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DPChallenge Forums >> Tips, Tricks, and Q&A >> Art of the Curve
Showing posts 1 - 11 of 11, (reverse)
07/08/2005 07:11:40 PM · #1
Okay: now we tackle curves. Let me get out the basics so that we can move on. Like levels, curves control tonal values. Unlike levels, curves is able to target a specific tonal value and change its representation. When you open curves you are presented with a square graph which has a diagonal line running from bottom left to top right. You have a bar on the left which gives you info on the output of the operation. The bottom bar represents what you started with. Note that it starts dark and grows lighter along the bar. In the bottom bar you have a division. If you click here you are able to reverse the values of the bar. This is just a useless feauture because what you want is the following: On the left bar you want the dark at the bottom and on the bottom bar you want the dark on the left. This means that when you pull the diagonal line up, the values will lighten. Pull them down and the values darken. The feature to reverse this orientation is more for the perverse who want to pull down to lighten and up to darken. Most professional think, "up light and down dark."

Now, by pressing alternate and clicking anywhere inside the graph you toggle between the amounts of lines you want. I prefer ten because it brings it near to the zone system. Always keep in mind that all adjustments must be made in modest amount because you could easily create a monster. If you click anywhere on the diagonal line you will create a control point. This is a dot. drag it up, down and notice the immediate change in the image. Let us say you want the control point higher or lower on the line, simply drag it up or drag it down. Say you no longer want this control point at all: simple drag it right off the graph and presto it disappears. Let us suppose that you have worked your image and made a mess: simple press alt and reset and you are back to where you started. Now, if you click anywhere on the image you are working on, you will see a small control appear on the diagonal line. This is a neat feauture and one that we will use in a real example. In the meantime, if you see a trouble area in your image and suppose you find this small area too dark or too light. Then all you need do is click on the image area that bothers you, observe where it falls on the diagonal. Go click on the approximate area to establish a control point. Of course, we need another step because if you were to drag this point up or down the entire diagonal will shift like a snake and affect all tonal values in ways that you do not want. First you need to nail down what is right with your image. This is done by simply clicking control points from top to bottom. More like pinning down the original line. How many? it all depends on how much correction is needed. First put the first control point in the center. This insures that the midtones do not change. Go two squares down and place another. And another near the bottom. Do the same on the upper part. What you done here is to secure the integrity of your image as you started. Now you are free to target the area you want and moving the control point to lighten or darken the area in question. Please understand that what you are in control is not only the specific portion you selected from the image but all values that are similar in your image. This means open eyes since you do not want to correct something and mess something else up. Also, like I said, be gentle. If you horse the line, the entire diagonal grows jittery much like a snake and look at the havoc it causes on your image. Go easy.

You should not be reading this if you are still struggling with levels. First master levels because levels should always be your first adjustment. In levels you see that lovely helpful graphical representation of the values in your image. In curves, you do not.

Before we proceed let me show you a neat way to tint an image. Take the image and convert to grayscale and then to RGB again. Do whatever it needs but do not sharpen. Now open curves and open the channels. A simple drag of each of the channels will bring about the tint you want. Simply place a control point in the middle and drag up or down, go to the next color and do the same. You can have any tint you want. Just a little practice is needed but this is learned quickly.

To show you the power of curves I am going to use my image, "Miracle in basic Editing." Without curves this image could have never won the ribbon. The image was shot with my crude double exposure box. Look at the image and you can not help observe the diagonal line. Certainly a job for advance editing but curves will bail us out in basic editing.

Image one is what we are starting with. First we do a levels and our concern is the actual subject. We make sure we get as much good values in the wine, water, glass and bottle. This will have a tendency to worsen the ugly dividing line. Now that we have secured the integrity of the image we use curves. It is important to realize that not all corrections can be rendered in one application. We adjust, then we flatten, then we adjust again. There is no way to fix this image in one step. I will not even bother to go through all the steps but show you enough to convince you.

To simplify we are going to convert the image to grayscale. This will save us explanations that you can easily reconstruct on your own. First, I click on the value I want to retain as the background. You can choose the dark or you can choose the light. We note where it falls on the diagonal line of the graph. We click there. Now we pin the areas that we do not want movement in. Then we drag the target point either up or down, depending on your original choice. Like I said, we must go slowly and not too far from the point of origin. We then press okay and flatten. We open curves again and make another similar adjustment and then click okay and flatten and then do it again, until you are satisfied.

If you recall earlier I said that when you adjust one specific set of tonal values you affect all such values. This is evident in this image because it robs or changes these same values wherever they happen to be. Being such a popular value it will eat into areas where you do not want. Hence, always, eyes wide open and attention to what you are doing. And be ready for compromise but be persistent as magic can be done if you are patient.

There is one more outstanding feauture of curves which many professionals use constantly. It is the very last adjustment before sharpenning. It is refered to as the "S" curve adjustment. It livens the middle tones with a richer contrast. Here is a simple procedure: Open curves. Make sure you have the graph with the ten lines (we need ten only for reference of illustration) click on the center. This creates a pivotal point while securing dead center. Now click two squares from the bottom and click another two squares from the top. Now you are all set. I am assuming your lights are at the top and darks on the bottom as we explained earlier. GENTLY pull top top control point up a nudge and then drag the bottom a nudge down. Look at your image and find its most attrractive setting. You are finish. Now, in the event that your histrogram is crashing on the right with too much white: you can reverse the S curve. This will tone down the ultra light tones in your image and move the back end of darker tones to a lighter setting to match the overall balance.

Well, go and play with it. each houe you invest will bring you back untold control in making a good image better and in saving some images such as I did with my crappy wine to water image. Good luck and study. Read up on the subject.

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The first image is the original. The 2nd after levels. The 3rd after the first curves adjustment. 4th the second adjustment. Here we merely click on the dark line on the left then pinned down around it and move this area up: the last is the image almost there.

Od course, whenever you are working any image you should always save the effort such as imageone, two and so on. In case you get in trouble with subsequent edits you can revert back to an earlier version.

Message edited by author 2005-07-09 00:06:58.
07/08/2005 08:08:39 PM · #2
Thank you for sharing Dan :)

07/08/2005 08:23:56 PM · #3
Yea thanks for that.I've been using curves like I swim - sort of synchronised drowning. Now I have something to guide me, and I really appreciate the time and effort you took to do it;)
07/08/2005 08:27:40 PM · #4
Seconding what amber said! Thanks Dan, I'll print this out along with your LEVELS tutorial. Very, very much appreciated.
07/08/2005 09:38:44 PM · #5
Thanks Dan! I stopped abusing my images with curves after your Levels post. That helped me immensely. Can't wait to work on curves. Sure appreciate the time you've taken to enlighten us.
07/08/2005 09:56:36 PM · #6
just marking the thread to read it later, thanks for the story book of info.
07/08/2005 11:12:10 PM · #7
Originally posted by graphicfunk:

This is just a useless feauture because what you want is the following: On the left bar you want the dark at the bottom and on the bottom bar you want the dark on the left. This means that when you pull the diagonal line up, the values will lighten. Pull them down and the values darken. The feature to reverse this orientation is more for the perverse who want to pull down to lighten and up to darken. Most professional think, "up light and down dark."

I really appreciate the writeup, but I have to disagree a bit with this one little part. A lot of people, including myself to look at a graph this shape with the 0,0 origin in the lower-left, and positive numbers in the upper-right.

Maybe because that's because I always work with either grayscale values (0-100% black) or CMYK values (0-100% of each color) in the Info Window -- RGB values go the opposite way: from 0 (black) to 255 (white), and are really confusing. It's much easier to think of a "percentage" of a tone than 1/255th of a tone ...

I have some examples of what can be done with Curves in this pBase Gallery including some screen shots, which have the zero-point in the lower-left.

I totally agree that they are your most important basic tonal control, and personally use almost nothing else.

07/08/2005 11:17:59 PM · #8
Nice write-up. Now how about buy me Photoshop so I can try it. <g>
07/08/2005 11:27:32 PM · #9
Well done Dan. I use curves all the time and I learned quite a bit by your example. Should be written up into a tutorial with images embedded at the appropriate places in the text.
07/08/2005 11:27:47 PM · #10
Try Curvemeister. $30 I think. Plugs in to Elements, which comes free with many cameras.

07/08/2005 11:33:35 PM · #11
Very good post - wish I had read this awhile ago as opposed to taking me forever to finally get down a skim a book on Photoshop :P
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