Although Photoshop provides numerous color adjustment tools, the most basic and commonly used are actually curves. Others, such as brightness/contrast, etc., are derived from this. Curves are the most commonly used adjustment tools in Photoshop. Understanding curves can help you access many other color adjustment commands.
Open the photo below in Photoshop. You can convert it to grayscale to see the distribution of brightness (you can cancel the conversion operation with [CTRL+Z] after reading it), as shown in the second picture below. It can be seen that the nearby mountains belong to the dark tone area, the sky belongs to the high light area, and the distant mountains belong to the mid-tone area.
Now use the curve command [Image>Adjustment>Curve] [CTRL M], and you will see the settings box as shown on the left, with a line segment at 45°, which is the so-called curve. Note that there is a channel option at the top. Now we select the default RGB.
We often see tone adjustment buttons in music playback software or on the combination audio palette. As shown below on the right. Moving up is strengthening, moving down is weakening. The sound is divided into treble, midrange and bass. Generally, the left end controls the bass part and the right end controls the treble part.
We can use the same thinking to look at curves. Photoshop expresses the dark tones, midtones and highlights of the image through this line segment. As shown in the picture below, the endpoint in the lower left corner of the line represents the dark tone, the endpoint in the upper right corner represents the highlight, and the transition in the middle represents the midtone.
Notice the two gradient bars from black to white to the left and below. The gradient bar at the bottom represents the range of absolute brightness, with all pixels distributed between 0 and 255. The double-headed arrow in the middle of the gradient bar inverts the highlights and shadows of the curve. To maintain consistency we use the default gradient bar of black on the left and white on the right.
The gradient bar on the left represents the direction of change. For a certain point on the line segment, moving up is to brighten, and moving down is to darken. The limit for brightening is 255 and the limit for darkening is 0. Therefore its range also belongs to absolute brightness.
Note that the "Preview" option in the lower right corner of the curve setting box needs to be checked. Then click in the middle of the line segment, a control point will be generated (red arrow in the left picture below), and then drag upward to the position in the left picture below. You will see the image become brighter, as shown on the right below. You can compare the effect before and after the adjustment by checking or unchecking "Preview". There is an underlined letter P to the right of "Preview", which is the shortcut key. Correspondingly, there are also the loading shortcut key L and the storage shortcut key S. Whenever such underlined letters appear, you can press the ALT key on the keyboard to turn a certain function on or off. However, they are only effective when the Curve Settings box appears. It is not a global shortcut key. If you want to delete a control point that has been created, drag it outside the curve area, just like deleting a guide line.
So why does this change make it brighter?
Let's analyze it. As shown in the figure below, suppose there are three points abc on the line segment. Combining the previous knowledge, we can know that a is the darker part of the image, c is the brighter part, and b is located in the middle of the two. After adjustment, they all moved a certain distance above the Y-axis. Since moving up is equivalent to brightening, the summary is: brighten the darker parts, brighten the middle parts, and brighten the brighter parts. The image certainly looks brighter.
Don’t settle for less, take a closer look. The moving distances of the three points abc in the Y-axis direction are different, b is farther while a and c are closer. what does that mean? It means that the three points are highlighted to different degrees.
Expanding the idea, b located in the mid-tone has the largest increase, while a near the dark tone and c near the highlight have a relatively smaller increase. This means: the darker or lighter the part in the original image, the smaller the amount of lightening.
The points at either end of the curve do not move, which means that if there are areas in the original image that are pure black or white, they are not highlighted. But I can definitely say that there are no pure black and pure white pixels in this image. As for why I know this, I will explain it later.
In order to verify the above theory, we put the two images together before and after the change, and determined the highlight area, dark tone area and mid-tone area, as shown below. 12 is highlights, 34 is midtones, and 56 is shadows. Switch the information palette 〖F8〗 to HSB mode, and use the mouse to compare the brightness values of 12, 34, and 56, which is the value of B. See what changes.
