1. Correction of image color
The reproduction of color refers to a complex process of color decomposition, transmission and synthesis. The restoration of color is also a main aspect of print reproduction. In the process of color reproduction, it is subject to light source such as scanning, lens, color filter and photoelectric conversion. The influence of system, photosensitive material, paper, ink and other factors, color error is inevitable, especially the compression of the print level and the problem of ink, which has a crucial influence on the reproduction of color, in order to obtain the ideal color reproduction. You must try to correct these color errors to achieve the desired color reproduction. Color correction has always been the biggest headache for computer graphics and printing, because many colors on the screen are not printed at all, or there is a serious color shift. If it can't be corrected at the design stage, it will not be too late after the finished product is printed. It is.
1 Preparation before color correction
First, equipment calibration and system calibration are required. These devices include scanning equipment, display equipment, output equipment, and proofing equipment. These equipments must undergo strict professional correction. In addition, there must be a set of perfect colors between these equipments. Management solutions, these are the basis for our correction of color. Here, we must pay special attention to the display device. In image processing, the appearance color of the image is reproduced by the display before printing. The display is based on the RGB mode, and the final product we want is to use the ink to restore the printed matter on the paper. In CMYK mode, using RGB display devices to reproduce CMYK images will inevitably affect the appearance of colors, so the conversion (color management system) in the display should be accurate, and the light source uniformity and stability should be maintained. Sex, in order to make the screen display and proof as consistent as possible.
Second, a level of correction is required before color correction is performed. Because according to the color rendering mechanism, the color is presented on the basis of the neutral gray level, so the color correction should be performed after the level is corrected first. Otherwise, after the color correction is completed, the color will change when the level is corrected.
It is reasonable to correct the color in which color mode. In Photoshop, regardless of whether the image is RGB mode or CMYK third mode, the tone and color can be corrected. In which color mode, the correction is different, and the RGB color space is used. The advantage of the correction is that the color gamut space is larger and the color space of the display is the same, but since it must be converted to the CMYK space when used for printing output after correction, some colors cannot be displayed in the CMYK color gamut. That is, the color of the image is beyond the printing color gamut, called the overflow color. The advantage of color correction in the CMYK color space is that the corrected image is used directly for printing without color overflow. In addition, since the CMYK color space is a color space that conforms to people's visual habits, it is easier to grasp the color change when expressing a certain color and its changes. In view of these aspects, the image can generally be corrected in the RGB color space, and then Fine-tuning the image in the CMYK color space.
2 color deviation discrimination
Gray balance is a very important concept for correcting colors. When we discriminate the color shift of the scanned digital image, we need to use the concept of gray balance. If we know the primary color components needed to generate neutral gray of various brightness, we can use the neutral in the original. The gray area is color corrected. In Photoshop, the screen density tool (Info) is used to measure the color value in the digital image. If the value is supposed to be neutral gray, the value is not the gray balance value, indicating that the image has a color shift, according to the proportion of gray balance. It's easy to tell which color is more and which color is less.
The diffuse highlight area in the image is the best area for checking neutral gray. The highlight area is not all neutral gray, but the gray component is more than the other brightness colors, so check it from here. Judging the color cast, the calibration of the high-light pole mentioned above is also started from here.
In addition, there are many other experiences in judging the color color deviation, such as memory color, like blue sky and white clouds, grass green space, etc. These colors have deep memory in people's minds, which is more important for professional image processing personnel. It is necessary to remember the proportion of CMYK of these colors. The more you remember, the better you can judge whether the color of the image is accurate or not.
3 should pay attention to the color shift problem of RGB to CMYK
Once again, the printing uses four colors of magenta, cyan, yellow and black. Therefore, the colors are all specified by CMYK. As long as CMYK is determined, it means that the proportion of printed ink is determined, and the color usually does not have much problem. However, the color in the computer is expressed in RGB mode. Therefore, to print the RGB color, it is necessary to go through a color conversion step. Two problems will occur here:
The first is that there is no way for the ink to print out all the RGB colors. Usually high-brightness colors, fluorescent colors, and metallic luster are not printed. At this time, the program has to be replaced with the closest CMYK. If you use Photoshop, you will find that the selected color square has a triangle exclamation mark, or an exclamation point after the CMYK value indicates that the color cannot be printed.
The second question is, what conversion standard is based on RGB to CMYK? Even if an RGB color can be converted to CMYK, it does not mean that the actual ink color will be the same as the color displayed on the screen. Usually the biggest problem is that it occurs on cyan. In the uncorrected case, the cyan (R:0 G:255 B:255) seen on the screen is a very bright color, but the actual printing cyan (C: 100% M: 0% Y: 0% K) : 0%) is a completely different feeling, with a lot of cyan colors, such as blue and green, have serious color cast problems.
4 solutions
Using CMYK to specify color Some software, such as CorelDRAW and Photoshop, will especially consider the correctness of conversion between RGB and CMYK. As long as CMYK is used directly in the software to specify the color, you can get results close to the actual printing, so Use CMYK mode as much as possible to operate. Photoshop can convert the image file into CMYK mode for editing (MENU:Image>Mode>CMYK color), so you can completely avoid using unprintable colors. The disadvantage is that most filters can't work in CMYK mode. The method has to be temporarily converted to RGB mode, use the filter and then switch back to CMYK mode.
Another way is to use the printed color table to query the actual printed color. The color table is a reference template for color values, which lists various combinations of CMYK for query. So when you want to choose a specific color, first find the desired color from the color table, and then input the CMYK value marked on the table into the software. For the average person, this is the most effective color correction method. In actual use, it is of course impossible to check all the colors appearing in the layout, as long as the focus (such as text, frame line), or large-area coloring is confirmed. There is another thing like the color chart, called the standard color standard. This thing is to name the ink used in the printing system and print the samples one by one. The color inside must be printed with special ink and can't be used in CMYK printing.
In summary, modern designers should have the corresponding pre-press knowledge, and even the best design ideas will eventually become the finished products in our hands, so color as an important element in graphic design, not only in the print The effect of the pre-image has a decisive effect, and the correctness of the pre-press correction is more related to the quality of the finished product.
1. Correction of image color
The reproduction of color refers to a complex process of color decomposition, transmission and synthesis. The restoration of color is also a main aspect of print reproduction. In the process of color reproduction, it is subject to light source such as scanning, lens, color filter and photoelectric conversion. The influence of system, photosensitive material, paper, ink and other factors, color error is inevitable, especially the compression of the print level and the problem of ink, which has a crucial influence on the reproduction of color, in order to obtain the ideal color reproduction. You must try to correct these color errors to achieve the desired color reproduction. Color correction has always been the biggest headache for computer graphics and printing, because many colors on the screen are not printed at all, or there is a serious color shift. If it can't be corrected at the design stage, it will not be too late after the finished product is printed. It is.
1 Preparation before color correction
First, equipment calibration and system calibration are required. These devices include scanning equipment, display equipment, output equipment, and proofing equipment. These equipments must undergo strict professional correction. In addition, there must be a set of perfect colors between these equipments. Management solutions, these are the basis for our correction of color. Here, we must pay special attention to the display device. In image processing, the appearance color of the image is reproduced by the display before printing. The display is based on the RGB mode, and the final product we want is to use the ink to restore the printed matter on the paper. In CMYK mode, using RGB display devices to reproduce CMYK images will inevitably affect the appearance of colors, so the conversion (color management system) in the display should be accurate, and the light source uniformity and stability should be maintained. Sex, in order to make the screen display and proof as consistent as possible.
Second, a level of correction is required before color correction is performed. Because according to the color rendering mechanism, the color is presented on the basis of the neutral gray level, so the color correction should be performed after the level is corrected first. Otherwise, after the color correction is completed, the color will change when the level is corrected.
It is reasonable to correct the color in which color mode. In Photoshop, regardless of whether the image is RGB mode or CMYK third mode, the tone and color can be corrected. In which color mode, the correction is different, and the RGB color space is used. The advantage of the correction is that the color gamut space is larger and the color space of the display is the same, but since it must be converted to the CMYK space when used for printing output after correction, some colors cannot be displayed in the CMYK color gamut. That is, the color of the image is beyond the printing color gamut, called the overflow color. The advantage of color correction in the CMYK color space is that the corrected image is used directly for printing without color overflow. In addition, since the CMYK color space is a color space that conforms to people's visual habits, it is easier to grasp the color change when expressing a certain color and its changes. In view of these aspects, the image can generally be corrected in the RGB color space, and then Fine-tuning the image in the CMYK color space.
2 color deviation discrimination
Gray balance is a very important concept for correcting colors. When we discriminate the color shift of the scanned digital image, we need to use the concept of gray balance. If we know the primary color components needed to generate neutral gray of various brightness, we can use the neutral in the original. The gray area is color corrected. In Photoshop, the screen density tool (Info) is used to measure the color value in the digital image. If the value is supposed to be neutral gray, the value is not the gray balance value, indicating that the image has a color shift, according to the proportion of gray balance. It's easy to tell which color is more and which color is less.
The diffuse highlight area in the image is the best area for checking neutral gray. The highlight area is not all neutral gray, but the gray component is more than the other brightness colors, so check it from here. Judging the color cast, the calibration of the high-light pole mentioned above is also started from here.
In addition, there are many other experiences in judging the color color deviation, such as memory color, like blue sky and white clouds, grass green space, etc. These colors have deep memory in people's minds, which is more important for professional image processing personnel. It is necessary to remember the proportion of CMYK of these colors. The more you remember, the better you can judge whether the color of the image is accurate or not.
3 should pay attention to the color shift problem of RGB to CMYK
Once again, the printing uses four colors of magenta, cyan, yellow and black. Therefore, the colors are all specified by CMYK. As long as CMYK is determined, it means that the proportion of printed ink is determined, and the color usually does not have much problem. However, the color in the computer is expressed in RGB mode. Therefore, to print the RGB color, it is necessary to go through a color conversion step. Two problems will occur here:
The first is that there is no way for the ink to print out all the RGB colors. Usually high-brightness colors, fluorescent colors, and metallic luster are not printed. At this time, the program has to be replaced with the closest CMYK. If you use Photoshop, you will find that the selected color square has a triangle exclamation mark, or an exclamation point after the CMYK value indicates that the color cannot be printed.
The second question is, what conversion standard is based on RGB to CMYK? Even if an RGB color can be converted to CMYK, it does not mean that the actual ink color will be the same as the color displayed on the screen. Usually the biggest problem is that it occurs on cyan. In the uncorrected case, the cyan (R:0 G:255 B:255) seen on the screen is a very bright color, but the actual printing cyan (C: 100% M: 0% Y: 0% K) : 0%) is a completely different feeling, with a lot of cyan colors, such as blue and green, have serious color cast problems.
4 solutions
Using CMYK to specify color Some software, such as CorelDRAW and Photoshop, will especially consider the correctness of conversion between RGB and CMYK. As long as CMYK is used directly in the software to specify the color, you can get results close to the actual printing, so Use CMYK mode as much as possible to operate. Photoshop can convert the image file into CMYK mode for editing (MENU:Image>Mode>CMYK color), so you can completely avoid using unprintable colors. The disadvantage is that most filters can't work in CMYK mode. The method has to be temporarily converted to RGB mode, use the filter and then switch back to CMYK mode.
Another way is to use the printed color table to query the actual printed color. The color table is a reference template for color values, which lists various combinations of CMYK for query. So when you want to choose a specific color, first find the desired color from the color table, and then input the CMYK value marked on the table into the software. For the average person, this is the most effective color correction method. In actual use, it is of course impossible to check all the colors appearing in the layout, as long as the focus (such as text, frame line), or large-area coloring is confirmed. There is another thing like the color chart, called the standard color standard. This thing is to name the ink used in the printing system and print the samples one by one. The color inside must be printed with special ink and can't be used in CMYK printing.
In summary, modern designers should have the corresponding pre-press knowledge, and even the best design ideas will eventually become the finished products in our hands, so color as an important element in graphic design, not only in the print The effect of the pre-image has a decisive effect, and the correctness of the pre-press correction is more related to the quality of the finished product.
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