How to distinguish the color cast of photos in photoshop, not by feeling?

It should be to check the values of red, blue and green (RGB). Not very good at using it. . .

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First, the significance of color correction:

Now there are many sources of pictures, such as digital camera shooting, online downloading, scanning and so on. For various reasons, most pictures have different degrees of color cast. Before using these pictures, the first thing you have to do is to observe whether these pictures are color-cast. If it is color cast, you should carefully analyze what color cast is, and then use a series of means to correct it.

Color correction is to restore the original color of the picture. Color correction and color matching are two fundamentally different concepts. The former is to correct the color of a picture with color deviation to the original color, which must follow certain scientific standards and cannot be arbitrary, and belongs to the category of special technology; The latter is the "artistic rendering" of the picture by the author to express some intention or some emotion. It is arbitrary and does not need to follow strict technical standards. Therefore, the color correction discussed here has nothing to do with color matching and its "artistic rendering" and must not be confused.

When discussing the color problem, it is natural to think of the human eye, because color is to be felt with the eyes, and the human eye is sometimes so unreliable. If human eyes could be as accurate as instruments, movies would not have been invented! If you don't believe this statement, you can use the following figure to verify it, as shown in (Figure 0 1):

Question: Is A the same as B?

At first glance, you will say: "different"! But, you are wrong! Actually, A and B are exactly the same. Why is this? This "problem" is that the eyes are at work! A gray is surrounded by a square that is whiter and darker than it should be, but the shadow of that pillar is the key to make your eyes illusion! ! ! It is the four squares in the shadow that are darker than A that make B brighter than A, and the semi-dark part of the column itself participates in the rendering. .....

We can use Photoshop to verify, open this picture in Photoshop, and pick up the colors with the color picker to see what the RGB values of A and B are. The result is amazing! The value of R:G:B is107:107:107.

This shows that the human eye is not a precise part, but scientific data is the most important, and everything is correct if it can stand scientific verification!

We can also cut off the gray blocks in area A and area B, so that we can separate them from the rendering of the surrounding environment and see what the difference is. When A and B are alone, they look exactly the same, because there is no environmental interference to cause the illusion of the eyes.

It can be seen that our eyes are so haggard and unreliable, so any problem involving image color can never be decided by eyes and feelings alone!

The human eye is an image pickup organ with biconvex Dan Toujing structure, and the scene is imaged on the optic nerve of the fundus through the original and simple corneal biconvex lens. It is difficult for us to understand how a biological lens such as an ordinary magnifying glass can perfectly correct aberration distortion and why the scene seen by human eyes is 10 thousand times higher than that of any precision camera. In fact, the knot image of the human eye is perfectly corrected by the brain, and this process is completed together with the correction of the color seen by the human eye! Since the invention of television, the glass of kinescope has been spherical and has been used for more than half a century. The visual nerve of human brain has gradually become accustomed to spherical image, but the aberration curve caused by spherical image can still be clearly seen by human eyes. When the flat picture tube appeared, the human eyes mistakenly thought that the flat picture should have sunk in when they got rid of the habit of long-term spherical image. Everyone has experienced this process of brain correction and adaptation, but they soon got used to it. In fact, the human eye is better and more perfect than chromatic aberration adaptation correction in correcting pre-aberration, because you know that things that are originally straight lines will never bend. When the distorted and bent image is corrected on the monitor, you will agree with it in a short time. It is easy to prove the illusion of color to the human eye. Please look at the picture below. In fact, the orange in the middle is the same, but it seems that the orange on the right is obviously brighter. This phenomenon is called simultaneous contrast because of the contrast of red.

In addition, when you look at a color (such as red) for a long time and then look at the white wall, the complementary color of red will appear on the white wall, which is also a defect of the human eye. This phenomenon is called time contrast.

In addition, the human eye has a strong adaptability and habit to yellowish pictures. There are two main reasons for seeing yellow but not yellow. First, because the incandescent light bulb we commonly use emits yellow light (the light with low color temperature is warm yellow light), most people will have yellowish dyeing when they look at things, especially at night, which will form a habit in the long run. Second, because yellow is the brightest color in all tones, it gives people a strong sense of brightness. Therefore, when an object is dyed yellow, people will feel bright, even brighter than pure white light. This is purely a feeling, but it is actually inaccurate. Therefore, it is not accurate to color by feeling alone.

Remember, people perceive the world through light and shade and express their feelings through color, so this is why people are more likely to accept yellow skin.

Understand the above truth, and then look around the internet, you will find that most pictures have color cast problems. Since we can't correct the color by feeling, what standard should we use to measure it? That's the fairest: neutral gray. Color harmony still follows the neutral gray theory, so the color difference adjustment of the picture should be based on neutral gray.

The purpose of color shift correction is to adapt the color information recorded by the machine to the "adjusted" state seen by human eyes. When the RGB of the picture is equivalent, the color cast problem is solved.

Second, the scientific basis of color correction:

In color theory, Munsell believes that color harmony can only be achieved when all the colors that make up a painting are mixed together to produce neutral gray. He thinks that color harmony in a strict sense is that all colors in a painting are mixed in proportion to get neutral gray (quoted from the book Color Theory published by Southwest Normal University). Color harmony still follows the neutral gray theory, so the color difference adjustment of the picture should be based on neutral gray.

Adobe discusses two basic principles of gray balance control:

(1) High point, middle tone and dark tone determine the tone of the image.

(2) Only under the adjustment of gray balance can color matching be correctly implemented, and gray balance is the basis of color existence.

The postman teacher's description of "neutral gray";

Natural objects are black and white and gray. Under normal light (white light), the three RGB parameters reflected by the image should be equal.

After a photo is developed and scanned, the RGB values of the original black, white and gray objects in the image are not equal, indicating that the image has color cast. Which value is higher, which color has deviation.

Under dark light, white objects will appear gray. So the white object in the image is not necessarily the standard white with RGB=255.

I think: in RGB, black, white and gray are the brightness relations of colors. Any color (red, green, blue, cyan and yellow) is white at its brightest and black at its darkest.

Therefore, in the gray gradient, except pure black and pure white, as long as RGB is equal to standard gray, it can belong to the category of neutral gray.

As for 127 or 128, I don't think it is necessary to be true. The absolute value should be 127.5, but the decimal point cannot be set, so 128 is usually taken as an example. RGB= 128 is called "absolutely neutral gray".

Third, several typical color casts:

(1) A manuscript that rains on a cloudy day looks like it is covered with a layer of light blue, but it lacks red because there is no sunshine on a cloudy day.

(2) Positive films taken by fluorescent lamps as light sources sometimes appear green. This is because the light emitted by fluorescent lamps looks white, but in fact, white contains strong colors. If you shoot directly with color negative, it will definitely lead to color cast.

(3) The color deviation caused by the film itself is due to the different manufacturers and production dates, so the film has different color tendencies. This color cast is a small amount of color cast, which will not cause overall color cast like the first two.

(4) Most manuscripts have colors in their memories, such as the familiar sky, various trees and flowers. If these colors change, it is easy for human eyes to find them.

(5) Tiny color shift is a color shift that is not perceived by human eyes. The solution to these color deviations is to find neutral gray in the image or the color in the memory as the standard. Four, the principle of correcting color cast should be followed:

(1) Color cast is not limited to a certain color in the image.

(2) When there is a potential color cast in the image, the bright part should be checked first, because the human eye is most sensitive to the color cast of the bright part.

(3) When correcting color cast, neutral gray should be selected first, because neutral gray is an important means to make up for color cast. When correcting the gray of the color part, don't trust the color presented by human eyes, because other colors in the image will change human eyes' perception of gray, which is what we call the influence of environmental colors. In this case, we should use the pipette tool to check.

(4) When correcting the color deviation, try to adjust the complementary color of the color.

(5) According to the specific requirements of the image, HLS mode can be used for adjustment. (6) The color deviation of many images is quite serious in some tonal ranges. If we simply adjust this part of the timbre, the timbre outside the adjustment will change greatly, so we must coordinate the overall timbre range. V. Preparation before color calibration:

1, display calibration: This is a very important step before color calibration. If no color correction is performed before the image is output, it is likely that the colors seen on the screen are inconsistent with the printed colors, or the same image displayed on another monitor is far from each other. If the color deviation of your monitor is serious, it will directly affect scanning, printing and various color processing operations. Therefore, before manipulating the image, you must use Adobe Gamma software (installed with Photoshop and located in the control panel of the system) to calibrate the display.

2. If color management and accurate ICC profile are used, the display will display colors more reliably. Using ICC display profile can help you to eliminate the color shift in the display, make the gray scale of the display as neutral as possible, and standardize the image display in different displays. In Photoshop, color management calls Adobe RGB( 1998). It is suggested that you change the gray scale in color management to Gray Gamma2.2 and save it as your own scheme. For detailed information about the calibration and color management of the monitor, please search on the Internet yourself, which is easy to find.

See the figure below:

Six, the general methods and skills of color correction:

(a) The first two questions that need to be clarified:

Firstly, when neutral gray is RGB= 1: 1: 1, the gray scale from black to white is over-colored, which is the legal industrial standard for determining image hardware and the ideal value for restoring color images. However, in the process of recording color images, there are many uncertain factors that affect color restoration and reproduction, resulting in the neutral color distribution is not as absolute as the ideal equilibrium value. But as long as we follow the hardware requirements in the process of color reproduction, the result is affirmative. The smaller the neutral color deviation when the image is allowed to be restored, the higher the fidelity between the image and the scene color, which is a standard pursued in color correction. For example, cyan in a snow scene is the off color. According to the standard of neutral gray, we can't achieve absolute RGB = 1: 1 when restoring colors, but as long as you work hard, try to make the gray areas gray, even if the data doesn't reach RGB balance, it is already gray visually. We can use the insensitivity of eyes to color changes to realize the function of color correction. In fact, the human eye's ability to adapt to color is at work at any time. For example, a long piece of white paper is illuminated by incandescent lamp or fluorescent lamp, and what you see is always white, but the color recorded by the machine is very different. We can make use of the "faults" of our eyes to reduce the strict requirements for the ideal data of neutral colors in color correction. Why not? There is no doubt that the method of finding neutral colors in unadapted color images is simple, that is, it is not biased towards neutral colors of any color in memory-black, gray and white! Neutral gray is not inconclusive in color correction and color reproduction of color imaging equipment, but an iron law! To put it bluntly, the white place must be white and the black place must be black in the revised picture.

Second: Memorizing color, using one's own memory to restore the original color of an image in color correction is not everyone's skill! Like music, piano learners need to keep practicing their listening skills to remember the pitch. Without a piano, people who have practiced listening have a good memory. By knocking on the glass, they can tell exactly which key on the piano. Memory color is accumulated by years of visual training and continuous work experience, and excellent post-producers are much more sensitive to color than ordinary people. There is no different understanding of color here. As long as you keep in mind the concept of neutral gray and the restoration law of color images, then the colors of memory should be relatively uniform. In the process of color correction, the critical point of color change is a process of observing and exercising vision. This change is very subtle. People with keen eyesight can't tolerate any color-cast sand. We pursue visual balance, and the visual balance we pursue on the machine is to find the true meaning seen by human eyes and humanize the distorted color of the machine. The cultivation of memory color depends on your careful observation in real life, and no one can help you But the quickest way is:

First, colorless things. Pay more attention to the colors of the surrounding objects and environment, and remember them deliberately. Especially colorless things, such as black, white and gray, the color of these things is the zero point of colorful colors in everything, and neutral colors are equal to zero. Please remember it.

B, skin color. No matter yellow skin, white skin and black skin, there is a rule in color images, and that is blood color (healthy color). That is, there is red in yellow, red in white and red in black. If the skin color is yellow, you will feel sick. Even if some people's skin is yellow, you should correct it to a healthy color in color photos.

(2) Four cardinal principles:

(1) It is correct to accept neutral gray (skepticism will eventually lead to giving up halfway);

(2) Patiently learn basic theoretical knowledge and persevere;

(3) do a lot of exercises (there is no qualitative leap without quantitative accumulation);

(4) Observe carefully in reality (this is very important for stepping into a profound realm).

The above four points must be experienced by all experts. This is a long process step by step. If you treat color correction technology calmly and realistically, you won't find it difficult. If you can really do the above four points, you should find that "neutral gray" is a very lovely thing because it is very elastic.

(3) General methods:

Correcting color deviation is to correct neutral gray first, and only in this way can the correct color reproduction be obtained. When the gray level in the color image is restored to reality, the color of the whole picture is closer to nature, and the gray level ratio in the color image closest to the natural color is exactly the case when the RGB values of the three primary colors are equal. When using a straw for neutral gray operation, we should first determine the black and white field of the image with a black and white straw, and then determine the neutral gray of the image with a gray straw. However, it is also possible to determine the neutral gray correction color with a gray straw first, and then directly pull the color scale to determine the black and white field of the image to ensure the color balance of the image.

Most photos can find neutral gray pixels, which can be found in the following aspects:

1. According to the colors you know from experience and real life, look for something that should be gray, such as the highlight gradient on your hair;

2. Shadow of white wall or white clothes;

3. Asphalt road;

4. Natural scenery should be gray matter, etc. These parts are where the neutral gray is located, and the neutral gray can be compared with the gray level of the border of the nearby software window. You have noticed that in Photoshop, its background is neutral gray.

Verification and correction of data schema

1. Open the PS information board, where the mouse goes, the data board will display color data in time, and correct the places that should be gray in the image according to the parameter ratio of neutral gray to RGB =1:1,such as computer desk, wall shadow, metal tube, silver gray object, etc.

2. White clothes are irradiated with light without chromatic aberration, and the places that are not completely white should be within the range of neutral gray. At this time, you can directly choose a point on the white clothes as neutral gray, correct the color, and then adjust the image tone appropriately;

3. If it is a close-up of a single color face, it can be corrected with reference to the skin color standard;

4. The main color trend of skin color is not determined by anyone's feelings, but the standard skin color tested by neutral gray balance characteristics on the color information recording medium.

Because neutral gray can't be changed in tone distortion correction, it supports and consolidates our experience in correcting color deviation according to standard skin color. When the original image is corrected according to the complementary law of RGB and CMY, and the gray level is relatively balanced, we make full use of the "hue" function of PS to change the color shift in this image except the gray level balance. This correction is an advanced processing method when other colors are abnormal in neutral gray balance.

In a word, according to neutral gray theory, under normal illumination, R = G = B in an object image, neutral gray is the most "notarized", which is an important basis for us to correct color cast images; When selecting neutral gray points, "pay attention to finding places that are not affected by ambient light"; If the picture contains RGB= 1: 1: 1 information, but other parts have color cast, it is caused by hue shift. If it is an object or a landscape picture, you can observe whether the color deviation conforms to the color seen in reality, and if it does not, use hue instruction to correct it.

Color cast identification method

In order to correct the color, we must first be able to identify the color deviation of the picture. There are many ways to identify color cast, and the following methods are usually used:

1. Take advantage of first impressions

When you observe color photos, it is usually helpful to use the first impression to identify the degree of color cast. This method is especially necessary for some photos with less serious color cast. This is because people's eyes have a strong adaptability to color, and the results of repeated observation of photos with slightly different colors will gradually weaken their resolution, so we must grasp the first impression.

2. Observe the gray part of the scene image

The gray scene in color photos is the most difficult part to restore correctly, and it is also the part that can best explain the degree of color balance, so it is often used as the basis for distinguishing color cast. If the gray part of the scene is corrected, other colors can be more realistic. On the other hand, if the gray part of the scene has a color cast, other colors will certainly not be truly represented. When shooting color films, why do each group of lenses have to take some gray plates? The reason is for the need of color correction.

3. Balance correction method

When explaining the reason of color correction, I talked about the different performance of color photographic materials's three-layer emulsion characteristic curve. According to the situation, from low density to high density, the color difference of photos will be the same. In order to get rid of some common colors hidden in the scene with different levels of density in the whole picture, and thus restore the true colors of the scene, the correction method is balance correction.

4. Color cast should be identified by the middle density part of the photo.

A color photo, in a series of density grades from low density to high density, the color of the middle density part is the most full and rich, which accounts for the largest proportion in the picture and is often the main part, so it is very representative to identify the color difference of the photo mainly in the middle density.

In the low-density part of the photo, although the color cast can also be identified, the degree and severity of color cast are not as full and obvious as those in the medium-density part because of the shallow level. The high-density part of the photo is prone to color cast, which leads to misjudgment. For example, the appearance of blue in the shadow of the blue world is a natural phenomenon and cannot be misjudged as color cast. For another example, if the negative is too thin, high-density photos will be slightly green, which is a feature caused by poor blackness. If it is consistent with the medium density color deviation, it can be corrected, otherwise it cannot be corrected. The above situation shows that identifying the color cast in the low-density part and the high-density part of the photo is only of reference value and cannot be used as the basis for correcting the color cast of the photo.

5. Observe human skin color

Where there are pictures of people, especially those with people as the main characters, color cast can be carried out according to people's skin color, because people's skin color is familiar to everyone. As long as the color of human skin can be truly restored, other scenes in the whole picture will also be photographed under the condition of balanced color temperature.

Get a better color effect. However, the correction and restoration of skin color must also consider factors that vary from person to person, such as race, occupation, gender, age and so on. They have different skin colors.

6. Distinguish the primary colors and secondary colors of photos.

In some photos, the two colors may be different, but there are often differences in degree. For example, part green is composed of part yellow and part cyan. Except for positive green, yellowish and cyan are always unequal. Therefore, it is necessary to distinguish who is dominant first, and then use color filter combinations with different density values to make corresponding corrections.

7. Distinguish the color cast from the relationship between the colors of the scene.

We can't only pay attention to the color of a scene when identifying the color cast of a photo, because the color cast exists in different colors of each scene on the screen. Only by analyzing the colors of two or more scenes and their relationship can we find the colors with deviation in the photos. For example, the red and green scene in the picture is a little yellow, and the blue sky looks gloomy, so it can be determined that the overall tone of the photo is yellow.

The main purpose of identifying and correcting color deviation is to balance colors and truly restore the original colors of the scene. However, films shot under certain environmental conditions need to emphasize and exaggerate the atmosphere of the scene, and even use exaggerated methods to enhance its artistic effect. For example:

(1) the sunrise, sunset, steelmaking, tapping, fire and other scenes can be red, and warm colors should be used.

② Snow scenes and moonlit nights can be slightly bluer. Green mountains and green waters, forests and grasslands can be slightly turquoise, and cool colors should be used.

Of course, the rendering and exaggeration of the scene atmosphere should also have certain limits, and should not be excessive.

Seven, color correction commonly used instructions:

Color scale and curve, color balance, hue/saturation, optional color, channel mixer (rarely used).

1, color scale and curve:

A, "color scale" is mainly used to adjust the lightness, darkness and contrast of colors, so as long as you think the canvas is too dark and too bright and the contrast is not obvious enough, you can consider using it to adjust the picture. The role of the black-gray-white triangle slider in the dialog window of "Levels" is that the brightness of the black slider will decrease when it moves to the right, and the brightness of the white slider will increase when it moves to the left. The gray slider controls the midtones. Moving to the left will increase the brightness, and moving to the right will decrease the brightness. As shown in the figure below:

B. Curves: Similar to the Levels dialog box, the Curves dialog box allows you to adjust the entire tonal range of an image. However, the curve can be adjusted at any point in the range of 0-255, instead of using only three variables (highlight, dark and midtone), while keeping other values of 15 unchanged. You can also use curves to precisely adjust individual color channels in an image. See the figure below:

2, color balance:

For normal color correction, the Color Balance command changes the overall color mixture of the image.

To use the color balance command:

(1) Make sure that the composite channel is selected in the Channels palette. This command is only available when viewing composite channels.

(2) Open the color balance dialog box.

(3) Select "Dark Tone", "Intermediate Tone" or "Highlight" to select the tone range to be changed emphatically.

(4) Select "Keep Brightness" to prevent the brightness value of the image from changing with the change of color. This option can maintain the tonal balance of the image.

(5) Drag the slider to the color to be added in the image; Or drag the slider away from the color you want to reduce in the image.

The value above the color bar shows the color change of the red, green and blue channels. (For laboratory images, these values represent channels A and B.. ) The value range can be from-100 to+100. See the figure below:

3, hue/saturation:

This function is mainly used to control hue, saturation and brightness. See the figure below:

4. Optional colors:

Optional color correction is a technique used by high-end scanners and color separation programs, which increases and decreases the amount of printed color for each increased and decreased original color pixel in the image. Even if "Optional Color" uses CMYK color to correct images, it can also be used to correct RGB images and images to be printed.

Optional color correction is based on a table that shows the quantity of each printing ink used to create each primary color. By increasing and decreasing the number of printing inks related to other printing inks, you can selectively modify the number of printing colors of any primary color without affecting any other primary colors. For example, optional color correction can be used to significantly reduce cyan in green pixels of an image while keeping cyan in blue pixels unchanged. To use optional color commands, do the following: Make sure that a composite channel is selected in the Channels palette. Optional color commands are available only when viewing composite channels.

Opens the Optional Colors dialog box. Select the color you want to adjust from the color menu at the top of the dialog box. This group of colors consists of additive primary colors and subtractive primary colors, as well as white, neutral colors and black.

For the method, select an option:

Relatively change the percentage of cyan, magenta, yellow or black in the total. For example, if you add 10% from 50% magenta pixels, 5% will be added to magenta. The result is 55% magenta (50% x 10% = 5%). (This option cannot adjust pure anti-white light because it contains no color component. )

Adjust the color absolutely according to the absolute value. For example, if 10% is added from 50% of magenta pixels, the total amount of magenta ink will be set to 60%.

Note: Adjust the proximity between the color-based and the options in the color menu. For example, 50% magenta is between white and pure magenta, and the correct proportion mixture defined for these two colors will be obtained. Drag the slider to increase or decrease the pixels of the selected color.

The change of optional color instructions also has similar effects, such as the shadow of people's faces in the sun and the connection between blue sky and white clouds. If you use a medium-gray filter, you can distinguish them better and will not be confused because of the "color shift" between pixels. But the effect of these measures is difficult for ordinary people to see. Optional color command can obviously distinguish or improve the image color, but the operation should be moderate. It belongs to an expert-level advanced operation that understands photography. As long as the color that needs to be emphasized exists in the original information, we can be promoted in the "optional color" instruction.

5. Channel mixer:

The Channel Mixer command allows you to modify a color channel using a mixture of the current color channels. With this command, you can do the following: Realize creative color adjustment, which is not easy for other color adjustment tools.

Choose a different percentage from each color channel to create a high-quality grayscale image.

Create high-quality sepia tones or other color images.

Convert images in an alternate color space, such as YCbCr used in digital video.

Switch or copy channels.

In a word, "hue command" and "optional color" have strong exponential change characteristics, and we can play with all kinds of colors at will. Since we can make all kinds of colors, they can also correct all kinds of color casts at will, but no matter how we play with these commands, the premise is that the three primary colors and the recognition of color casts are excellent! The change of "channel" in color gradation instruction is different from that in "channel mixing" instruction, which is a single color "density" adjustment (proportion), while the color "channel" in color gradation instruction is the change of color level, that is, the change of color contrast, and the color information can change on any section; The "Optional Color" command can change the "black and white density" of a single color; In the Tone command, you can change the brightness of a single color. Therefore, the comprehensive correction of color deviation can be realized by using these instructions comprehensively.