4.5 Color correction
Color correction is the process of changing the colors in an image to be more accurate or to be more subjectively pleasing. Typical color correction is used to correct a color cast across an image that is causing grey colors to not appear grey. You can also change colors to help convey a specific mood.
All of the nodes in this section are located under “Color” on the Nodes Toolbar.
Before you start, it’s instructive to review some properties of working with color. When a camera captures light to make a photograph or when you view an image on a computer’s display, colors are produced by adding select primary colors. This is called “Additive Color” mixing and the primary colors are almost always red, green, and blue. Secondary colors are produced by mixing combinations of primary colors. Adding equal parts of each primary color produces a grey color.
When you correct the color in an image, you’ll usually add the opposite color to shift toward a neutral grey. By adding a color to an image, by definition of “Additive Color” mixing, you will have made the image brighter. This means you might have make further changes to your tone correction nodes so your image doesn’t look over exposed. You should still perform tone correction first because it’s common for color issues to disappear after tone correction.
The Levels node can be used for more than changing the tones of your entire image, it can also be used to change the tones of each channel individually. Left click the “Channel” combo box and left click the channel you want to adjust in the pop-up menu. The fields will change to reflect the selected channel. You don’t need to add a separate node for each channel you want to adjust. The RGB, Red, Green, and Blue channels are all processed together.
To correct a color cast in an image, you will want to adjust the Gamma field in one or more of the channels. Use the color wheel as a guide to figure out which primaries need to be adjusted. Adding the color opposite of the color cast in your image will make your image appear more grey. For example, to remove a blueish color cast, you want to add some yellow. To add some yellow, select the “Blue” channel and lower the Gamma.
Adding your own color cast to an image is done the same way. Adjust the Gamma field for individual channels. You should correct any existing color cast first or else your image will probably look flat and washed out.
Like the Levels node, the Curves node can also be used for color correction. Left click the “Channel” combo box and left click the channel you want to adjust in the pop-up menu. This changes the curve you will be able to interact with. Also like the Levels node, you don’t need to add a separate node for each channel because the RGB, Red, Green, and Blue channels can be adjusted individually with one Curves node.
Using the Curves node to do color correction gives you all of the same power as when you did tone correction in the previous section. A particularly useful application is to change a specific tonal range while leaving other parts unchanged. To do this, add a couple knots along the diagonal to surround the tonal range you want to change. Then add a knot in the middle that can be raised above or below the diagonal to change the color. The two knots along the diagonal “pin” the center knot so its changes are restricted to the specified range. Finding where to place the knots might require experimentation.
White Balance Node
The easiest way to color correct an image is to use the White Balance node. It’s designed to quickly make grey objects appear grey, regardless of lighting. This works great for many photos but for more creative control, you’ll want to use one of the other nodes in this section.
The White Balance node only has two fields which helps to make it simple to use.
- The temperature in kelvin of the lighting the photograph was shot under. This can be a little unintuitive at first because “warm” looking temperatures actually have a lower temperature value and “cold” looking temperatures have a higher temperature value.
- Green or magenta color cast to apply. Recall from our review on color earlier that you can apply green to a magenta image to make it appear more neutral and vice versa.
Both of these fields often require subtle adjustments. You can hold down the Shift key while dragging the slider to make more precise changes.
Upon selection of a White Balance node, you’ll find a White Balance tool on the Tools Toolbar that can be used to select a grey spot in your image. The node will then use this region to automatically adjust the Temperature and Tint fields. The White Balance tool can be left clicked on the Canvas to select a specific pixel or left clicked and dragged to select a rectangular region. You should try selecting a mid-grey area when correcting a color cast. Ideally, you’ll have taken an image with a grey card designed for this purpose. You can also select a non-grey area for an inaccurate but creative color change.
HSL Channels Node
The HSL Channels node adjusts an image using the HSL (Hue, Saturation, and Lightness) color model. The HSL color model is an intuitive way for working with colors compared to RGB. Instead of balancing red, green, and blue adjustments to make colors appear stronger but not lighter, in the HSL model, you can just adjust the saturation.
- The hue is mapped to a 360 degree spectrum. Changing the hue essentially finds where the current color is in the spectrum and then rotates it by the specified amount.
- Increase or decrease the “colorfulness” of colors.
- Increase or decrease the brightness of colors relative to white.
In practice, the HSL Channels node is primarily used for changing the saturation in an image to give it more impact.