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The Curves Tool

The Curves Tool

One of the most powerful retouching tool in Photoshop or Gimp is called Curves. In this chapter I'm going to describe how to use the tool with Black and White images but all the concepts here explained can be easily extended to color images. Assume we have a Black and White image where pure Black is represented with a value of 0 and pure White with a value of 100. All numbers between 0 and 100 represent different scales of Grey.

In digital retouching all tones close to black are called Shadows, the ones closed to white are called Highlights and the one between are called Middle Tones. In a Black and White image the Curves Tool operates an indiscriminate transformation of tone per every single pixel in the image.

Let's describe how the Curves Tool is working using the following diagram. On the x-axis we represent the brightness of a pixel in the original picture. On the left side with have the Shadows, in the middle the Middle Tones and on the right side the Highlights. We do not know from the diagram which pixel has a given tone because the tools works indiscriminately on all the pixels having the specified tone.

On the y-axis we display the tones of the resulting images. On the bottom the Shadows, in the middle the Middle Tones and on the top the Highlights.

In the diagram, the pixels tone transformation is represented by a red curve. The change of a pixel tone into another is obtained in the following way. Keep for example all the pixels having tone X0 (a Middle Tone) as displayed in the diagram. Draw a vertical line passing throw X0 and intersecting the curve on X1. Then draw an horizontal line passing throw X1 and intersecting the y-axis on X2. This is the resulting pixel tone and it means that all pixels, in the original Black and White image, having tone equal to X0 are now mapped into tone X2.

As you can see from the diagram, tone X2 is closer to 0 than tone X1. Pixels having tone equal to 0 (black) are mapped to the same tone, while pixels having tone equal to 100 (white) are mapped to the same again.

The transformation reduces the brightness of all Middle Tones while keeping Shadow and Highlights almost the same.

The result of the above transformation is shown in the following pictures: on the left side we have the original Black and White picture while on the right side we have the image after applying the curves transformation. We can immediately notice that the resulting image is darker than the original one.

In the following example I'm showing a different Curves transformation which effect is increasing the Middle Tones of the images. It can be though as the opposite of the previous transformation.

In the left side the original image and on the right side the images with the Curves trasformation applied. The resulting image is clearly lighter than the original one.

The following tone trasformation is a little bit trickier. This time Middle tones remain almost the same while Highlights are increased and Shadow are decreased.

The result is an enhancement of the image contrast as shown by the following pictures.

As a conclusion I want to show the identity transformation that maps an image into itself without altering the original tones.

In the following chapters I'm going to explain how all Blending Modes can be explains in terms of Curves Tools with a major important difference. While the Curves Tool works on all the pixels of the source image without discrimination, Blending Modes can work on specific image frequencies.

The concept of image frequency will be explained later but the key idea is that we will be able to use Blending Modes and the concept of Curves Tool e.g. to remove vignetting, sharpen the image, painting with light over an image, add contrast without altering the sharpness and so on.