Color correction is a very important item in the image modification. Color correction includes fine-tuning the hue and changing the contrast and color of the image.
Obtaining a good scanned image is a good start for all work. Although Photoshop can modify a defective image, if the scanned image does not obtain sufficient color information, excessive color adjustment in Photoshop will cause more details to be lost. So try to get high-quality images while scanning, the control before scanning is often important.
Before starting the color correction, you can check the pixel values ​​of the highlight and dark parts of the image, select the Info command under the Windosws menu to pop up the Info palette, move the mouse position on the image, and observe the brightest and darkest on the Info palette Part of the pixel value, such as the RGB pixel value of the brightest part is 240, and the RGB pixel value of the darkest part is 10, it means that the tonal range containing all these pixel values ​​has enough details to obtain a rich layered image.
The commands in the Image Adustments submenu are all used for color adjustment. These commands modify the distribution of pixel values ​​in the image and allow the tone to be adjusted within a certain range of accuracy. The curve commands provide the most accurate adjustment. When executing the color scale and curve commands on the individual channels of a color image to modify the color balance in the image, the curve commands provide the most precise control over the distribution of pixel values ​​within the channel. The Hue / Saturatlon, Replace Color, and Selective Colol commands can modify specific colors in the image. The Varations command is often used to adjust the colors that are too precise. It can adjust the color balance, contrast, and saturation of the image.
How to correctly reproduce the gray balance in the original in the process of scanning and copying is a very important work. It is a difficult work but it cannot be done well. It is the basis of real tone, gradation and color reproduction. Through the so-called color management system (CMS) is also based on achieving gray balance.
When color-adjusting images in color scanning and Photoshop, users usually only notice how to adjust the entire image displayed on the screen to make it closer to the original in terms of hue, brightness, and contrast. However, it often overlooks a very important idea, that is, the images on the display are displayed in RGB mode. From the theory of the three primary colors, the neutral gray will appear in the RGB mixture of equal proportions. Presenting various levels of light gray and dark gray.
Understand the following two basic principles when scanning:
(1) The highlight point, middle tone and dark tone determine the color tone of the image.
(2) Only under the adjustment of the gray balance can the color combination be correctly implemented. Gray balance is the basis for the existence of color.
(Note: When explaining the origin of gray balance, Adobe pointed out that "gray balance is derived from the color deviation of printing four-color inks", but this does not mean that people working in the professional printing industry have reason to ignore this idea when correcting images. Problem. Because C, M, and Y inks in four-color printing cannot be overprinted into pure black, and neutral gray cannot be accurately reproduced, a K version is added to compensate to make it more faithful to RGB It reproduces the true colors of gray-balanced originals. Therefore, it is wrong to say that neutral gray is only useful in printing. Professional color correction personnel must observe the RGB values ​​and CMYK colors at the same time when correcting the color cast of the image. Value, because anyone using the "color balance" command is actually manipulating the complementary relationship between RGB and CMY, any professional color correction can not get rid of CMYK).
(1) Correction of color balance
Color balance refers to the adjustments that can be made in the color space. These adjustments can be accomplished through curves and tone gradations. Color balance is to change the individual colors, but the changes will also indirectly affect other colors in the image. Color correction is for color balance, it only changes one color in the image without affecting other colors.
The image can be adjusted for all colors or for a single channel. The use of all color adjustments is often not effective, because the color cast of the image is not all color cast, most of them are one to two colors. With a single channel, the colors and seat levels can be adjusted according to specific Sui conditions to achieve satisfactory results.
Do not adjust the colors in the fixed area. If the image has a color shift in a certain area or range, it means that the entire image is color shifted, not the area that is visually seen. The correction of the whole image can not only eliminate the obvious color shift, but also adjust the unnoticed details.
Color shift means that the color of the image is different from the original hue. When you see a picture tending to a certain color, don't blindly remove this color, you should first consider the characteristics of the image. For example, landscape pictures are always biased toward cyan or yellow. Color shift can highlight or convey some information. Before removing the color shift, you should first clear the position of the color shift. The general color shift can be visually identified to determine its position. Highlight.
(2) The following principles should be followed when correcting color shift
(1) The color shift is not limited to a certain color in the image.
(2) When there is a potential color shift in an image, the bright tone part should be checked first, because the human eye is most sensitive to the color shift of the brighter part.
(3) When correcting the color shift, first choose neutral gray, because neutral gray is an important means to compensate for color shift. When correcting gray in the color part, do n’t believe the color presented by the human eye. It is because other colors in the image will change the human eye ’s perception of gray. Pipette tool to check.
(4) When correcting the color shift, try to adjust the complementary color of the color as much as possible.
(5) According to the specific requirements of the image, the HLS mode can be used for adjustment.
(6) The color shift of many images is quite serious in certain tonal ranges. If you only adjust this part of the color tone, the color tone outside the adjustment will change drastically, so you must coordinate the entire color tone range.
(3) Several typical color casts
(1) The manuscript that rains on a cloudy day looks like it is enveloped by a layer of light blue. Because there is no sunlight on a cloudy day, it lacks red.
(2) The positive film taken by the fluorescent lamp as the light source may sometimes appear greenish. This is because the light emitted by the fluorescent lamp looks self-colored, but in fact, white contains a strong color. If you use a color negative film Direct shooting must cause color shift.
(3) The color deviation caused by the film itself is due to the different production date of the manufacturer, so the tendency of the film to have color is also different. This color shift is a small amount of color shift, and will not cause color shift as a whole as the first two.
(4) Most manuscripts have colors in memory, such as the sky, trees, flowers and grass that are familiar to everyone. If these colors change, it will be easy for human eyes to find.
(5) Small color shift is a color shift that is not noticed by human eyes. The solution to these color shifts is to find neutral gray in the image or the color in memory as a standard.
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