Prepress gray balance knowledge review--application in digital image

9, the use of gray balance in digital imaging

The concept of gray balance comes from the traditional printing industry. We all know that printing uses the mixture of C (cyan) and M (magenta) Y (yellow) K (black) to restore the images we need. Theoretically, C and M The Y and Y blends can be black in the sense that the colors, levels, and contrasts of the images are truly restored, but in fact printing inks (printer inks) cannot be very pure for various reasons. , so introduce black. The same amount of cyanine yellow blending just got our visual gray, this gray is the gray balance we say, the gray balance is not alone, but there is a gray tone to a dark gray tone.

Regardless of the method used, there are highlights, highlights, midtones, shadows, and black points.

1. High light (white field): Theoretically, it should be pure white, without any level, but actually the white point of different hue (high-key, mid-tone, low-key) images is different. Therefore, the true white field should be determined based on the images of different tones.
2, bright tone: bright area contains a certain level of detail, usually in the overall 5% -15%.
3, Midtones: occupy the main part of the image, is the most abundant part of the level, about 15% -75% in the whole.
4, dark tone: from 75% - 95% called dark tone.
5. Black spots (black courts): More than 95% darker than dark places.

No matter which kind of tune above, there should be a certain level, including white spots and black spots. Affected by the environmental color, the theoretical black and white color does not exist. This shows that there is a corresponding gray balance value in the tone from the white point to the black point.

The correct determination of the black (balanced) white field (equilibrium) and neutral gray balance plays a decisive role in the exact reproduction of the image:

The white field CMY values ​​should not all be 0, and the white field values ​​of the majority of the images can be set as C5, M3, Y3. It can be understood that the white field's cyan is greater than the magenta and yellow 2-3%. How does this value come from? From Figure 1, we can see that the curves of MY are basically coincident, and the curve of C is obviously higher than MY, then the value of MY3 comes from where? Because 2% dots are often not displayed during printing, 2% dots are ignored and 3 dots are started. We know that the pure white color does not exist, and the white paper under the direct light is obviously blue (C), which is consistent with them.

Note: In fact, the mixture of these three colors is the gray balance of the highlight region. This white field is not every image. The CMY ratio of different image white fields is analogized by percentage. (The values ​​5, 3, and 3 of the white field CMY are personal understandings and are subject to discussion).

The CMY value of the black field adds black K to achieve a visually pure black due to the warm dark red color obtained by the three-color mixing. Therefore, the value is C95, M90, Y89, and K80. The cyan of the black field is greater than the magenta and black. Yellow -80%. In order to make the dark part of the image have a very delicate layer and color saturation, when the total amount of CMY exceeds 90%, black is used instead.

The black and white fields of different tones are different. To determine the black and white field, use the Levels tool in the Adjust submenu under the PS Image menu to view: Move the black and white triangle to the first black or white.

Middle ash balance: The middle ash balance is often the part of the image subject to be represented. Values: C50, M36, Y36, we just have to remember: In the midtone part, the cyan is 10-13% larger than the magenta and yellow.

The concept of gray balance plays a very important role in the adjustment of images. We usually do not use the gray balance to adjust the hue, contrast, and saturation of the image, but we do not pay attention, and the deviation of the gray balance will Directly affects the bright contrast and color cast of the entire image. Here are some rules for adjusting images using gray balance:

1. When using the curve tool to adjust the color cast of the image, use the bright part of the image as a reference, because the human eye is sensitive to the bright part of the image.
2. When darkening the color cast, we'd better adjust the black version because CMY is actually replaced by a black version when it exceeds 90%.
3, because the image color cast is not the whole, but a certain color deviation, so do not correct the entire image color, in addition to color correction should be outside the bias color (complementary) to add or subtract.

The precise adjustment of the gray balance is a complex and difficult task, but at the same time the adjustment of the gray balance is very important. Here, as far as my understanding is concerned, what we are talking about is just some furry things. It is not entirely correct. Please refer to the appropriate details for specific details. data.

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