Adobe's Implications for Color Correction and Gray Balance Control

Color correction is a very important part of image modification. Color correction involves fine-tuning the hue, changing the contrast and color of the image.

Getting a good scanned image is a good start for all work. Although Photoshop can modify the defective image, if the scanned image does not acquire enough color information, excessive color adjustments in Photoshop will result in more detail loss. So as far as possible to obtain high-quality images during scanning, control before scanning is often important.

Before starting the color correction, check the pixel values ​​of the highlights and dark parts of the image. Select the Info command to pop up the Info palette in the Windosws menu, move the mouse over the image, and view the brightest and darkest on the Info palette. Part of the pixel values, such as the brightest part of the RGB pixel value is 240, the darkest part of the RGB pixel value is 10, then the tone range contains all these pixel values ​​have enough detail to obtain a rich layer of the image.

The commands in the Image Adustments submenu are all commands for color adjustment. These commands modify the distribution of the pixel values ​​in the image and allow adjustment of the hue within a certain accuracy range. The curve command provides the most accurate adjustment. When performing color scale and curve commands on individual channels of a color image to modify the color balance in the image, the curve command provides the most precise control over the pixel value distribution within the channel. The Hue/Saturatlon, replac Color, and Selective Colol commands modify specific colors in the image. The Varations command is often used to make adjustments to very accurate colors. It adjusts the image's color balance, contrast, and saturation.

How to correctly reproduce the gray balance in the original during the scanning and copying process is a very important task, and it is a job that is difficult to do well but does not work well. It is the basis for real tone, gradation, and color reproduction. Through the so-called color management system (CMS) is also based on the realization of the gray balance.
In color scanning and Photoshop color adjustment of the image, the user usually only pay attention to how to adjust the image displayed on the screen to make it more similar to the original in terms of hue, brightness and contrast. But often overlooked a very important idea, that is, the images on the monitor are displayed in RGB mode. From the theory of the three primary colors, the neutral gray color will appear in an equal proportion of RGB mixture. There are various levels of light gray and dark gray.

There are two basic principles to understand when scanning:

(1) High light, midtones and dark shadows determine the hue of the image.
(2) The color combination can only be correctly implemented under the adjustment of the gray balance. Gray balance is the basis for the existence of color.

(QGTO note: Adobe explained the origin of the gray balance, pointed out that "gray balance is derived from printing the color cast of four-color ink," but this does not mean that people working in the professional printing industry have reason to ignore this when correcting images. Because of the four-color printing, C, M, Y three-color inks cannot be overprinted into pure black, and it is impossible to accurately reproduce neutral gray, so only a K-plate is added to make it more faithful to The true color of gray-balanced originals is reproduced in RGB.Therefore, the view that neutral gray is only useful in printing is wrong.Professional color correction personnel must observe the RGB value and CMYK at the same time when correcting the color cast of the image. The color value, because anyone is using the "color balance" command, is actually also in the manipulation of the complementary relationship between RGB and CMY, any professional color correction can not get rid of CMYK).

(a) Correction of color balance

Color balance refers to the ability to make adjustments in the color space. These adjustments can be made using methods such as curves and tonal adjustments. Color balance is the change of individual colors, but the change also indirectly affects other colors in the image. Color correction is for color balance, it only changes one color in the image without affecting other colors.

The adjustment of the image can be done for all colors, or for individual channels. The use of all color adjustments is often not effective, because image color shifts are not all color shifts, most are from one to two colors. With a single channel, the color and seat level can be adjusted according to specific conditions to achieve satisfactory results.

Do not adjust the color in the fixed area. If the image has a color cast in a certain area or range, it indicates that the entire image is color cast, not the area that is visually seen. Correcting the entire image not only eliminates obvious color casts, but also adjusts details that are not noticed.
Color shift means that the color of the image is different from the original color tone. When you see a picture tending to a certain color, do not blindly remove this color. You should first consider the characteristics of the image of the purpose of the image. For example, a landscape picture is always blue or yellow. When color deviation can highlight or transfer some kind of information, you should first find out the color shift position before removing the color shift. The general color shift can be distinguished by visually identifying its position. The color shift mainly focuses on the main color tone and reflection of the image. High light on.

(b) Follow the following principles when correcting color cast

(1) The color cast will not be limited to only one color in the image.
(2) When an image has a potential color cast, check the highlights first because the human eye is most sensitive to color shifts in the lighter part.
(3) Neutral gray should be selected when correcting color shift, because neutral gray is an important means to compensate for color shift. When correcting the gray color part, do not believe the color of the human eye, as other colors in the image will change the feeling of the human eye to gray, this is what we call the influence of the ambient color, should be used to encounter this situation Pipette tool for inspection.

(4) When correcting color shift, try to adjust the complementary color of the color.
(5) According to the specific requirements of the image, adjustment can be made using the HLS mode.

(6) The color shift of many images is quite severe in some tonal ranges. If you simply adjust this part of the hue, it will make changes in the hue than drastic changes, so be sure to coordinate the entire tone range.

(c) Several typical color deviations

(1) The manuscript of rainy days looks like it is covered with a light blue layer. Due to the lack of sunlight on cloudy days, it lacks red.

(2) Positive films shot with fluorescent light as the light source sometimes produce a greenish phenomenon. This is because the light emitted by a fluorescent lamp appears to be self-colored, but actually white contains a strong color, if a color negative film is used Direct shooting will inevitably cause color cast.

(3) The color shift caused by the negative film itself is due to the difference in the manufacturer's production date, so the tendency of the negative film to have a different color is also different. This color cast is a small amount of color cast that does not cause color cast overall like the first two.

(4) Most of the manuscripts have the colors in memory, such as the sky, trees and flowers that are familiar to everyone. If these colors have changed, the eyes of the people will easily find out.

(5) A small color cast is a color cast that is not noticed by the human eye. The solution to these color casts is to find neutral grey in the image or the color in the memory as a standard.