Learn Phoduit

4.4 Tone correction

Tone correction is the process of changing the range of dark and bright tones in an image. It’s often used to correct photographs that were shot with a poor exposure or to add more contrast. There are two nodes commonly used for making tone corrections. First is the Levels node which provides a simple way to change the black level, white level, and grays of an entire image. The other node is the Curves node which lets you precisely change select parts of your image’s tonal range.

Levels Node

The Levels node takes a range of tonal values and then stretches or compresses that range. It’s located under “Color” in the Nodes Toolbar. It only takes one image as input and the corrected image is produced as a single output so it’s easy to use.

Channel
The channels to operate on. By default, it’s set to “RGB” so it works on red, green, and blue equally but you can change this to make tone corrections to a specific channel.
Input
The input tonal range. Set the left side to the black point and the right side to the white point. Tones outside of the selected range are clipped to full black or full white respectively.
Gamma
How much to darken or brighten all of the mid tones in the input tonal range. Set to 1.0 to leave unchanged.
Output
The output range the input tones will be stretched or compressed to fit. This is useful when working with masks but it will make a typical photograph appear hazy.

A histogram of the input image is displayed inside of the node. While it’s up to you to decide how your image should look, the histogram is a useful guide that you should consider using as a starting point. Line up the Input field with the inner parts of the gaps on the far left and right sides of the Histogram to maximize contrast while minimizing clipping. From there, it’s a trade off between adding contrast at the cost of further tonal clipping. Some tonal clipping is often desired to make your image appear sharper.

Some images will display breaks in their histogram after being modified. This is a sign that there isn’t enough information in the input image for an adjustment. It can lead to ugly banding where parts of your images appear flat. The only effective way to work around this is to use a higher quality input image like a raw image. If you intend to print your image, keep in mind that minor banding is usually not noticeable with most printing methods.

Curves Node

The Curves node is similar to the Levels node but it gives you more control so you can make adjustments to specific tones. The gradient along the bottom represents the tones of the input image and the gradient along the left represents the tones of the output image. You draw a curve to define how the input tones are mapped to the output tones.

The Curves node is located under Color in the Nodes Toolbar. Like the Levels node, it only takes one image as input and the corrected image is produced as a single output.

Channel
The channels to operate on. By default, it’s set to “RGB” so it works on red, green, and blue equally but you can change this to make tone corrections to a specific channel.
Curves
The curves define how the input tones are mapped to the output tones. Knots are hollow or solid circles that are added to the curve to change its shape.

The initial “curve” has one knot in the bottom left and one in the top right with a straight 45 degree line between them. This means all input tones are mapped to the same output tones – thus are left unchanged.

Left click anywhere in the dark gridded area to add a knot. Knots that have just been added are solid circles to show that they are selected. You can select a different knot by left clicking it.

You can reposition a knot by left clicking and dragging it. Moving a knot past another knot to its left or right will delete the knot being moved. You can also delete a knot by selecting it and then pressing the Delete key on your keyboard. The left most and right most knots cannot be deleted.

Adjusting curves can be tricky at first but it does get easier with practice. A very useful curve to start with is a subtle “S” shape which adds contrast throughout an image. To create an “S”-shaped curve, add one knot slightly below the diagonal about 1/3rd from the left and another knot slightly above the diagonal about 1/3rd from the right. You’ll notice the darker tones will get darker and the lighter tones will get lighter. You can experiment by moving these knots vertically to change the amount of contrast applied to your image.