decreasing it in another (by making it flatter). Flattening out the curve will make it more difficult to see detail, which can be problematic in many images. The histogram can help you cheat in some cases by allowing you to sacrifice detail in brightness ranges that are not very prevalent in the image, so that you can get away with boosting the contrast in the rest of the image.

When you notice a histogram that contains a low flat area, try adding a dot on both ends of that area on the curve above to mark where the flat part starts and stops. Then move the upper dot down and the lower dot up until they are at approximately the same height to flatten the curve above the flat part of the histogram. This will increase the contrast in the rest of the image (by making the rest of the curve steeper) while sacrificing it in the flat area. In these situations think of it as being able to have your cake (contrast in this case) and eat it too (by not messing up your image). It will only work on those special images that feature a low flat area, so don’t expect it to work on most images.

Intersection Line

Turning on the Intersection Line checkbox will cause a light gray horizontal and vertical line to extend from your cursor when moving a point on the curve. This is to help you see how the point relates to the horizontal and vertical gradients that appear in the Curves dialog box. The horizontal gradient indicates which brightness level you’re adjusting in your image, while the vertical gradient indicates how the brightness level will change as the result of your adjustment. These intersection lines can also be useful when trying to line up a point with a particular area of the histogram (like what we discussed above about sneaky contrast adjustments).

I personally find the intersection line makes the Curves dialog box look too cluttered and complex, so I rarely turn it on.

Curves As A Levels Replacement?

In previous versions of Photoshop, I’d often choose Levels over Curves when adjusting grayscale images or when working on masks. The reason for this was three-fold:

  1. Levels felt simpler with only five sliders to deal with and most of the time only three of them were necessary.
  2. Levels offered a histogram that was not available in Curves and the histogram was useful in determining if an image contained any black or white.
  3. Levels offered a Clipping Display that indicated exactly which areas in an image were becoming solid black or white.

All those features are now available in Photoshop CS3’s updated Curves dialog box, which makes Levels less essential. In fact, I’ve completely stopped using Levels in CS3, and I’m now campaigning for people to give up Levels altogether, and go for Curves instead. If you take some time now to really understand Curves and learn to use it with confidence, I promise you will never regret saying goodbye to Levels. But enough campaigning for now, let’s continue with Black/White Point sliders.

Black/White Point Sliders

The two sliders found near the lower right and lower left corners of the Curves grid are known as the Black and White Point sliders. They replicate the upper left and right sliders that are found in the Levels dialog box.

When you move the black slider toward the right, you’ll force the shade it points to in the gradient to black along with all the shades that are found to the left of the slider. Moving the white slider toward the left will force the shade it points at to white along with all the shades found to the right of the slider. Here’s an example of when you might use them:

If you have an image that does not contain the full brightness range (no blacks or whites as indicated by a histogram that does not extend all the way across the Curves grid), consider using the new Black and White Point sliders to adjust the image. Start by dragging the black triangle that appears in the lower left corner of the Curves grid toward the right until it touches the left edge of the histogram. That will cause the darkest area of the image to become black. Next, move the white slider that appears in the lower right corner of the Curves grid toward the left until it touches the right edge of the histogram. That will cause the brightest area of the image to become white.

Once you’ve done that with both sliders, you should end up with an image that contains the full brightness range available. With that accomplished, you can start to adjust the overall brightness and contrast of the image by manipulating the rest of the curve.

I mainly use the Black and White Point sliders on grayscale images because performing color correction on a color image will usually cause it to contain the full brightness range available, which makes these sliders less than essential on color images. In fact, the eyedroppers in the Curves dialog box (which are used for color correction) use the Black and White Point sliders to do their work. They simply do it on the individual color channels instead of the main curve.

Clipping Display

A clipping display can show you which areas of your image are becoming solid black or solid white as the result of your adjustment. It’s an especially useful little feature that tells you if you’re pushing your adjustment too far. There are three ways to get a clipping display in the updated Curves dialog box:

  1. Hold Option (Mac), or Alt (Win) when moving the Black Point slider to see which areas are becoming solid black.
  2. Hold Option (Mac), or Alt (Win) when moving the White Point slider to see which areas are becoming solid white.
  3. Turn on the Clipping Display checkbox near the bottom of the Curves dialog box and then press Control-Tab to cycle between the black and white clipping displays (or click on the Black and White eyedropper icons in the Curves dialog box).

When using the clipping display, areas that appear in color contain partial detail (also known as saturation clipping) while areas that appear as black (when viewing the black clipping display) or white (when viewing the white clipping display) contain no detail at all.

I prefer my images to contain a small area of solid black and solid white to insure that I’m using the full contrast range available. There are a few exceptions to this rule such as images that contain fog or mist, which might lose their natural -looking quality when adjusted to contain the full brightness range.

I find that light sources and reflections of light sources on very shiny surfaces (like glass, water or polished metal) look best when they are solid white.

Baseline

Turning on the Baseline checkbox will cause Photoshop to overlay a light gray diagonal line over the Curves grid. This line indicates where the curve was positioned before any changes were made and is useful in determining exactly how a curve will affect an image. When working with the Light setting, moving the curve above the baseline will brighten the image, while moving it below the line will darken the image (the opposite is true when using the Pigment/Ink setting). When working on the individual color curves (via the Channels pop-up menu near the top of the Curves dialog box), moving the curve above the baseline will cause the color of the image to shift toward the color indicated in the pop-up menu, while moving the curve below the baseline will move the overall color of the image away from that color and toward its opposite color (we talked about the concept of opposite colors earlier in this chapter).

I always have the Baseline visible because I find it much easier to analyze how a curve is affecting an image if I can compare it to the original, unadjusted image (as represented by the baseline).

Channel Overlays

When you adjust the individual color channels in Curves (via the Channels pop-up menu near the top of the dialog box), you’ll cause the overall color in an image to become warmer or cooler. In previous versions of Photoshop, you had to manually switch between the color curves to see if any color shifts were being applied to the image. But now you can simply turn on the Channel Overlays checkbox, which will cause all of the color curves to be overlaid on top of each other making it easy for you to see if color shifts are being made. You’ll still have to switch to the individual color curves to make adjustments though.

This overlay can be useful when you want to determine what type of color shift a particular Curves adjustment is making. If one of the color curves ends up above the baseline (the gray diagonal line that appears after turning on the Baseline checkbox), then the image will appear brighter (since more light is being used) and will shift toward the color of the curve. If a color curve appears below the baseline, the image will appear darker (since less light is being used) and will shift toward the opposite of the color of the curve. Red light is absorbed by cyan ink (they are opposites), just as green light is absorbed by magenta ink and blue light is absorbed by yellow ink.

Presets

If you find yourself applying the same curve to many images, you might want to consider saving the curve as a preset. After creating the curve you desire (but before clicking the OK button), click the icon that appears just to the right of the Presets pop-up menu at the top of the Curves dialog box and choose Save Preset from the menu that appears. Saving the preset into the default location will cause it to appear in the Presets pop-up menu at the top of the Curves dialog box, making it easy to quickly apply the same adjustment to future images. If you don’t use the default location, you’ll have to click on the icon that appears to the right of the Presets pop-up menu and choose Load Preset to be able to access the preset.

If you plan to simply apply the same curve to two or three images, there is no need to save a preset. Instead, use one of the following techniques:

  1. Use a Curves Adjustment Layer to adjust the first image and after clicking OK, drag the Adjustment Layer from the Layers palette and release your mouse button while it is on top of another open document. This drops a copy of the Adjustment Layer in the other document.
  2. Apply a curve by choosing Image>Adjustments>Curves and then after clicking OK, switch to another document and hold Option (Mac), or Alt (Win) while choosing Image>Adjustments>Curves, which will get Curves to remember the last settings you applied.

Adobe really listened to users when they revamped Curves for CS3. They implemented 99% of the suggestions I made for Curves and I’m happy to say that I have little to complain about. Now, let’s move on and explore the other changes Adobe made to the adjustments available in Photoshop.

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