The curves tool is an interesting piece of kit, and it does seem a little daunting when you first approach it, however if you understand what exactly you are adjusting and what the general effect of the tool is you should have no trouble at all approaching it. Of course overuse can ruin an image, but when used appropriately the Curves tool acts as an intuitive method of controlling the exposure and colour of an image that can correct colour casts, restore detail and even change the mood of an image if used well. Here I'll use an image of my birthday cake from a few years ago to demonstrate how you can improve the exposure of an image without losing much (if any) detail.
Step 1. Open your image and then open the Curves tool. In Paint Shop Pro there are two ways to do this; either through Adjust, Brightness, Contrast, Curves or through right-clicking a layer and opening Adjustment Layers, Curves. I'm using the latter method as it allows me to easily alter the exposure as I like by double-clicking the adjustment layer, whereas otherwise I'd have to open the window every time I wished to change anything.
Step 2. There are some automatic configurations available along the right side of the window but I'll leave you to try those out for yourself. It's best to learn to do it yourself to get the best results so I'll focus on teaching you that for now. Before we change anything, we need to understand what exactly we will be changing. The histogram we are presented with has an X and a Y axis with a black and white gradient running alongside it, the horizontal (x) axis shows us the tone at each point in the original photo, the vertical (y) axis refers to the tone that each point is actually being output and displayed on screen/in print at. So if we take a point exactly in the middle of the image and move it straight up we will be increasing the output brightness of the middle tone of the image, and if we move it across we will be making it so that a brighter tone in the image will be represented as the middle tone on screen.
This screenshot shows the effect of vertical and horizontal movement on the central point of the graph; moving the point up brightens the image and moving it down will darken it, as it will make the central point represent itself on screen as brighter or darker, with every tone above it brighter and below it darker. Moving it down, therefore, will maintain the point we selected as the middle tone, but by moving it downwards we have given all the tones darker than it less shades to be represented by thus reducing the contrast in the darker tones.
Step 3. With this in mind it is possible to use the Curves tool to recreate the effect of moving the arrows in the Levels tool. All we have to do to ensure that our image is displaying the full dynamic range is move the top and bottom points horizontally so as to make sure the maximum and minimum output values remain in use. Move the points so that they are in line with the start and the end of the histogram in the background of the chart now to see how this increases the range of tones in the image. In Paint Shop Pro you will see 2 numbers in the top left as you change the position of the points; the left figure represents the input value and the output value is on the right. So in mine I moved the left point to 11 > 0 meaning that values of 11 or below in the input range will be shown as tone 0 (black) and my right point is now 208 < 255 so that values of 208 or above will display as tone 255 which is the white point.
Step 4. Where the Curves tool differs from the Levels tool is that you can add and move as many points as you like, which allows you to create a variety of shapes and customisable exposures as a result. The two most common types of exposure created by the Curves tool are the S and Inverted S. Here I'll demonstrate the difference; all you have to do to add a point is left click anywhere along the line, you can then drag it and try out these exposures for yourself. Notice that the S curve increases the number of output tones for a narrow band of the mid-tones, while the Inverted S curve does the opposite. Taking a large number of mid-tones and forcing them to be displayed in a fewer number of output tones, thus reducing contrast.
Step 5. So if we think of the central point of the graph as representing the mid-tones, and the apex of each corner heading along the same diagonal as the default line that opens in the Curves tool as the Highlights and Shadows respectively we can see how we can start to use the tool to increase the contrast of certain areas of the image using the Curves tool. The only downside is that if you push this tool too far you will end up getting artefacts in your image, so for every area that receives a boost in its output range another must lose some dynamics. Here I've coloured each point to show the input tone it is currently configuring to demonstrate how each point can correspond to different parts of the image; here it progresses from pure black to shadows to mid-tones to highlights and finally pure white.
Step 6. Looking at my image it is clear that there is a lack of contrast; there are no real highlights to speak of and the entire image appears washed out and grey, so an S curve seems the way to go. This will increase the number of input tones that will become highlights as well as adding some extra depth by doing the same for the shadows. When I apply a gentle S curve (remember that you can't push this tool too far without creating artefacts) you can see that the dog's nose is now darker, and the white cake and tablecloth are now actually white in some areas. Also notice how the pattern on the tablecloth becomes more clearly defined and the image as a whole looks better. Here you can see the curve I applied as well as the before and after effects showing the increased brightness of the cake and definition in the tablecloth.
Step 7. As I said earlier, the Curves tool can also be used to correct colour cast or even change the mood and appearance of an image. My image doesn't have a colour cast, but the image has a generally cold feeling due to the amount of blue and white in it, yet if you look in the top left of it you can see a faint orange hue. This is because the restaurant we were eating in has orange lighting; it's a cosy little Italian place, so I'm going to try reintroducing the warmth that the flash removed using the curves tool. In the Curves window, next to Channel, where it says RGB open the drop-down menu and select the colour channel you wish to increase; to make the image warmer I'm adding red and removing some blue so open the red first if you're doing this.
Step 8. Now place a tag on the central point and drag it slightly up to increase the curve, this will stretch the number of red input shades and increase contrast in them whilst removing some contrast from the vivid reds, which luckily there are pretty much none of in my image. Your image should change hue, and will most likely not look its best right now, but don't worry this will soon be fixed.
Step 9. Because we have increased the contrast within the red channel we need to reduce another channel in the same area, so head over to the Blue channel, which is the one responsible for the coldness in the image originally. Move the central point downwards slightly, as always less is more here. You will probably have to switch between the two channels a few times to remove any noticeable colour bias but once you have fixed it your image should have become warmer.
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Step 10. When you have finished making all your adjustments press OK to apply your Curves settings to the image. Remember, just like other adjustment layers you can still paint areas within it to mask them against the changes that are being made, and double-clicking the layer will allow you to once again play with the settings until you are happy. That's the basics of the Curves tool covered, see what you can do with it and you might figure out some neat little tricks to bring out the best in some of your images. Here are my final blue and red curves (as you can see tiny changes can bring about great change) as well as a comparison shot of before and after the curves were applied to my image so you can judge for yourself just how useful it can be.