PS Curve adjustment Easy to explain: the first curve

In Photoshop, open the following photo. It can be converted to grayscale to see the distribution of brightness (after reading can undo the conversion operation).

After adjusting the following figure. Can see near the mountain belongs to darkened area, the sky belongs to the high light area, the distant mountain belongs to the middle tune.

Now we use the curve command "image > Adjust > Curve" 〖ctrl_m〗, we will see the following diagram setting box, which has a 45° line segment, which is called the curve. Note that there is a channel at the top of the option, now we first select the default RGB.

We often see the tone adjustment button in the music player software or the Zuheyinxiang palette. The following figure. Move up to enhance, down is weakened. The voice has a treble, alto, bass points. General left-side control bass, right side control treble part.

We can use the same thinking to look at the curve, Photoshop, the dark tone of the image to mediate the high light through this line segment to express. In the following figure, the end of the lower-left corner of the segment represents the darkened, the upper-right endpoint represents the highlight, and the middle transition represents the middle tone.
Note that there are two gradient strips from black to white on the left and below. The gradient bar below represents the range of absolute brightness, and all pixels are distributed between these 0 and 255. The two-headed arrow in the middle of the gradient bar acts as a high light and dark tone of the inverted curve. To maintain consistency we use the gradient bar of the default left-right white in the diagram.
The gradient bar on the left represents the direction of change, and for a particular point on the line, moving up is highlighted, and moving down is dimming. The highlight limit is 255, and the dimming limit is 0. Therefore its range also belongs to the absolute brightness.

Note the preview option in the lower-right corner of the curve setup box needs to be checked. Then click in the middle of the line to produce a control point, and then drag up the position of the left figure below. You'll see the image brighten, as shown on the right. You can compare the effect before and after the adjustment by checking or canceling the preview. The right side of the preview has an underlined letter p, which is the shortcut key. There are also load shortcuts L and storage shortcuts s, and so on. Whenever this type of underlined letter appears, you can turn on or off a feature by pressing the appropriate key on the keyboard. However, they are only valid when the Curve Setup box appears. is not a shortcut key for the global. Shortcut keys for the global are generally F4, F5, or letter keys combined with Ctrl/alt/shift.
So why is this change brightening?

　

Let's analyze it. The following figure assumes that there is an ABC three point on the line segment, combining the preceding knowledge to know that a is the darker part of the image, that C is brighter, and b is in between. After adjusting, they moved a distance above the y-axis. Because moving up is equivalent to brightening, so the combination is: the darker part is highlighted, the middle part is highlighted, the brighter part highlights. The image certainly looks brighter.

Don't be content with it, take a closer look. The distance of the ABC three points in the y-axis direction is different, B is farther and a and C is nearer. What does that mean? It means that the three points are different in brightness.
By extending the idea, the b increase in the middle is the largest, while the near-darkened a and c near the high light are relatively small. This means: the darker or brighter part of the original artwork, the smaller the brightness.
The points at both ends of the curve do not move, which means that if some of the original artwork is black or white, they are not highlighted. But I can tell you with certainty that there are no black and white pixels in the sample image. As to why I know, I will explain later.

To verify the above theory, we put together the two images before and after the change, and set the high light area, darkened region and midtones area, as shown below. 12 is the high light, 34 for the middle, 56 for the dark tone. The Information palette 〖f8〗 switch to HSB mode, using the mouse to compare the brightness of 12, 34, 56, that is, the value of B. See what's changed.

For more intuitive comparisons, we can use the Color Sampler tool 〖i or Shift_i〗, and click 1234 sequentially in the diagram to see the color values of 4 sampling points (also known as sampling points) in the information palette. Turn them all into HSB, roughly the following figure. Note that the hotspot of the color sampler tool is at the end of the eyedropper, which is the lower-left corner of the cursor, and if not, you can press the case conversion key 〖caps Lock〗 switch to the cross cursor mode.
The Color sampler tool and the eyedropper tool are very close to each other, and don't confuse the difference. The Eyedropper tool is, the color sampler tool is.

Now compare the brightness of 12, 87% and 95% respectively, and increase by 8%. Compare the brightness of 34 to 57% and 78% respectively, with an increase of 21%.
Because you can only have up to 4 sampling points at a time, you can use the Color Sampler tool to move existing sampling point locations if you need data from other places. You can move the sampling cursor on the 34 point to the 56 position and you will see an increase of about 8%.

Through the actual comparison above, we verify the reasoning that we have done according to the curve change. That is, the amplitude of dark harmonic and high light changes is small, and the amplitude of intermediate adjustment is large.
Go deeper. Look at the picture below, in fact, we can get the ABC three points before and after the approximate B value, without relying on the sampler.

We know that the X axis value represents the absolute brightness of the original image, and the Y axis represents the adjusted absolute brightness, then observe the starting position of point B on the X and Y axes, all 50%. This represents "the original 50% after 50%", that is, there is no change. The y-axis position of point B is about 71% to 73% after the light is highlighted. This means "50% after 71%", in other words, the original image of 50% brightness pixels, in the adjustment should be 71% to 73% or so. We can compare the 143,144 coordinates in the two figure respectively.

The same way to observe the C point, should be "original 75% after 87%". The adjustment is approximately 86% to 89%. You can compare the 253,125 coordinates of the two figure respectively.
A closer look at the point of a on the Y axis is not equal to the C point, but slightly larger than c. Should be "original 25% after 40%", about 39至41% around. It can be compared with the 366,260 coordinates of two figure respectively.

Let the mouse in the original image has not been adjusted around, pay attention to the information plate B value, you will find the highest brightness of about 90%. After adjusting the image upstream, you will find that the highest brightness is around 96%. This shows that there are no pixels in the image that are brightest (that is, B value 100%, or pure white) before and after the adjustment. You can make it appear by increasing the amplitude of the adjustment.

We can put the information palette in a place that won't be obscured by other things, then use the curve adjustment, the adjustment of the mouse to move to the image, the information palette will appear two sets of values, the middle with "/" separated, the left is adjusted before the value, the right is the adjusted value. The following figure. At coordinates 487, 29, the brightness increases from 89% to 100%. At this point if you confirm the curve adjustment, this place is pure white.

This kind of parameter comparison function of information palette, also apply to other kind of color adjustment way, it is very convenient. Therefore, it is generally convenient to view the information palette in the corner of the interface.
Of course, most of the time is based on the eyesight to determine whether the overall effect is appropriate, the need for such a precise pursuit of local effects of the opportunity is not much. But understanding this way is good for deepening the understanding of color principle.

Although it is more intuitive and convenient to use a percentage of B to indicate and contrast brightness, remember that this percentage is based on the underlying value of a brightness, which is 256. This principle has already been mentioned in the first section of this lesson. That is, the percentage value of the B that we get above should multiply by 256 if you want to correspond to the actual brightness level. The following figure.
Also remember a problem where the total number of levels is 256, but the highest number is 255. This principle we have already mentioned in the course #01.

Before we reasoned that the ABC three point changes the amplitude, the unit used is the percentage, that is to correspond with the B value in the HSB. In fact, the curve setup box itself has a color level value, when the mouse in the curved frame move, the lower left corner will appear a set of dynamic values. The following figure.

When a point is established on the curve, the input and output indicate the currently selected point. The following figure. The input represents the change before the color order value, and the output represents the changed color level value. You can enter numbers directly to adjust the positioning accurately. At this point two values are no longer moving with the mouse, if you want to restore dynamic values, you can use the mouse in the position away from the curve (the right picture below the mouse) click on it. If the distance curve is too close and the cursor changes to a cross arrow, clicking will increase the curve control point.

We have deduced that the range of B points is "original 50% after 71%", the conversion of which is 256x50%=128,256x71%=182 (approximate) respectively. You can see that the values in the above figure are the same. The error is mainly because 71% is based on the approximate location of the visual.

You may not understand why I have to go around a corner like this instead of introducing the color bands at the beginning. As explained earlier, it is to correspond with the B value in the HSB color mode. Then why do we have to correspond to the B value? Because in the actual use, it is more convenient to use the information palette to view the brightness. And the information palette does not have a color level, the best way to measure the brightness of the HSB is the B value.

After understanding the principle that the curve rises can brighten the image, you should also be able to understand that if the curve drops, it will darken the image, and this is no longer repeated here.