Talking about Digital Image Quality Control Method
2024-09-14 11:00:45
With the continuous development of computer technology, prepress technology is continuously replenishing fresh blood. It should be said that prepress technology is the most vigorous in the entire printing field. Digital image quality control in the prepress area is a prerequisite for ensuring the quality of prints, and scanning is the most commonly used image digitization method. The quality of digital images is closely related to the performance of the scanner. The prerequisite for obtaining higher quality digital images is to accurately debug the scanner and control the data.
The scanning process begins with preparations, including scanner installation and document preparation. After the scanner is installed, in order to ensure the stability of the scanning quality, at least let the scanner warm up for a period of time before the scanner is used (refer to the scanner's production manual) in order to obtain better lighting conditions and a better tonal range. The preparation of a manuscript primarily refers to the comprehensive observation of the type, content, color, gradation, and clarity of the manuscript in accordance with the characteristics of the image reproduction to determine the process parameters that are compatible with the purpose of copying, manuscript content, and art reproducibility. When placing the original on the scanner's original holder, be careful to keep it flat and clean, parallel to the edge of the original holder.
Followed by the basic parameters of scanning settings, including scanning mode (professional mode, home mode, automatic mode, if the scanned image is applied to printing, it is recommended to use professional mode), original type (transmission, reflection, etc.), scan image mode (RGB color mode, CMYK color mode, grayscale color mode, etc.). After the basic parameters of the scan are determined, prescans are performed at low resolution to determine the area for fine scans.
The final step is to adjust the main control parameters of the scan quality and complete the scan. The main control parameters for scan quality include resolution, color and gradation, sharpness, and descreening. After scanning, if the resulting image is insufficient, a series of post-processing must be performed. For example, you can use Photoshop software to complete image trimming, rotation, size and resolution adjustments to further adjust the image's color, level and clarity, use trimming tools to eliminate layout scratches, smears, etc., and then select the appropriate file format for the image storage.
Adjustment of scanning quality control parameters Selecting the black and white fields of the original document correctly selects the black and white fields in the original image, with the purpose of finely adjusting the brightness level of the image. Black field refers to the darkest place in the manuscript, but it is not necessarily the darkest place in the manuscript. White field refers to the brightest place in the original, but it is not necessarily the brightest place in the manuscript. If the brightest part of the manuscript has no hierarchy, it should not be selected as a white field, and vice versa. In general, the density range between black and white fields covers all levels of the original, so black and white fields should be selected in the main part of the original document that needs to be scanned. For example, a color photograph, which includes people and landscapes, should in principle be selected on the portraits of people in black and white fields. In order to highlight the main characters, they should generally not be selected as scenery on the back.
In comparison, the selection of white fields is particularly important, and the scanning of the highlights and midtones of the original has the greatest impact. The two parts are usually regarded as the keynote of the manuscript and are the main focus of the scan. It will affect the level and color of the image. Therefore, the selection of the white field should be careful, you can choose more than a few white fields, try to scan, compare the results, and finally determine a suitable white field. If the black and white fields can be correctly selected, the scanned image will be very clear, evenly distributed, and clearly structured.
Neutral gray balance curve Neutral gray balance is one of the most important factors in prepress image scanning. It determines whether the scanned image produces overall color cast and partial color cast. Since the confirmation of the neutral gray balance has a certain degree of psychological factors, at the same time the printing conditions have a very sensitive influence on the neutral gray balance. Therefore, neutral gray balance must depend on the stability of printing conditions. Therefore, it is very important to stabilise the printing department's paper, ink, and the dot enlargement value of each color sequence. In the operation of the scanner, neutral gray balance data is the main basis in the image adjustment operation. Neutral gray balance can be measured using the following two methods:
Gray ruler test: After using Kodak or Fuji reflective ruler to perform normal scanning and color separation, the film is output and printed for proofing. Then use the reflection densitometer to measure the density of the reflection ladder at all levels as the density of the various levels of the abscissa, and at the same time, the grayscale rule points of the C, M, Y, and K color films corresponding to the gray ladder ruler As the ordinate data. This will plot the state of the three CMY curves, and correct the CMY tri-color curve data based on the gray-scale ruler's performance on the neutral gray region. The corrected neutral gray balance data is input to the scanner, and the scanned film is output again after being scanned for proof printing until a satisfactory neutral gray balance data is obtained.
Neutral gray spectrum method: The chromatogram includes two different parts. Some of these include ten color gamut zones, each with a fixed dot and different magenta and yellow colors. This part is designed for gray balance evaluation, and the other part is designed for the design of the black version of the network cable. Each color gamut is divided into 25 areas with black ink printed graticules for the purpose of evaluating neutral ash, and to obtain a density similar to the gray portion formed by the three-color overprint. If the two are the same, the grey color of the gray and black ink of the three-color mixture at equal brightness will be indistinguishable within a reasonable visual distance. This produces a simple reference to determine which part is trichromatic mixed grey.
A color patch is selected in each color gamut region in the neutral gray spectrum. This color block should be the closest to visual gray. The assessment should be conducted under standard visual conditions. When formulating a neutral gray curve, there may be a deviation from a certain point on the curve due to the observer's visual deviation. At this point, the data of a few extreme points should be corrected to keep the neutral gray balance data as a smooth curve state.
The hierarchical representation of the hierarchical curve image is also the brightness representation of each region of the image. The highest density exhibited by the printed matter is between 1.8 and 1.9, and the gamma (Y) of the best level curve of the printed matter is 1.8. Therefore, for different types of originals, the scope of the copied level curve should be different. In addition, factors such as printing conditions, ink, and paper will greatly affect the printed dot gain value. Therefore, the state of the level curve should also be taken into account. Of course, the above factors usually have the greatest impact on the area of ​​50% of the area, so it is necessary to control the state of the middle point of the hierarchy curve. Due to the quality of the image hierarchy, basically relying on the naked eye to observe, so the final determination of the state of the level curve is often confirmed by a gray ladder and a standard manuscript with rich levels and colors for multiple scanning and proofing.
Adjustment of the color system Due to the color error caused by the ink and the paper, and when the R, G, and B filters in the scanner separate colors, it is not possible to completely filter all the colors, resulting in color errors. In order to make the printed image conform to the original as much as possible, the color must be adjusted to correct the color error caused during the scanning and printing process, and the color should be adjusted to the standard recognized by human psychology for certain colors. In professional scanners and Photoshop software, the adjustment of the colors is performed in the six color systems of R, Y, G, C, B, and M, rather than one color block in the image. Since the color hue change is a continuously changing process, when the hue of a certain color system is adjusted, it will inevitably affect the color change of its adjacent two color systems. In addition, as the opposite color of a certain color system, the brightness of the color will be affected. It should be noted that the brightness of the color is also the hierarchical representation of the image.
At present, the color is calculated and adjusted by LCH or Lab on some professional scanner software. Therefore, it requires the operator to understand the meaning of the above two methods of color description so that the color can be adjusted accurately and quickly.
The false light mask is used to produce a very fine white border or black border between the density differences of the image, so that the clarity of the image produced by the visual effect is improved. But in fact it does not play a role in image resolution. Due to its effect on the difference in the intensity of the image brightness, proper use of the false light mask will be a good representation of the finer levels of the image. On the other hand, an excessively intense false light mask will coarsen the image grain and create a strong sense of relief, giving the image a rough feeling. In some roller scanners, the dummy mask can be controlled in each channel. In the control software of the flatbed scanner, it is basically the same as the setting of the PhotoShop software. The basic data settings of the virtual light mask are constrained by factors such as image magnification, printed network lines, paper, and dot gain values.
Quality is the life of a company. The quality of a product is directly related to its market competitiveness. Improving the quality of digital images is of great significance for improving the quality of prints. As a key factor in the printing production process, the quality control of digital images is an important issue that should be given high attention and serious research in the entire printing process. Only in this way can we ensure that printed materials are of higher quality and strive for more customer resources. Strong position in the fierce printing competition.
The scanning process begins with preparations, including scanner installation and document preparation. After the scanner is installed, in order to ensure the stability of the scanning quality, at least let the scanner warm up for a period of time before the scanner is used (refer to the scanner's production manual) in order to obtain better lighting conditions and a better tonal range. The preparation of a manuscript primarily refers to the comprehensive observation of the type, content, color, gradation, and clarity of the manuscript in accordance with the characteristics of the image reproduction to determine the process parameters that are compatible with the purpose of copying, manuscript content, and art reproducibility. When placing the original on the scanner's original holder, be careful to keep it flat and clean, parallel to the edge of the original holder.
Followed by the basic parameters of scanning settings, including scanning mode (professional mode, home mode, automatic mode, if the scanned image is applied to printing, it is recommended to use professional mode), original type (transmission, reflection, etc.), scan image mode (RGB color mode, CMYK color mode, grayscale color mode, etc.). After the basic parameters of the scan are determined, prescans are performed at low resolution to determine the area for fine scans.
The final step is to adjust the main control parameters of the scan quality and complete the scan. The main control parameters for scan quality include resolution, color and gradation, sharpness, and descreening. After scanning, if the resulting image is insufficient, a series of post-processing must be performed. For example, you can use Photoshop software to complete image trimming, rotation, size and resolution adjustments to further adjust the image's color, level and clarity, use trimming tools to eliminate layout scratches, smears, etc., and then select the appropriate file format for the image storage.
Adjustment of scanning quality control parameters Selecting the black and white fields of the original document correctly selects the black and white fields in the original image, with the purpose of finely adjusting the brightness level of the image. Black field refers to the darkest place in the manuscript, but it is not necessarily the darkest place in the manuscript. White field refers to the brightest place in the original, but it is not necessarily the brightest place in the manuscript. If the brightest part of the manuscript has no hierarchy, it should not be selected as a white field, and vice versa. In general, the density range between black and white fields covers all levels of the original, so black and white fields should be selected in the main part of the original document that needs to be scanned. For example, a color photograph, which includes people and landscapes, should in principle be selected on the portraits of people in black and white fields. In order to highlight the main characters, they should generally not be selected as scenery on the back.
In comparison, the selection of white fields is particularly important, and the scanning of the highlights and midtones of the original has the greatest impact. The two parts are usually regarded as the keynote of the manuscript and are the main focus of the scan. It will affect the level and color of the image. Therefore, the selection of the white field should be careful, you can choose more than a few white fields, try to scan, compare the results, and finally determine a suitable white field. If the black and white fields can be correctly selected, the scanned image will be very clear, evenly distributed, and clearly structured.
Neutral gray balance curve Neutral gray balance is one of the most important factors in prepress image scanning. It determines whether the scanned image produces overall color cast and partial color cast. Since the confirmation of the neutral gray balance has a certain degree of psychological factors, at the same time the printing conditions have a very sensitive influence on the neutral gray balance. Therefore, neutral gray balance must depend on the stability of printing conditions. Therefore, it is very important to stabilise the printing department's paper, ink, and the dot enlargement value of each color sequence. In the operation of the scanner, neutral gray balance data is the main basis in the image adjustment operation. Neutral gray balance can be measured using the following two methods:
Gray ruler test: After using Kodak or Fuji reflective ruler to perform normal scanning and color separation, the film is output and printed for proofing. Then use the reflection densitometer to measure the density of the reflection ladder at all levels as the density of the various levels of the abscissa, and at the same time, the grayscale rule points of the C, M, Y, and K color films corresponding to the gray ladder ruler As the ordinate data. This will plot the state of the three CMY curves, and correct the CMY tri-color curve data based on the gray-scale ruler's performance on the neutral gray region. The corrected neutral gray balance data is input to the scanner, and the scanned film is output again after being scanned for proof printing until a satisfactory neutral gray balance data is obtained.
Neutral gray spectrum method: The chromatogram includes two different parts. Some of these include ten color gamut zones, each with a fixed dot and different magenta and yellow colors. This part is designed for gray balance evaluation, and the other part is designed for the design of the black version of the network cable. Each color gamut is divided into 25 areas with black ink printed graticules for the purpose of evaluating neutral ash, and to obtain a density similar to the gray portion formed by the three-color overprint. If the two are the same, the grey color of the gray and black ink of the three-color mixture at equal brightness will be indistinguishable within a reasonable visual distance. This produces a simple reference to determine which part is trichromatic mixed grey.
A color patch is selected in each color gamut region in the neutral gray spectrum. This color block should be the closest to visual gray. The assessment should be conducted under standard visual conditions. When formulating a neutral gray curve, there may be a deviation from a certain point on the curve due to the observer's visual deviation. At this point, the data of a few extreme points should be corrected to keep the neutral gray balance data as a smooth curve state.
The hierarchical representation of the hierarchical curve image is also the brightness representation of each region of the image. The highest density exhibited by the printed matter is between 1.8 and 1.9, and the gamma (Y) of the best level curve of the printed matter is 1.8. Therefore, for different types of originals, the scope of the copied level curve should be different. In addition, factors such as printing conditions, ink, and paper will greatly affect the printed dot gain value. Therefore, the state of the level curve should also be taken into account. Of course, the above factors usually have the greatest impact on the area of ​​50% of the area, so it is necessary to control the state of the middle point of the hierarchy curve. Due to the quality of the image hierarchy, basically relying on the naked eye to observe, so the final determination of the state of the level curve is often confirmed by a gray ladder and a standard manuscript with rich levels and colors for multiple scanning and proofing.
Adjustment of the color system Due to the color error caused by the ink and the paper, and when the R, G, and B filters in the scanner separate colors, it is not possible to completely filter all the colors, resulting in color errors. In order to make the printed image conform to the original as much as possible, the color must be adjusted to correct the color error caused during the scanning and printing process, and the color should be adjusted to the standard recognized by human psychology for certain colors. In professional scanners and Photoshop software, the adjustment of the colors is performed in the six color systems of R, Y, G, C, B, and M, rather than one color block in the image. Since the color hue change is a continuously changing process, when the hue of a certain color system is adjusted, it will inevitably affect the color change of its adjacent two color systems. In addition, as the opposite color of a certain color system, the brightness of the color will be affected. It should be noted that the brightness of the color is also the hierarchical representation of the image.
At present, the color is calculated and adjusted by LCH or Lab on some professional scanner software. Therefore, it requires the operator to understand the meaning of the above two methods of color description so that the color can be adjusted accurately and quickly.
The false light mask is used to produce a very fine white border or black border between the density differences of the image, so that the clarity of the image produced by the visual effect is improved. But in fact it does not play a role in image resolution. Due to its effect on the difference in the intensity of the image brightness, proper use of the false light mask will be a good representation of the finer levels of the image. On the other hand, an excessively intense false light mask will coarsen the image grain and create a strong sense of relief, giving the image a rough feeling. In some roller scanners, the dummy mask can be controlled in each channel. In the control software of the flatbed scanner, it is basically the same as the setting of the PhotoShop software. The basic data settings of the virtual light mask are constrained by factors such as image magnification, printed network lines, paper, and dot gain values.
Quality is the life of a company. The quality of a product is directly related to its market competitiveness. Improving the quality of digital images is of great significance for improving the quality of prints. As a key factor in the printing production process, the quality control of digital images is an important issue that should be given high attention and serious research in the entire printing process. Only in this way can we ensure that printed materials are of higher quality and strive for more customer resources. Strong position in the fierce printing competition.
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